Early Career Conference in Trapped Ions (ECCTI) 2024

7 Jul 2024, 17:30 → 12 Jul 2024, 19:00 Europe/Vienna

Hörsaal A (Technik) (Viktor-Franz-Hess Haus)

ECCTI Mail, Elise Wursten (RIKEN), Fabian Anmasser, Manuel John, Silke Auchter (Infineon Technologies Austria AG), Simon Lechner (CERN), Tommaso Faorlin (Universität Innsbruck)

Join us at ECCTI from the 7th-12th of July 2024!

ECCTI is intended to connect a broad community with very diverse scientific goals with common technical challenges.

We invite graduate students and early career researchers (within 5 years of completing a PhD) to share their cutting-edge work with a global audience. Dive into the physics research of today with a focus on:

• Atomic Clocks
• Quantum Information & Computation, Quantum Simulation, Quantum Technologies
• Antimatter Physics
• Precision & Molecular Spectroscopy
• Nuclear Physics

Why Attend? Engage in fruitful discussions shaping the future of physics. Connect with potential colleagues, broaden your perspectives, and partake in interactive sessions to develop skills essential for a successful career in research or industry. In addition, we will have the honor to host a lecture by Nobel prize laureate, David Wineland, who received the prize for his pioneering work on ground state cooling of trapped ions and for opening the door to the experimental study of the interaction between light and matter.

🌈 We encourage applications from a diverse community. Additional support is available for those facing attendance barriers. Part-time PhD students and those having been on career breaks are exempt from the 5-year post-PhD limit. Each application is assessed individually.

poster_ECCTI24_rev07.pdf

Sunday, 7 July

17:30 → 20:30 Conference Practicalities: Registration & welcome reception

Monday, 8 July

09:00 → 09:20 Conference Practicalities: Welcome to ECCTI 2024

09:20 → 10:30 Nuclear Physics

09:20 Precision mass measurement of proton-dripline nucleus $^{22}$Al and implications on suspected halo nature in the ground state

Halo nuclei exist at the extremes of nuclear structure where a isotopes’ mass distribution extends far outside the compact core: a consequence of a weakly bound nucleon(s). The unique properties of these isotopes provide stringent tests for nuclear structure models. These nuclei are positioned on the nuclear driplines, often restricting experimental access due to low production rates or short half-lives. Proton-halo nuclei are further suppressed due to the confining effect of the Coulomb barrier. The Facility for Rare Isotope Beams (FRIB) has extended the reach towards these isotopes, including $^{22}$Al whose halo nature has recently been suggested based on observed isospin-symmetry breaking effects in the sd-shell region [1]. The level scheme found in this work, however, contains significant uncertainties as a result of its unmeasured mass, thus impacting the mirror asymmetry parameter. Precise knowledge of these isotopes’ binding energy, i.e. mass, is paramount due to the role of weak binding in the emergence of the halo structure. The Low Energy Beam Ion Trap (LEBIT) facility at FRIB used Penning trap mass spectrometry to determine a mass excess for the $^{22}$Al ground state of $\text{ME}=18\;093.6(7)$~keV, a factor of thirty improvement in uncertainty to the last measured value [2]. This result agrees well with the predicted binding energy from $\textit{sd}$-shell USD Hamiltonians, which also predicts restricted halo formation due to minimal $1s_{1/2}$ occupation in the proton shell. A particle-plus-rotor model additionally investigates the possibility of enhanced s-wave occupation from the interplay of weak binding. Ultimately, our findings suggest the existence of halo structure in the $^{22}$Al ground state would require strong continuum-induced deformation, similar to the suspected situation for $^{29}$F [3].

This work was conducted with the support of Michigan State University and the National Science Foundation under Grants No. PHY-1102511, PHY-1126282, PHY-2111185, and PHY-2238752. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics and used resources of the Facility for Rare Isotope Beams (FRIB) Operations, which is a DOE Office of Science User Facility under Award Number DE-SC0023633, and under the FRIB Theory Alliance Award No. DE-SC0013617.

[1] J. Lee, et al., Phys. Rev. Lett. 125, 19 (2020)
[2] M.Z. Sun et al., Chinese Phys. C 48, (2024)
[3] K. Fossez and J. Rotureau, Phys. Rev. C 106, 3 (2022)

Speaker: Mr Scott Campbell (Michigan State University, Facility for Rare Isotope Beams)

09:43 Towards Measurements of Electroweak Nuclear Properties using Single Molecular Ions in a Penning Trap

We present the development of a novel Penning ion trap for precision spectroscopy of symmetry-violating electroweak properties using single trapped molecular ions [1]. The high magnetic field of the Penning trap can be used to Zeeman shift two molecular states of opposite parity to near degeneracy, enhancing the sensitivity of parity-violating nuclear properties by more than 11 orders of magnitude [2]. Hence, our proposed experimental setup is expected to provide highly sensitive measurements of symmetry violating nuclear properties across the nuclear chart. This contribution will describe the status of a cryogenic Penning trap for performing measurements in SiO+ and TlF+ molecules, as well as discuss future prospects of this technique.

[1] J. Karthein, S. Udrescu, S. Moroch et al. arXiv:2310.11192 (2023)
[2] Altuntas, E. et al. Phys. Rev. Lett. 120, 142501 (2018)

Speaker: Scott Moroch (Massachusetts Institute of Technology)

10:05 Progress of the LMU $^{229m}$Th$^{3+}$ Trapped-Ion Nuclear Clock Project

The $^{229}$Th nucleus assumes a unique role in the nuclear landscape for its low-lying isomeric first excited state $^{229m}$Th with an excitation energy of $8.338 \pm 0.024$ eV [1], accessible with modern VUV-laser systems. A nuclear clock based on this thorium isomer holds promise not only to push the limits of high-precision metrology with a fractional uncertainty expected in the range of $10^{-19}$ [2], but also to contribute to dark matter and other fundamental physics research as a novel quantum sensor.
It will also be able to contribute to the search for theoretically expected temporal fluctuations of fundamental constants like the fine-structure constant $\alpha$ [3].

The cryogenic Paul-trap experiment currently operated at the LMU Munich is primarily designed for long ion-storage times, allowing for the measurement of the still unknown ionic lifetime of the isomer. The lifetime is expected to be several thousands of seconds and its determination is essential for the realization of a nuclear frequency standard. In a second step, the setup will be a platform for VUV-comb spectroscopy of the $^{229}$Th nuclear transition, paving the way towards a first nuclear clock prototype.

In this poster, the building blocks of the experimental setup for trapping and sympathetic laser cooling of $^{229m}$Th$^{3+}$ by $^{88}$Sr$^{+}$ are presented and the status of first measurements, such as trapping, storage, and Doppler-laser cooling of $^{88}$Sr$^{+}$, are discussed.

This work was supported by the European Research Council (ERC)
(Grant agreement No. 856415) and BaCaTec (7-2019-2).

[1] Kraemer, S., Moens, J., Athanasakis-Kaklamanakis, M. et al., Observation of the radiative decay of the 229Th nuclear clock isomer, Nature 617, 706–710 (2023)
[2] C. J. Campbell et al., Single-Ion Nuclear Clock for Metrology at the 19th Decimal Place, Phys. Rev. Lett. 108, 120802 (2012)
[3] E. Peik et al., Nuclear clocks for testing fundamental physics, Quantum Sci. Technol. 6, 034002 (2021)

Speakers: Georg Jakob Holthoff (Ludwig Maximilians Universitat (DE)), Markus Wiesinger (Max Planck Society (DE))

10:30 → 11:00 Coffee break

11:00 → 12:35 Quantum Information & Computing

11:00 Quantum Information Processing with Trapped Ion Qudits

In the current stage of its evolution, Quantum Information Processing is
following the precedent set by classical computing and generally encodes information in binary form, thus relying on so-called qubits. For many of the systems used to process this quantum information, this is however a rather artificial constraint, limiting the Hilbert space available for computation and introducing leakage errors that slip through the most common error correction schemes. Going from qubits to qudits, non-binary logical base states, to leverage a larger Hilbert space for computation is therefore one path towards building more powerful and reliable Quantum Information Processors that can be used to solve real-world problems. We use trapped Calcium ions to encode information in higher-dimensional qudits up to d = 7, demonstrate comparable performance to a qubit-based processor and set out to build a new experimental setup dedicated exclusively to QIP with trapped Calcium qudits. This will enable us to carry out multiple-different entanglement gates on qudits, reducing the experimental overhead required when entangling higher-dimensional systems.

Speaker: Peter Tirler (University of Innsbruck, Quantum Optics and Spectroscopy Group)

11:18 Stimulated Raman 2-qubit logic gates in metastable trapped-ion qubits

A proposed scheme for implementing trapped-ion quantum computing encodes qubits in different types of electronic levels where logic gates can be implemented with low cross-talk, know as the omg architecture [1]. One type of qubit this scheme employs is the metastable (m) qubit, which has not been widely studied. We have implemented m qubits in the D$_{5/2}$ manifold of $^{40}$Ca$^+$ and performed one- and two-qubit stimulated Raman gates, one of the first entangling gates performed in m qubits. We perform these gates using laser beams tuned 44 THz red of the 854 nm D$_{5/2}$ to P$_{3/2}$ transition with increased power using a fiberized injection-locked 976 nm diode laser system. The injection-locked scheme allowed for a three-fold increase in gate speed compared to using a single free-space laser diode setup by increasing the power in each of the two beams from 80 mW to 250 mW. We have measured the spontaneous Raman scattering rate from these beams, and comparing these results to scattering models we have developed that account for effects relevant at large detunings [2], we find that spontaneous Raman scattering error rates at this wavelength can be made low enough that they are no longer a limiting factor in achieving fidelities needed for fault-tolerance.
[1] D. T. C. Allcock et al., Appl. Phys. Lett. 119, 214002 (2021)
[2] I. D. Moore et al., Phys. Rev. A 107, 032413 (2023)

Speaker: Sean Brudney (University of Oregon)

11:36 Snapshotting Quantum Dynamics at Multiple Time Points

Measurement-induced state disturbance is a main challenge in obtaining quantum statistics at multiple time points. We propose a method to extract dynamic information from a quantum system at intermediate time points, namely snapshotting quantum dynamics. In order to do this, we introduce a multi-time quasi-probability distribution (QPD) that correctly recovers probability distributions at respective times and construct a systematic protocol to reconstruct the multi-time QPD from measured data. Our approach can also be applied to extract correlation functions for various time orderings. We provide a proof-of-principle experimental demonstration of the proposed protocol using a dual-species trapped-ion system. We employ 171Yb+ and 138Ba+ ions, respectively, as the system and the ancilla to perform multi-time measurements that consist of repeated initialization and detection of the ancilla state without directly measuring the system state. The two- and three-time QPDs are reconstructed, where the dynamics of the system are faithfully monitored. We also observe negativity and complex values in multi-time QPDs which clearly indicate a contribution of quantum coherence in the dynamics. Our scheme can be applied to any multi-time measurements of a general quantum process to explore the properties of quantum dynamics.

[1] arXiv:2207.06106

Speaker: Pengfei Wang (Beijing Academy of Quantum Information Sciences)

11:54 Novel tweezer assisted sub-Doppler cooling of trapped 171Yb+ ion crystal

We propose a new sub-Doppler cooling scheme in trapped ion crystals in Paul traps which utilizes a Sisyphus-like cooling mechanism to simultaneously cool all the motional modes of the crystal.
We use a hollow tweezer, tuned near resonance with the transition from the qubit manifold to a short-lived excited manifold, to generate a state-dependent tweezer potential. This introduces a position dependent quench rate for the qubit states. The cooling scheme is completed by using a microwave field to drive the magnetic dipole transition between the qubit states, creating a Sisyphus-like cooling mechanism which is augmented by the position dependent effective lifetime.

We identify the optimal cooling parameters for one and two-ion crystals exactly, and use a variational ansatz to extract the cooling rate for larger ion crystals. We also show that this cooling scheme is relatively robust against tweezer pointing errors. Furthermore, the scheme allows for the entire crystal to be cooled sympathetically by adressing a single ion with the tweezer, while not destroying the internal qubit state of the other ions.

Speaker: Bas Gerritsen (University of Amsterdam, ITFA)

12:35 → 14:00 Lunch break

14:00 → 16:00 Skill session: Seminar by Nobel-laureate David Wineland

16:00 → 16:30 Coffee break: Conference Photo

16:30 → 19:00 Poster session

Tuesday, 9 July

09:00 → 10:30 Antimatter

09:00 ALPHA-g data analysis: determining the gravitational acceleration of antihydrogen

The ALPHA-g experiment at CERN recently made the first direct observation of the effect of gravity on the motion of antimatter [1]. The result – that antihydrogen falls towards the Earth – is consistent with Einstein’s Weak Equivalence Principle.

In ALPHA-g, antihydrogen is produced by combining antiproton and positron plasmas, each confined in Penning-Malmberg traps. Antihydrogen is subsequently confined in an Ioffe-Pritchard magnetic trap with its axis aligned parallel to the Earth's gravitational field; an octupole provides radial confinement and two solenoids (one above and one below the trapping region), provide axial confinement. The gravitational potential adds to the magnetic potential; when the magnetic fields from the upper and lower solenoids are equal, the gravitational potential results in an up-down asymmetry in the total potential.

An imposed difference in magnetic field in the upper and lower solenoids, known as a bias, is delicately adjusted over a range of values. At each value of the magnetic bias, the magnetic fields of the solenoids are ramped down slowly compared to the antiatom motion, releasing the antihydrogen, and leading to annihilations on the walls of the apparatus which are detected by a position and time sensitive detector. If the imposed bias cancels the gravitational potential, antihydrogen escapes upwards or downwards with equal probability.

Determining the downward, $p_{\text{dn}}$, escape probability from observed annihilations is non-trivial because the efficiency with which antihydrogen annihilations are detected in the upper and lower regions may be different, some small fraction of antihydrogen escaping downwards may be detected in the upper region (and vice versa) and the precise number of trapped antihydrogen atoms is unknown. In addition, cosmic rays passing through the apparatus lead to a background annihilation rate which may also be up-down asymmetric.

A Bayesian method is employed to determine $p_{\text{dn}}$. The likelihood analysis assumes annihilations detected in the upper and lower regions are independently Poisson distributed, with Poisson mean expressed in terms of the relative detector efficiency, the efficiencies with which annihilations are detected in the incorrect region, the cosmic background annihilation rates, and $p_{\text{dn}}$. We solve for the posterior $p_{\text{dn}}$ using the Markov-Chain Monte-Carlo integration package, Stan [2].

Further, we determine the gravitational acceleration of antihydrogen and a statistical error by modifying the likelihood analysis described above to include results from simulations of the experimental procedure. In the modified analysis, $p_{\text{dn}}$ is replaced by the simulated probability of downward escape, which is a function of the antihydrogen gravitational acceleration.

Future increased precision measurements of antimatter gravity will involve transferring the trapped antiatoms to shallower confining potentials. Adiabatic cooling [3] during magnetic transfer will reduce antiatom loss and further increase sensitivity to gravity.

[1] Anderson, E.K., Baker, C.J., Bertsche, W. et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023). https://doi.org/10.1038/s41586-023-06527-1
[2] Stan Development Team. 2023. Stan Modeling Language Users Guide and Reference Manual, 2.31. https://mc-stan.org
[3] D. Hodgkinson, On the Dynamics of Adiabatically Cooled Antihydrogen in an Octupole-Based Ioffe-Pritchard Magnetic Trap, Ph.D. thesis, The University of Manchester (2022).

Speaker: Danielle Louise Hodgkinson (University of California Berkeley (US))

09:23 Sympathetic cooling of a Be+ ion by a Coulomb crystal of Sr+ ions: a test bed for taming antimatter ions (GBAR)

The GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN, situated on the antiproton decelerator ring (AD), is aimed at investigating the free fall of antihydrogen atoms prepared at rest, as suggested by J. Walz and Th. Hänsch [1]. This experiment employs two trapped ions: one is a Be+ ion cooled via laser, while the other, an Hbar+ ion, undergoes cooling through interactions with the Be+ ion (sympathetic cooling).

We are developing a test-bed experiment designed to explore sympathetic cooling of a light ion using a cloud of laser-cooled heavy ions, mimicking the conditions anticipated in the GBAR project [2]. The experimental setup involves the pairing of 88Sr+ (laser-cooled ion) and 9Be+ (sympathetically-cooled ion). The choice of these ions offers two advantages: the ability to optically address the 9Be+ ion for thermometry measurements and their mass ratio (88/9 ≈ 9.8) closely resembling that in the GBAR project (9/1).

Preliminary experimental results [3] demonstrate sympathetic cooling of 9Be+ ions by laser-cooled 88Sr+ ions, forming a Coulomb crystal. Detection of sympathetically cooled ions relies on analyzing Coulomb crystal images, revealing dark areas where non-laser-cooled Be+ ions reside. Molecular dynamics simulations showed strong spatial segregation for both species, due to their high mass ratios [4].

However, the initial experiment did not include laser addressing of the 9Be+ ions for measuring cooling dynamics and control over the initial energy of Be+ ions. The next stage of the project therefore involves adding the 313nm laser for addressing Be+ ions and utilizing a 2-zone trap to control the initial energy of a single Be+ ion. This will enable us to measure for the first time the capture dynamics of a light ion by a Coulomb crystal and to follow its cooling over several decades (typically from 10000K to mK). We will compare these measurements with numerical simulations [4].

Initially, only Sr+ ions were employed to validate trapping and cooling conditions, characterize photon collection optics, and test ion transport protocols between trapping zones. A method utilizing Doppler recooling was developed to characterize the initial energy of Sr+ ions upon arrival in the target trap. Subsequently, using the Be+ ion cooling laser at 313 nm, it will be possible to cool a single Be+ ion and transport it with controlled kinetic energy to the second trapping zone, already loaded with a Sr+ Coulomb crystal. The kinetic energy loss of the Be+ ion is quantified by measuring laser-induced fluorescence rate at resonance, which produces no laser cooling or heating. Thermalization of the light ion via coulomb interactions will then be studied for different heavy-ion crystal temperatures, shapes and ion numbers.

REFERENCES
[1] J. Walz and T. Hänsch, General Relativity and Granvitation, (2004) 561
[2] P. Perez and Y. Sacquin, Classical and Quantum Gravity, 29 (2012)
[3] A. Douillet et al., 1st North American Conference on Trapped Ions (NACTI 2017) Boulder, USA, 2017
[4] N. Sillitoe et al., JPS Conf. Proc. 18, 011014 (2017)

Speaker: Derwell Drapier

09:45 Studies of highly charged ions formed using antiprotons at AEgIS

Antimatter Experiment: gravity, Interferometry, Spectroscopy (AE$\bar{\hbox{g}}$IS) achieves pulsed production of antihydrogen using a charge-exchange reaction between antiproton and Rydberg positronium (an electron and a positron in a bound state). The AE$\bar{\hbox{g}}$IS experiment is used to probe antimatter bound systems for measurements of gravitational free fall and precision studies of positronium. More recently a new program has been started which focuses on the controlled formation and studies of antiprotonic atoms - a bound state consisting of antiproton in an orbit around a matter nucleus.

Ongoing developments strive to achieve controlled formation of antiprotonic atoms by co-trapping anions with cold antiprotons. Then one laser would neutralize the ions and subsequently another laser would excite the formed neutral atom to a Rydberg state for a charge-exchange reaction with the antiprotons.

The controlled formation of antiprotonic atoms within the trap allows a detailed spectroscopic study of antiprotonic bound states in a trap.
In the case that an antiprotonic atom is formed, the antiproton would cascade down the electronic energy levels causing emission of Auger electrons and x-rays until the antiproton is close enough to annihilate on the nucleons, resulting in highly charged nuclear fragments. Capturing highly charged positive ions would be of further interest for nuclear structure studies.

A new procedure for proof-of-principle measurement was developed. Low pressure nitrogen gas was introduced into the AE$\bar{\hbox{g}}$IS apparatus. Antiprotons were then trapped inside the apparatus and a nested trap was formed to capture positive ions. Subsequently, the antiprotons were released, and the nested trap was reshaped for time-of-flight (TOF) measurement. Then, the ions in the nested trap were released, and TOF spectra were captured using an MCP. To analyze the TOF spectra, we compared them with simulations. This work has shown that it is possible to trap ions formed by the antiproton interaction with a gas and use the AE$\bar{\hbox{g}}$IS apparatus as a TOF spectrometer capable of giving insights into mass to charge ratios of the ions.

Speaker: Mr Valts Krumins (University of Latvia (LV))

10:08 Towards a 10-fold improved measurement of the antiproton magnetic moment

The standard model of particle physics provides one of the currently best descriptions of nature but fails to account for the asymmetry between matter and antimatter that is observed on cosmological scales. One way to investigate this problem is the test of CPT-Invariance by comparisons between fundamental proton and antiproton properties. [1]

The BASE collaboration is specialized in the use of advanced Penning trap setups as well as cryogenic superconducting detection systems with single particle resolution to perform high precision measurements on protons and antiprotons. Past measurements include the comparison of antiproton and proton charge-to-mass ratio with a fractional precision of 16 parts per trillion (ppt) [2] and the antiproton g-factor with a fractional precision of 1.5 parts per billion (ppb) [3]. This 3000-fold improvement of the g-factor precision helps to constrain possible CPT-odd interactions and sets limits on possible exotic particle interactions [4]. BASE is dedicated to further improving the precision of these measurements by continuously refining its Penning trap setup and is aiming to increase the g-factor precision to the 100 ppt level.

This contribution will give an overview of the BASE antiproton experiment located at CERN and aims to explain the unique challenge that sub-ppb precision g-factor measurements in accelerator halls pose. Focus is put on the understanding, characterization and if possible, the suppression of systematic corrections due to imperfections in the magnetic and electric fields.

[1] Hori, Masaki, and J. Walz, Progress in Particle and Nuclear Physics 72 (2013)
[2] M. J. Borchert et al., Nature 601, 35 (2022)
[3] C. Smorra et al., Nature 550, 371 (2017)
[4] Smorra, C., Stadnik, Y.V., Blessing, P.E. et al., Nature 575, 310 (2019).

Speaker: Bela Peter Arndt (Max Planck Society (DE) / GSI Helmholtzzentrum für Schwerionenforschung GmbH)

10:30 → 11:00 Coffee break

11:00 → 12:45 Quantum Technologies

11:00 Towards Sideband Cooling and Thermometry on an X-Junction Surface Trap with Integrated Current Carrying Wires

Trapped ions have proved to be a promising way of realising a large-scale quantum computer. They allow for simple reproducibility and modular architectures which is crucial for a scalable, universal quantum computer. Our blueprint for a trapped-ion based quantum computer outlines operating with global microwave (MW) fields to dress the ground-state hyperfine manifold of 171Yb+ ions [1].

Borrowing knowledge from the semiconductor industry, we have produced microfabricated ion traps with embedded current-carrying wires (CCWs) which provide a controllable, high magnetic field gradient [2]. By applying a stable, fast switching current source to these wires, we measure an accurate local magnetic gradient using Ytterbium 171 and demonstrate the current working operation of this chip. The local magnetic gradient is important to provide regions of the chip where entanglement is performed, and regions where qubits can be held in a memory zone, which do not utilise magnetic gradients.

Static magnetic gradients coupled with global microwave fields enable high spin-motion coupling parameters. This allows spin-motion coupling which allows more accurate energy measurements to be performed on the motional sidebands and track the heating rate of the ion which is very important for measurements of gate infidelities and characterizing transport and reconfiguration protocols. With this scheme, we are then able to perform sideband cooling to the motional ground state, which is required for certain gate schemes, and to perform diabatic transport.

[1] B. Lekitsch, S. Weidt, A. G. Fowler, K. Mølmer, S. J. Devitt, C. Wunderlich, and W. K.Hensinger, “Blueprint for a microwave trapped ion quantum computer”, Science Advances 3 (2017).
[2] M.S.Brown et. Al. Fabrication of surface ion traps with integrated current carrying wires enabling high magnetic field gradients (2022) Quantum Sci. Technol. 7 034003

Speaker: Sahra Ahmed Kulmiya

11:23 Test and characterization of multi layer ion traps on fused silica

Quantum computing has emerged as a promising frontier with the potential to revolutionize computation by effectively tackling classically intractable problems. Among the various platforms for realizing a universal quantum computer, trapped ions have demonstrated their capabilities, allowing for quantum gate operations on quantum bits (qubits) by manipulating single or multiple ions. This approach offers notable advantages such as low error rates and long storage times [1]. However, the pursuit of a universal quantum computer inherently demands the scaling up of qubit numbers, which presents a significant engineering challenge. One such challenge is the construction of ion traps capable of storing many ions while keeping the qubit-to-qubit connectivity sufficiently high.
To tackle this scalability problem, our primary focus is on an industrially microfabricated surface ion trap designed to accommodate larger numbers of ions arranged in a two-dimensional grid [2]. Furthermore, we have developed an electrical wafer test as an example of how increasingly complex ion traps can be tested before their integration into a setup.
We use a surface ion trap with the capacity to confine 18 ions within two adjacent 1D crystals. The trap comprises three aluminum layers separated by silicon oxide, constructed on a fused silica substrate manufactured at the industrial fabrication site of Infineon Villach. Additionally, the ion trap incorporates an integrated resistance-based temperature sensor with sensitivity of 2.5 Ohm/K @ 10K to monitor the ion trap during operation. Furthermore, a comprehensive room temperature electrical wafer test concept comprising 540 measurements per chip was developed. This verifies the functionality of the trap before insertion in the setup. Heating rates below 10ph/s at an axial frequency of 1.2MHz on different trapping sites were measured to benchmark the trap in a cryogenic environment. The presentation will cover the trap concept, the electrical wafer test procedure, the characterization of the temperature sensor, and the results achieved with the ion trap in a cryogenic setup.

[1] C. Bruzewicz, Trapped-ion quantum computing: Progress and challenges, arXiv:1904.04178
[2] P. Holz, Two-dimensional linear trap array for quantum information processing, arXiv:2003.08085

Speaker: Matthias Dietl

11:45 Industrially fabricated ion trap chips for radial coupling experiments

We present and discuss industrially fabricated ion trap chips [1, 2] on the dielectric substrates Fused Silica and Sapphire.

Surface ion trap chips offer a promising platform for the scaling of ion trap quantum computers. We investigate shuttling in the radial direction as element of a scalable architecture [1]. For this, we present chips that are designed to trap ions in two-well potentials in the large separation and in the radial coupling regimes. The chips presented are capable of trapping ions or ion chains in separate rf potential wells. The design parameters of a surface ion trap in the radial coupling regime with fixed ion height and ion-ion distance are investigated. Based on this, we present improved trap designs. We discuss the simulation, design and fabrication challenges involved in creating such chips. The traps are or will be fabricated on single- and multi-layer stacks. The fabrication on multi-layer stacks enables a more complex electrode geometry and therefore more complex scalable ion trap layouts.

The ion traps are fabricated on the dielectric substrates Fused Silica and Sapphire, which are ultra-wide-bandgap materials and therefore promise excellent resilience against UV light and low rf losses. The status of industrial microfabrication on these materials is discussed, with a focus on the challenges of fabrication on different substrate materials and the fabrication of multi-layer stacks.

[1] Ph. Holz, S. Auchter et al., Adv. Quantum Technol. 3, 2000031 (2020)
[2] S. Auchter, C. Axline et al., Quantum Sci. Technol. 7, 035015 (2022)

Speaker: Michael Pfeifer (University of Innsbruck, Infineon Technologies Austria AG)

12:07 Novel ion trap with fibre cavity integration

The ion trap serves as a quantum platform with the potential to facilitate the realization of a scalable, fault-tolerant quantum computer, coupled with a straightforward photonic interface for connection to the so-called quantum network. In this system, multiple ions can be trapped within a single trap and individually controlled via laser manipulation. However, practical implementation faces several challenges, including low photon collection efficiency. Addressing this issue, integrating an optical cavity and establishing strong coupling with the ions in the trap emerges as a potent solution[1], aligning with our ultimate objective.

In this study, we have designed a prototype of a monolithic trap, characterized as a linear Paul trap. Notably, the trap can be monolithically fabricated, with the blades supplying distinct electric signals being insulated from one another through a trench structure.

The first trap has been fabricated using the selective laser etching method (SLE). Subsequently, we successfully endeavoured to trap both individual ions and ion chains within the trap and are currently engaged in the process of acquiring ion spectroscopy data.

However, during the implementation, several potential improvements are spotted, thus, the second generation has started to be designed.

In the second-generation trap, a significant modification involves the central section, where the endcap transitions into a spherical shape to enhance DC trapping efficiency. Additionally, four DC compensation blades have been incorporated to counterbalance the effects stemming from the cavity substrate. Comsol simulations on the trapping potential have been conducted to determine optimal compensation voltage and the distance between the blade and ion axis.

Furthermore, due to the reduction in the number of cavity modes resulting from the decreased mode volume, the size of the trap has been scaled down to approximately 1cm x 1cm, allowing for closer placement of the cavity mirrors. Another benefit of this reduction in size is the ability to shrink the cavity substrate accordingly, thereby enhancing the mechanical stability of the cavity system.

Moreover, the intricate wiring required to transmit electric signals via the feedthrough from outside of the chamber will be replaced by a neat printed circuit board (PCB) positioned beneath the trap itself. This PCB comprises two copper layers insulated by a dielectric layer, and the trap will be wirebonded to the PCB using gold wires. To create additional space for optical access, we intend to implement a type of PCB known as a rigid-flex PCB. In this configuration, the rigid part mirrors a standard PCB, while the flexible part, composed of polyimide, is foldable.[2] Leveraging this foldable feature, multiple dimensions within the chamber can be utilized to construct the most suitable configuration for laser beam alignment.

As of the conference date, the fabrication of the prototype for the second-generation trap is anticipated to be completed, as well as the final assembly with the new PCB.

[1]Takahashi, Hiroki, et al. "Strong coupling of a single ion to an optical cavity." Physical review letters 124.1 (2020): 013602.
[2]Sterman, Yoav. PCB Origami: Folding circuit boards into electronic products. Diss. Massachusetts Institute of Technology, 2013.

Speaker: Zhenghan Yuan

12:25 Monolithic Segmented 3D Ion Trap

We demonstrate the fabrication and operation of a linear Paul trap made from a single piece of fused silica. The glass is machined using a femtosecond laser assisted etching technique and subsequently coated with a conductive layer of gold. T-shaped trenches along the surface of the glass ensure insulation between neighbouring electrodes, without the use of shadow masks during the coating procedure. The monolithic design does not require alignment of individual components, reducing potential geometric imperfections and resulting anharmonicities.
The trap is designed to contain up to 50 ions for quantum computing and simulation experiments. The width of the electrode segments is optimised to facilitate strings of equidistant ions, which we demonstrate by confining strings of calcium ions. The trap geometry is versatile and designed to be suitable for other applications. Notably, the highly symmetric structure and axial access enables trapping of externally generated ions for spectroscopic experiments.

Speaker: Edgar Brucke (ETH Zurich)

12:45 → 14:30 Lunch break

13:45 → 14:15 Lab tours

14:30 → 16:30 Skill session: Lecture by Jean-luc Doumont

16:30 → 17:00 Coffee break

17:00 → 19:30 Poster session

Wednesday, 10 July

09:00 → 10:35 Quantum Technologies

09:00 Single ion dynamics in a phase-stable polarization gradient

Sisyphus cooling below the Doppler limit in polarization gradients has been a backbone of ultracold atom experiments for decades. It has recently been demonstrated for trapped ions as well. The potential advantage is that it can simultaneously cool multiple modes of a Coulomb crystal below the Doppler limit and could thus improve the time required to cool all modes of large ion crystals close to the motional ground state. So far, this has only been done using running wave polarization gradients. Localizing the ions at particular phases of the polarization gradient lowers the cooling limit by a factor of two. We demonstrate cooling in a phase-stable polarization gradient. The interferometric stability is created by splitting the light on-chip in integrated photonic waveguides and overlapping the emission from separate diffraction grating couplers. To our knowledge, this is the first experimental demonstration of the phase-dependence of polarization gradient cooling. This technique could cut down cooling times in large ion crystal quantum simulators and computers.

Speaker: Felix Knollmann

09:18 Chromatic suppression of spontaneous emission

Using reflecting boundary conditions, we can control the spontaneous emission of trapped $^{138}\mathrm{Ba}^+$ ions. By reflecting the ion's fluorescence light onto itself, the single photons emitted by the ions interfere with the ions themselves, allowing control over the emission rate. The control is dependent on the solid angle at which the produced photons are retro-reflected, and in order to accomplish total control, we use a hemispherical mirror that can monitor the ion from every direction of space. When the mirror radius is tuned to achieve destructive interference at the wavelength of the produced photons, fluorescence, and hence the accompanying energy transition, can be prevented. Here, I describe our current efforts to control the decay of the $6p_{1/2}$ state of the $^{138}\mathrm{Ba}^+$ ion, which can relax by emitting 493 nm or 650 nm photons. Our goal is to demonstrate control over the decay branching ratio, which could be useful in future studies, such as suppressing an undesired relaxation branch or simplifying the energy structure of ions.

Speaker: Thomas Lafenthaler

09:36 Motional spin-locking spectroscopy

Characterization of noise of a quantum harmonic oscillator is important for many experimental platforms. We experimentally demonstrate motional spin-locking spectroscopy, a method that allows to directly measure the motional noise spectrum of a quantum harmonic oscillator. In a spin-locking experiment, the free-evolution period of a Ramsey experiment is replaced with a continuous drive of a superposition of two states. Noise leads to depolarization of the initial state with a rate of depolarization that is determined by the noise strength. Probing the transition between two motional states gives access to the motional noise spectrum. We measure motional noise of a single trapped ion in a linear Paul trap in a frequency range from 200 Hz to 5 kHz with a power spectral density that resolves noise over two orders of magnitude. Coherent modulations in the oscillation frequency of the oscillator can be probed with a fractional frequency sensitivity at the $10^{-6}$ level.

Speaker: Florian Kranzl

09:54 Multi-Channel Quantum Scattering Calculation for Ultracold Ion-Atom Collisions

We focus on the theoretical modelling of the dynamics of ion-neutral systems at ultracold temperatures (<< 1K) in order to design ways for their full quantum control. Our aims are connected to experimental investigations of alkaline earth ion - alkali atom systems with hybrid traps. Due to the laser cooling scheme a metastable d-level of the alkaline-earth ion is considerably populated in these experiments, e.g. in case of 88Sr+ ion embedded in the cloud of ultracold 87Rb atoms [1] or 138Ba+ in 6Li cloud [2]. The large internal energy of the ion induces several inelastic processes like charge-exchange, spin-orbit change collisions or electronic excitation exchange.
We compute cross sections and rate coefficients for these processes within the framework of the quantum coupled-channel model considering the fine-structure of the colliding partners and the rotational coupling. Our calculations involve potential energy curves including the determination of R-dependent spin-orbit couplings (see Figs. 1, 2) following a diabatization approach [3].
Fig.1: Selected Hund's case (a) LiBa+ potential energy curves in the molecular frame. The red shadow shows the location of an avoided crossing of 21 Σ and 31 Σ responsible of the non-radiative charge exchange (NRCE) process (red arrow): Li(2s)+Ba+(5d) → Li++Ba(6s2,1S) .
Fig.2: The R-dependent spin-orbit couplings of the first 3 dissociation limits, based on Ω=0+/-, 1, 2, 3 the projection of the total angular momentum on molecular axis.

[1] R. Ben-Shlomi, R. Vexiau, Z. Meir, T. Sikorsky, N. Akerman, M. Pinkas, O. Dulieu, R. Ozeri, Phys. Rev. A 102 ,031301(R)
[2] P. Weckesser, F. Thielemann, D. Wiater, A. Wojciechowska, L. Karpa, K.Jachymski, M. Tomza, T. Walker, T. Schaetz, Nature 600, 429 (2021)
[3] X. Xing, R. Vexiau, N. Bouloufa, O. Dulieu et al, in preparation.

We acknowledge support from the CRNS International Emerging Action (IEA) - ELKH, 2023-2024; Program Hubert Curien ”BALATON” (CampusFranceGrantNo.49848TC)–NKFIHTE ́T-FR(2023-2024)

Speaker: Tibor JJónás (HUN-REN Institute for Nuclear Research (ATOMKI), Bem tér 18/c, 4026 Debrecen, Hungary; University of Debrecen, Doctoral School of Physics, Egyetem tér 1., 4032 Debrecen, Hungary;Université Paris-Saclay, CNRS, Lab. Aimé Cotton, Bat 505, Rue du Belvédére, 91400 Orsay, France)

10:35 → 11:00 Coffee break

11:00 → 12:30 Molecular Spectroscopy

11:00 Towards quantum logic spectroscopy of polyatomic molecular ions

Due to the complexity of the internal energy structure of molecular ions, control of their quantum state poses serious challenges. Contrary to atomic counterparts, most molecules lack cycling optical transitions, which prevents standard state preparation and detection techniques based on optical pumping as well as direct translational cooling schemes. These challenging problems were recently solved for some diatomic molecular ions with quantum logic spectroscopy (QLS) techniques [1, 2, 3]. Here we report on the progress in extending control capabilities of the quantum state to polyatomic molecules using co-trapped calcium ions in a new cryogenic ion-trapping apparatus.

Our approach involves the preparation of the internal molecular state with a resonance-enhanced photoionization technique [4]. Co-trapped calcium ions serve a dual purpose - sympathetically cooling translational degrees of freedom of the molecular ion and employing quantum nondemolition state detection for the molecule. Employed QLS protocol relies on the state-dependent motional excitation of molecular ions exerted by an off-resonant optical lattice [1]. The cryogenic ion-trapping setup design and the current progress in its implementation will be discussed.

The realization of the described framework opens a route to studying chemical reactions of collisions on the state-to-state level and conducting quantum logic spectroscopy with polyatomic species.

[1] M. Sinhal, Z. Meir, K. Najafian, G. Hegi, S. Willitsch, Science 2020, 367(6483), 1213.
[2] F. Wolf, Y. Wan, J. C. Heip, F. Gebert, C. Shi, P. O. Schmidt, Nature 2016, 530(7591), 457-460.
[3] C. W. Chou, C. Kurz, D. B. Hume, P. N. Plessow, D. R. Leibrandt, D. Leibfried, Nature 2017, 545(7653), 203-207.
[4] X. Tong, A. H. Winney, S. Willitsch, Physical Review Letters 2010, 105(14), 143001.

Speaker: Mikhail Popov (University of Basel)

11:23 Apparatus for deterministic ionization and loading of molecules

Our group studies the complex rovibrational structure of trapped molecular ions. These states are often inaccessible by standard quantum information readout methods, but can be explored by co-trapping them with an atomic ion for which a convenient cooling and qubit level scheme exists. The molecular states can then be coupled to an electronic state of the atomic ion via quantum logic spectroscopy. Our experiments are currently limited to investigating molecules containing Ca. In order to load arbitrary molecular species, we are building a setup where a molecular gas is leaked in, photoionized, and then axilally guided into an ion trap. We use time-of-flight mass spectrometry to map out the ion species produced from photoionization of various gasses like nitrogen or acetylene. Mass filters and ion optics will then be used to steer and focus the molecule of interest through a differential pumping region towards a linear Paul trap in a UHV chamber. Molecular ions can be injected into the trapping region through an aperture in the trap end cap, relying on interaction with a cool trapped ion string dissipate excess kinetic energy below the trapping potential.

Speaker: René Nardi (Universität Innsbruck)

11:45 Study of super-critical water through the Widom line using infrared spectroscopy

Infrared spectroscopy is an important tool to probe inter-molecular and
intra-molecular motions as it is sensitive to the details of inter-molecular interactions. In the present study, we use this tool to explore structure and dynamics of super-critical water (SCW) across the Widom line (i.e. by varying density at a constant temperature
just above the gas-liquid critical temperature of the phase diagram) to interpret the transient density fluctuations. Although such studies have been reported for liquid nitrogen, no such study has been carried out for water.
The important feature of this work is a combined molecular dynamics simulation with electronic structure calculation (ES/MD) approach. We use the discrete variable representation (DVR) scheme to construct the spectroscopic maps for transition frequencies and transition dipoles and obtain the infrared spectrum of the O-H stretch across the Widom line.

We find several new results. Below we summarize the main results of this work.
i) The line shapes show sharp changes as we cross the Widom line by varying the density while keeping temperature fixed slightly above the critical temperature of water. A crossover from the Lorentzian–like to a Gaussian-like line shape is observed as the Widom line is approached.

ii) The line broadening at critical density exhibits a divergent-like density dependence of the IR line width across the Widom line. This is in agreement with earlier studies on nitrogen. In supercritical water, the increase in frequency fluctuations on approaching the Widom line is found to be the origin of the anomalous rise in the lineshape. This effectively explains the role of density heterogeneity/inhomogeneity on vibrational spectra of supercritical water.
iii) In liquid water at ambient conditions, orientational correlation time is of the order of ps. In SCW, the time scale is found to be shorter. Because of high temperature, the dynamics is expected to be ultrafast.

The present study effectively demonstrates the strength of linear spectroscopic methods (like IR line width measurement) to capture important aspects of critical phenomena. This is the first theoretical infrared spectroscopic study of super-critical water (using ES/MD approach) across the Widom line.

Speaker: Dr Tuhin Samanta (Weizmann Institute of Science)

12:08 Photodetachment spectroscopic studies of cold, trapped negative ions

Photodetachment spectroscopy is a powerful spectroscopic technique for determining the internal state distribution of a molecular anion. Previously, our group studied the threshold photodetachment spectroscopy of CN$^−$ at both 16 K and 295 K in a 22-pole ion trap and measured the electron affinity of CN with great precision (EA: 3.864(2) eV) [1]. Here we present the threshold photodetachment spectroscopy study of C$_2^−$ , speculated to exist in the interstellar medium, in a radiofrequency 16-pole ion trap at 8 Kelvin. We investigated the behaviour of the cross section near the threshold for the ground state transition, C${_2}$X ${^1}$$\Sigma{^+_g}$$ $$\leftarrow $C${_2^-}$X ${^2}$$\Sigma{^+_g}$. We measured the electron affinity of C$_2$ which is consistent with the previously measured values [3][4].
We also present the status of the absolute cross section and near threshold photodetachment spectroscopic studies of the naphthyl anion (C$_{10}$H$_7^-$), a polyaromatic hydrocarbon anion (PAH), which may also play a role in interstellar chemistry [5].

[1]. M. Simpson et al., J. Chem. Phys. 153, 184309 (2020).
[2]. M. Nötzold, R. Wild, C. Lochmann, R. Wester., Phys. Rev. A 106, 023111 (2022). ¨
[3]. K. M. Ervin and W. C. Lineberger., J. Phys. Chem. 95, 2244 (1991).
[4]. B. A. Laws, S. T. Gibson, B. R. Lewis, R. W. Field., Nat. Commun. 10, 1(2019).
[5]. M. L. Weichman J. B. Kim, J. A. Devine, D. S. Levine, D. M. Neumark J. Am. Chem. S 137,4 (2015).

Speaker: Sruthi Purushu Melath

12:30 → 14:30 Lunch break

13:45 → 14:15 Lab tours

14:30 → 16:48 Quantum Information & Computing

14:30 Deterministic Quantum Gate Teleportation across a Trapped-Ion Quantum Network

Quantum gate teleportation utilises shared entanglement and local operations and classical communication to mediate logical gate operations between qubits that cannot directly interact, making it an essential tool for the modular quantum computing architecture [1]. In this work, we demonstrate the deterministic teleportation of a controlled-Z gate between two ${}^{43}\textrm{Ca}^+$ hyperfine clock qubits located in separated trapped-ion quantum processors, measuring an average gate fidelity of 86.2(8) %. We achieve this by combining state-of-the-art remote entanglement between two ${}^{88}\textrm{Sr}^+$ network ions [2] and local mixed-species entangling gates to mediate an interaction between co-trapped ${}^{43}\textrm{Ca}^+$ memory ions [3]. We discuss how this system enables distribution of a circuit comprising multiple instances of gate teleportation across our quantum network. Our results pave the way for distributed quantum computation based on networks of trapped-ion quantum processors.

[1] D. Gottesman, I. Chuang, Demonstrating the viability of universal quantum computation using teleportation and single-qubit operations. Nature 402, 390–393 (1999).

[2] L. Stephenson, D. Nadlinger, B. Nichol, S. An, P. Drmota, T. Ballance, K. Thirumalai, J. Goodwin, D. Lucas, and C. Ballance, High-Rate, High-Fidelity Entanglement of Qubits Across an Elementary Quantum Network, Physical Review Letters 124, 110501 (2020).

[3] P. Drmota, D. Main, D. Nadlinger, B. Nichol, M. Weber, E. Ainley, A. Agrawal, R. Srinivas, G. Araneda, C. Ballance, and D. Lucas, Robust Quantum Memory in a Trapped-Ion Quantum Network Node, Physical Review Letters 130, 090803 (2023).

Speaker: Dougal Main (University of Oxford)

15:15 Realization a phononic network with collective modes in trapped ion system

A network of bosons evolving among different modes while passing through beam splitters and phase shifters has been applied to demonstrate quantum computational advantage. Such networks have mostly been implemented in optical systems using photons. However, technical bottlenecks exist in photon systems. In particular, photon loss and non-deterministic generation and inefficient detection of photonic states hinder their further scalability and the demonstration of quantum advantage. It thus becomes desirable to explore new experimental platforms.
Quantized excitations of vibrational modes (phonons) of trapped ions are a promising candidate to realize such bosonic networks. Here, we demonstrate a minimal-loss programmable phononic network in which any phononic state can be deterministically prepared and detected[1]. We realize networks with up to four collective vibrational modes, which can be extended to reveal quantum advantage. This network has the capability that couple the ions with vibration modes to prepare, interfere, and measure phonons distributed in different modes, which achieves the ability to act as a boson sampling platform.
We experimentally demonstrate that phonons can be deterministically prepared and detected and that the number of phonons is nearly conserved while propagating in the network. By programmable operations throughout the boson sampling experiment, our system has demonstrated the ability to implement bosonic algorithms with high fidelity. We benchmark the performance of the network for an exemplary tomography algorithm using arbitrary multi-mode states with fixed total phonon number. We obtain high reconstruction fidelities for both single- and two-phonon states. Our experiment demonstrates a clear pathway to scale up a phononic network for quantum information processing beyond the limitations of classical and photonic systems.
Furthermore, we explore the possibility of realizing fault-tolerant quantum computation with error mitigation scheme, which can be realized with current system scale and fidelity. Based on quantum channel purification method, we are able to suppress all kinds of incoherent noise in unitary phonon operations. Our research will promote the development of quantum error mitigation in the spin-phonon hybrid system in ion trap, and provide a reference for the implementation of large-scale bosonic algorithms.

[1] Chen, W., Lu, Y., Zhang, S., Zhang, K., Huang, G., Qiao, M., ... & Kim, K. (2023). Scalable and programmable phononic network with trapped ions. Nature Physics, 19(6), 877-883.

Speaker: Dr Wentao Chen (Tsinghua University)

15:37 An ion trap quantum processor with integrated ion-photon interface

The aim of this project is to build a quantum computing processor with integrated ion-photon interface. It consists of an ion trap with zones for ion loading, QIP and a zone with an integrated optical cavity for enhanced communication. The electrode structure is designed for dual species operation, ion swapping and ion chain splitting. To achieve highly efficient high-fidelity quantum communication between processors, the system is equipped with an integrated cavity, strongly coupling to the trapped ion. To realize this, we designed a chip, which was manufactured using femtosecond laser induced selective etching (FLISE) from a fused silica substrate, and subsequently gold coated. Employing trenches between the electrodes the chip can be metalised without masks. The cavity is formed of fused silica rods instead of optical fibres as has been used previously [1] in order to improve the photon collection efficiency. In previous works, researchers have reported effective photonic entanglement by using high-numerical-aperture lens’ to couple two ions’ qubits into single-mode optical fibres to attain high rate and fidelity [2]. For our system, we expect significantly higher entanglement rates with high fidelity due to strong coupling operation.

[1] H. Takahashi et al., Phys. Rev. Lett., vol. 124, p. 013602, (2020).
[2] L. J. Stephenson et al., Phys. Rev. Lett., vol. 124, p. 110501, (2020).

Speaker: Maoling Chu (University of Sussex)

15:55 Quantum information processing with metastable states in trapped barium ions

Trapped $^{137}\textrm{Ba}^+$ ions possess two long-lived hyperfine manifolds in which quantum information can be stored: the ground $S_{1/2}$ level and the metastable $D_{5/2}$ level. The metastable level does not couple to the fluorescence beams, so information stored there is protected during dissipative operations such as cooling, state preparation or readout. This allows for these operations to be performed mid-circuit, a requirement for most error correction schemes. In addition, gates can be driven via two-photon Raman transitions using light at 532 nm in both levels, simplifying experimental setups.

Here, we present a system for quantum computation experiments with chains of $^{137}\textrm{Ba}^+$. A fibre network coupled to a novel photonic chip is used to generate an array of 532 nm beams that are individually focused on each of the ions. This enables the implementation of all logical and dissipative operations on a target subset of qubits. Furthermore, the ions are stored in a microfabricated linear trap with a segmented electrode structure [1] that can generate complex DC potentials. This allows us to modify the axial position of the ions to match the addressing beam array, as well as rotate the direction of the crystal's radial modes of motion to optimise high-fidelity two-qubit operations.

Additionally, we use the system and our control of the ground and metastable levels to implement a novel state-preparation and measurement (SPAM) protocol based on the detection of population leakages. We achieve SPAM infidelities as low as $5 \times 10^{-6}$, the lowest reported. We also discuss how information processing using multi-dimensional quantum systems could be implemented in this system.

[1] K. Choonee, G. Wilpers and A. G. Sinclair, doi: 10.1109/TRANSDUCERS.2017.7994124.

Speaker: Andres Vazquez Brennan (University of Oxford)

16:15 → 16:45 Coffee break

16:45 → 18:45 Skill session: Lecture by Jean-luc Doumont

Thursday, 11 July

09:00 → 10:30 Nuclear Physics

09:00 Recent measurements and developments at ISOLTRAP

High-precision mass measurements of radioactive ions are used to determine nuclear binding energies, which reflect all forces acting in the nucleus and are used to study among others nuclear structure, nuclear astrophysics, and weak interaction.
For this, the ISOLTRAP mass spectrometer at ISOLDE/CERN [1] uses various ion traps, including a tandem Penning-trap system and a multi-reflection time-of-flight mass spectrometer (MR-ToF MS), where the latter is suitable of both mass separation and fast, precise mass measurements.

In this contribution, the first direct mass measurements of neutron-deficient $^{97}\text{Cd}$ and the excitation energy of the $^{97\text{n}}\text{Cd}$ high-lying isomer along with a precise measurement of $^{98}\text{Cd}$ in the immediate vicinity of self-conjugate doubly magic $^{100}\text{Sn}$ ($N=Z=50$) will be presented together with measurements of neutron-rich $^{209,210}\text{Hg}$.
Additionally, the current setup of the ISOLTRAP experiment is introduced together with the future re-bunching system using a new Mini-RFQ behind the MR-ToF MS to enable measurements of extremely isobaric contaminated beams.

[1] D. Lunney et al., J. Phys. G: Nucl. Part. Phys. 44 (2017) 064008

Speaker: Daniel Lange (Max Planck Society (DE))

09:23 The stacked-ring ion guide and MR-ToF MS developed for the NEXT experiment

The NEXT experiment [1] is currently being built at the AGOR facility in Groningen. NEXT aims to study Neutron-rich EXotic, heavy nuclei around N=126 and in the transfermium region which are produced in multinucleon Transfer reactions. Precision mass spectrometry and decay spectroscopy will be used to characterize these nuclei.

The target-like transfer products are pre-separated from the primary beam and lighter projectile-like products within the magnetic field of a superconducting solenoid magnet. They are slowed down by use of a gas catcher. A continuous and divergent beam of low energy ions extracted from the gas catcher has to be transformed to well-focused bunches of ions with keV energy suitable for time-of-flight mass measurements. For this purpose a new ion guide consisting of a stack of ring electrodes has been developed [2]. A recently designed multi-reflection time-of-flight mass spectrometer (MR-ToF MS) [3] will be used for isobaric separation and mass measurements.

At the moment, the custom-made ion guide and MR-ToF MS are being commissioned and their performance are being studied using an alkali ion source. In this talk, the first tests of the setup will be presented and discussed.

[1] J. Even, X. Chen, A. Soylu, P. Fischer, A. Karpov, V. Saiko, J. Saren, M. Schlaich, T. Schlathölter, L.
Schweikhard, J. Uusitalo, and F. Wienholtz, The NEXT Project: Towards Production and Investigation of Neutron-Rich Heavy Nuclides, Atoms 10, 59 (2022).
[2] X. Chen, J. Even, P. Fischer, M. Schlaich, T. Schlathölter, L. Schweikhard, and A. Soylu, Stacked-Ring Ion Guide for Cooling and Bunching Rare Isotopes, Int. J. Mass Spectrom. 477, 116856 (2022).
[3] M. Schlaich, J. Fischer, P. Fischer, C. Klink, A. Obertelli, A. Schmidt, L. Schweikhard and F. Wienholtz, A multi-reflection time-of-flight mass spectrometer for the offline ion source of the PUMA experiment, Int. J. Mass Spectrom, 495, 117166 (2024).

Speaker: Marko Brajkovic (University of Groningen, the Netherlands)

09:45 MIRACLS: Laser spectroscopy of radioactive isotopes in an MR-ToF device

A host of techniques have been developed to study the nuclear properties of exotic isotopes produced at radioactive ion beam (RIB) facilities. One such technique is collinear laser spectroscopy (CLS), which provides a nuclear model-independent way of extracting observables such as nuclear charge radii, electromagnetic moments, and spins from the hyperfine spectrum of a particular atomic species.

The Multi Ion Reflection Apparatus for CLS (MIRACLS) is a new experimental setup in the ISOLDE RIB facility at CERN which aims to improve the sensitivity of conventional CLS by conducting it in a high-energy (> 10 keV) multi-reflection time-of-flight (MR-ToF) device [1, 2]. This type of ion trap utilizes two electrostatic mirrors to reflect ion bunches back and forth for several thousands of revolutions. In this configuration, we gain a sensitivity boost compared to conventional CLS since ion bunches are “recycled” after each revolution. As a result, exotic radionuclides with very low production yields become accessible, such as the magnesium isotope $^{34}$Mg, which will be the first physics case of MIRACLS and will give fresh insights on the so-called “island of inversion” around $^{32}$Mg.

Besides CLS, the high-energy MR-ToF device at MIRACLS can also be used for highly selective, high-flux mass separation to provide purified beams of radioactive isotopes [3]. These pure beams are a requirement for other experimental programs such as PUMA, which aims to exploit antiprotons to probe the surface effects of atomic nuclei such as halo nucleons or neutron skins [4].

This contribution will describe the operating principles of the Paul trap for ion beam preparation and the MR-ToF device at MIRACLS, discuss the latest commissioning results of the MIRACLS experiment, and give an outlook to the planned measurement of the charge radius of $^{34}$Mg.

References
[1] Simon Sels et al. “First steps in the development of the multi ion reflection apparatus for collinear laser spectroscopy”. In: NIMA B 463 (2020), pp.310-314.
[2] F.M. Maier et al. “Simulation studies of a 30-keV MR-ToF device for highly sensitive collinear laser spectroscopy”. In: NIMA A 1048 (2023).
[3] F.M. Maier et al. “Increased beam energy as a pathway towards a highly selective and high-flux MR-ToF mass separator”. In: NIMA A 1056 (2023).
[4] T Aumann et al. “PUMA, antiProton unstable matter annihilation". In: Eur. Phys. J. A 58.5 (2022), p. 88.

Speaker: Anthony Roitman (McGill University, (CA))

10:08 Probing the nuclear magnetic octupole moment of trapped Sr ions

At the Institute for Nuclear and Radiation Physics of KU Leuven (IKS) we started a project to measure data on the magnetic octupole moment ($\Omega$) of single valence radioactive nuclei. While currently this observable has only scarcely been measured, and is thus poorly understood, preliminary shell model and Density functional theory (DFT) calculations indicate $\Omega$ may display a strong sensitivity to nuclear shell effects, even stronger than the dipole moment. It may also be well suited to probe the distribution of neutrons within the nucleus, and study fundamental properties of nucleons of stable and radioactive isotopes. This objective presents several challenges, both technical and scientific, as there are presently no methods that reach the precision required to measure $\Omega$ for short-lived isotopes of any element. In this context, the first study will be performed on the stable $^{87}Sr^+$. With 49 neutrons, $^{87}Sr^+$ is characterized by a single hole in the N=50 closed shell, which makes it more easily compared with a variety of theoretical calculations. Once measurements with $^{87}Sr$ are demonstrated, it could be possible to extend them to the long-lived $^{83,85,89}Sr^+$ here at IKS. A non-zero $\Omega$ leads to small energy shift of the hyperfine structure. We aim to measure these splitting with a precision of the order of 1-10 Hz on the hyperfine intervals, which should result in a measurement of $\Omega$ with a precision of 10$\%$. This has been demonstrated feasible with stable
$^{137}Ba^+$, homologue of $Sr^+$, inside ion traps [1]. This contribution aims to offer a broad understanding of the project and present the latest developments in the laboratory.

References
[1] N. C. Lewty, B. L. Chuah, R. Cazan, B. K. Sahoo, and M. D. Barrett, “Spectroscopy
on a single trapped 137ba+ ion for nuclear magnetic octupole moment determination,”
Optics Express, Sep. 10, 2012. doi: 10.1364/OE.20.021379.

Speaker: Pierre Lassegues (KU Leuven (BE))

10:30 → 11:00 Coffee break

11:00 → 12:30 Precision Spectroscopy & Atomic Clocks

11:00 New physics searches with highly charged ions

Highly charged ions (HCI) are promising candidates for novel optical clocks with applications in frequency metrology and tests of fundamental physics [1]. Typically, megakelvin-range temperatures needed to produce HCI hinder high-precision spectroscopy. To overcome this, we extract HCI from an electron beam ion trap (EBIT) and transfer them to a cryogenic linear Paul trap. There, single HCI are sympathetically cooled by laser-cooled Be$^{+}$ ions down to millikelvin temperatures, thus enabling quantum logic state readout [2]. We demonstrated in this way an optical clock based on Ar$^{13+}$, and determined its absolute frequency with sub-Hz uncertainty [3] against the Yb$^+$ octupole ion clock at PTB [4]. Our techniques are readily applicable to many ions, e. g. Ca$^{14+}$ [5] as well as Xe HCI [6]. Recently, we determined the isotope shift of a narrow M1 transition in stable even isotopes of Ca$^{14+}$ with 150 mHz accuracy. We combine these results with available isotope-shift data of Ca$^+$ [7] in a King plot which is sensitive to a new force that would couple electrons and neutrons [8,9]. In this way, we strengthen the constraints on the existence of such a hypothetical interaction by a factor of about five as compared to previous studies [7]. We also estimate how far improved measurements of Ca isotope masses and isotope shifts of the Ca$^+$ S$_{1/2}$ - D$_{5/2}$ transition would enhance such constraints.

[1] M. Kozlov, et al., Rev. Mod. Phys., 90, 045005 (2018)
[2] P. Micke, T. Leopold, S.A. King et al., Nature 578 (2020)
[3] S. A. King, L. J. Spiess, et al., Nature 611, 43 (2022)
[4] R. Lange et al., Phys. Rev. Lett. 126, 011102 (2021)
[5] N. Rehbehn, et al., Phys. Rev. A 103, L040801 (2021)
[6] N. Rehbehn, et al., Phys. Rev. Lett. 131, 161803 (2023)
[7] T. T. Chang et al., arXiv:2311.17337v1, 123003 (2023)
[8] J. C. Berengut, et al., Phys. Rev. Lett. 120, 091801 (2018)
[9] J. C. Berengut, et al., Phys. Rev. Research 2 043444 (2020)

Speaker: Alexander Wilzewski

11:27 Towards XUV Frequency Comb Spectroscopy of Trapped He+ Ions

The energy levels of hydrogen-like atoms and ions are accurately described by bound-state quantum electrodynamics (QED). The frequency of the narrow 1S-2S transition of atomic hydrogen has been measured with a relative uncertainty of less than $10^{-14}$. In combination with other spectroscopic measurements of hydrogen and hydrogen-like atoms, the Rydberg constant and the proton charge radius can be determined. The comparison of the physical constants obtained from different combinations of measurements serves as a consistency check for the theory [1]. For QED tests, it is also interesting to measure hydrogen-like systems other than hydrogen, which are more sensitive to different terms of the theory. The measurement of the Lamb shift in muonic hydrogen, for instance, gave rise to the proton radius puzzle [2].
Another interesting spectroscopic target is the hydrogen-like He$^{+}$ ion. Ideal conditions for high-precision measurements can be achieved, since the He$^+$ ions can be held nearly motionless in the field-free environment of a Paul trap. Interesting higher-order QED corrections scale with large exponents of the nuclear charge, making this measurement much more sensitive to these corrections compared to hydrogen.
In this talk, we describe our progress towards precision spectroscopy of the 1S-2S two-photon transition in He$^{+}$ [3]. The transition can be directly excited by an extreme-ultraviolet frequency comb at 60.8 nm generated by a high-power infrared frequency comb using high-harmonic generation (HHG). The spectroscopic target is a small number of He$^{+}$ ions trapped in a linear Paul trap and sympathetically cooled by co-trapped Be$^{+}$ ions. After successful excitation to the 2S state, a significant fraction of the He$^{+}$ ions will be further ionized to He$^{2+}$ and remain in the Paul trap. Sensitive mass spectrometry using secular excitation will reveal the number of trapped He$^{2+}$ ions and will serve as a single-event sensitive spectroscopy signal. In order to perform Doppler-free spectroscopy, the frequency comb is split into counterpropagating double pulses which are overlapped at the ions. Possible signals to test and optimize the pulse-overlap are two-photon dissociation processes of BeH$^{+}$ using 204 nm and 255 nm light, which are generated as the 5th and 4th harmonic of the infrared frequency comb, respectively.

[1] T. Udem Nature Phys 14, 632 (2018)
[2] R. Pohl et al. Nature 466, 213 (2010)
[3] J. Moreno et al. Eur. Phys. J. D 77, 67 (2023)

Speaker: Florian Egli (Max Planck Institute of Quantum Optics)

11:54 Transportable optical clock for remote comparisons

We report on a transportable and user-friendly optical clock that uses the electric quadrupole transition ($\phantom{}^2S_{1/2}-\phantom{}^2D_{3/2}$) of a single trapped $\phantom{}^{171}$Yb$^+$ ion at 436~nm as the reference. The clock has been developed in an industry-lead collaboration (Opticlock) and is set up in two 19" racks. The main advantages of the system are its ability to robustly operate continuously over weeks and that it provides transportability. As a first step towards remote comparisons, Opticlock will travel to Finland in August 2024 to be compared with the $\phantom{}^{88}$Sr$^+$ clock at VTT MIKES. For this proof of concept campaign, Opticlock is aimed to demonstrate a short-term frequency instability below $5\times10^{-15}\sqrt{\tau}$ and a systematic uncertainty below $5\times10^{-18}$. The frequency instability of Opticlock has been improved by reducing the dead time required for magnetic field decay and its systematic uncertainty reduced by direct measurements of AC magnetic field and improved knowledge of the shift resulting from thermal radiation. Furthermore, a frequency comb generator is set up and integrated into the system. Prior to the remote measurement, the optical clock performance is assessed by local test transportation that identifies weak links in the system. The results pave the way for future key comparisons of high-performance optical clocks using transportable standards as an alternative to satellite-based techniques and optical fiber links.

Speaker: Mr Saaswath J. K.

12:12 Optical atomic clock based on entangled quantum states of trapped $^{40}$Ca$^{+}$ ions

Nowadays, optical atomic clocks based on trapped ions regularly reach relative systematic uncertainties in the region of $10^{-18}$. This resolution can be used to resolve height differences in geodesy and for the search for variation of fundamental constants, dark matter coupling to classical matter, or other new physics concepts [1], [2]. The fundamental quantum projection noise (QPN) limits frequency resolution of a single ion resulting in long averaging times to reach the systematic uncertainty level. This can be decreased by prolonging the interrogation time between the atomic system and the clock laser or scaling the number of ions being probed simultaneously. A third method is the use of entangled quantum states. However, it is still under investigation which measurement protocol based on entangled quantum states improves the statistical uncertainty compared to classical measurements and in the presence of noise and spontaneous decay [3], [4], [5]. Here we focus on the applicability of entanglement within a clock measurement scheme, investigating the region of sub-lifetime interrogation times on a two-particle maximal entangled state. Two $^{40}$Ca$^{+}$ ions are stored within a segmented linear Paul trap, cooled close to the ground state of motion by electromagnetic-induced transparency and resolved sideband cooling. The ions are prepared with a single-ion addressing beam into two opposing Zeeman states of the S$_{-1/2}$ ground state. Using a Cirac-Zoller gate, a Ramsey-like interferometer is opened, which is employed to measure the phase evolution of a superposition of two-ion Green-Horn-Zeilinger (GHZ) states against the clock laser. The accumulated phase is mapped onto GHZ states of different parity. We compare this scheme against a classical variant of this experiment, where the correlation between two independent Ramsey interferometers is used [6]. The phase evolution of the two ion states is protected against common-mode magnetic field noise, enhancing the coherence time of the GHZ-state close to the natural lifetime limited of those states (550 ms). These so-called decoherence-free substates (DFS) are prepared with a fidelity of greater than 95 %, mostly limited by the single ion addressing performance and the high-frequency phase noise of the clock laser. A clock servo operation is implemented with the measured parity signal. The frequency stability of the parity signal was measured against the $^{87}$Sr lattice clock at PTB [7], demonstrating a stability of 6.2 $\cdot 10^{-16} \sqrt{\frac{\tau}{1s}}$. This statistical uncertainty is still limited by technical noise, mostly fiber length fluctuations within the path of the clock laser to the experiment. In future experiments we aim for a demonstration of a purely quantum projection noise limited stability.

[1] Ludlow et al., Rev Mod. Phys. 87, 637 (2015)
[2] M.S. Safronova et al., Rev. Mod. Phys. 90, 025008 (2018)
[3] M. Schulte et al., Nat Commun 11, 5955 (2020)
[4] L. Pezzè et al., Rev. Mod. Phys. 90, 035005 (2018)
[5] S.F. Huelga et al., Phys. Rev. Lett. 79, 3865 (1997)
[6] M. Chwalla et al., Appl. Phys. B 89, 483–488 (2007)
[7] Work contribution by J. Klose, K. Stahl, S. Dörscher, E. Benkler, C. Lisdat

Speaker: Kai Dietze (Physikalisch Technische Bundesanstalt)

12:30 → 14:30 Lunch break

13:45 → 14:15 Lab tours

14:30 → 15:40 Quantum Simulation

14:30 Entanglement generation in 2D ion crystals

Quantum simulation is an approach to investigate a complex quantum system of interest by mimicking it with another, well-controllable and measurable system. One of the platforms that suitable for the task is trapped ions, held either in the form of a lattice in a Penning trap or as a linear chain in a Paul trap. The former can trap many more ions, while the latter has the important advantage of having site-resolved control and readout of individual ions’ electronic states. In our project we aim to overcome the limitations of a standard Paul trap and go beyond 1D chains, expanding ion crystals to 2D lattice while keeping individual addressing and readout. To do so, we have designed and built a monolithic Paul trap, capable of storing 100+ ions in a stationary 2D crystal. To create multiparticle entanglement in the system, state-dependent forces are induced via Raman transition within the ground state manifold of 40Ca⁺ ion. These forces make out-of-plane (drumhead) modes of a 2D crystal act as an entanglement mediator, bringing ions to a complex multiparticle state. We implemented Ising, transverse field Ising and XX spin-spin interaction models on our experimental setup, that led to creation of highly entangled collective spin state of up to 91 particles with a measurement variance below the standard quantum limit, also known as a spin-squeezed state. These states potentially provide an advantage in spectroscopy precision over classical states. Furthermore, improvements on search for optimal interaction parameters and achievable metrological gain given by variational quantum-classical approach were studied.

Speaker: Artem Zhdanov

14:55 Optical tweezer optimisation for trapped-ion quantum simulation

Trapped ion crystals offer a natural platform for quantum simulations. They provide advantageous conditions such as long coherence times and organization into lattice crystalline structures with fully connected interactions[1-2]. With this setup one can engineer spin-Hamiltonians whose interactions are mediated by the crystal’s phonon modes[3-6]. Our novel approach adds optical tweezers, i.e. tightly focused laser light, which is used to manipulate the ion crystals’ sound-wave spectra. This allows extra tunability over the ion interactions, paving the way to the simulation of a wide range of spin-Hamiltonians[7-9]. We show experimental work on a trapped-ion tweezer setup, detailing a tweezer optimization routine and alignment on the ions. We characterize the beam profile and observe tweezer-dressing of the ion states[10].

[1] Wang, P. et al., Nat Commun 12, 233 (2021).
[2] R. Blatt, D. Wineland, Nature 453, 1008-15 (2008).
[3] A. Bermudez et al., Phys. Rev. Lett. 107, 207209 (2011).
[4] H. Kaufmann et al., Phys. Rev. Lett. 109, 263003 (2012).
[5] P. Richerme, Phys. Rev. A 94, 032320 (2016).
[6] R. Nath et al., New J. Phys. 17, 065018 (2015).
[7] J.D.Espinoza et al., Phys. Rev. A 104, 013302 (2021).
[8] M. Mazzanti et al., Phys. Rev. Lett. 127, 260502 (2021).
[9] L.Bond et al., Phys. Rev. A 106, 042612 (2022).
[10] M. Mazzanti et al., in preparation.

Speaker: Maria Clara Robalo Pereira (University of Amsterdam)

15:18 Investigating interference with phononic bright and dark states in a trapped ion

Interference underpins some of the most unusual and impactful properties of both the classical and quantum worlds, from macroscopic systems down to the level of single photons. In this work a new description of interference, based on the formation of collective bright and dark states, is investigated experimentally. We employ a single trapped ion, whose electronic states are coupled to two of its motional modes in order to simulate a multi-mode light-matter interaction. We observe the emergence of phononic bright and dark states for both a single phonon and a superposition of coherent states. The collective dynamics of these systems demonstrate that a description of interference based solely on bright and dark states is sufficient to explain the light-matter coupling of any initial state in both the quantum and classical regimes.

Speaker: Robin Thomm

15:40 → 16:20 Quantum Information & Computing

15:58 Overcoming and harnessing non-commuting dynamics in trapped ions

A trapped ion forms a hybrid system consisting of the electronic spin and bosonic motional modes. The Hamiltonian describing the interaction between laser light and this hybrid system, when containing only commuting terms, leads to simple dynamics. However, the presence of non-commuting terms, either due to spurious off-resonant interactions or deliberate inclusion, leads to complex and rich dynamics. We present two regimes for dealing with this complexity: either by suppressing the unintended non-commuting terms and recovering simpler dynamics or by harnessing the rich dynamics to realize previously experimentally unexplored interactions.

In the first regime, an off-resonant non-commuting term present in Mølmer–Sørensen two-qubit entangling gates causes an error that increases with the drive strength. This manifests as an effective speed limit of two-qubit entanglement via this method. However, using phase stabilized standing waves, we can suppress this non-commuting term and break this speed limit [Saner, Bazavan et al. 2023].

In the second regime, we generate non-linear bosonic interactions by combining non-commuting spin-motion couplings [Sutherland et al. 2021]. Using two interactions linear in the bosonic mode with non-commuting spin conditioning, we generate effective non-linear interactions with more favourable scaling than conventional techniques driving higher order sidebands [Meekhof et al. 1996]. To maintain the non-commuting relationship between the spin-components, we actively stabilize the optical phase of the driving fields. As such, we are able to demonstrate nth order non-linear interactions: squeezing, trisqueezing, and quadsqueezing [Bazavan et al. 2024], the latter of which we believe is the first experimental demonstration.

The common underlying physics, involving non-commuting terms, and the experimental requirements, such as the active stabilization of the optical phase in the driving fields, allow for the utilization and concurrent optimization of the same experimental apparatus, which we shall discuss.

1. Saner, S., Bazavan, O. et al. Breaking the Entangling Gate Speed Limit for Trapped-Ion Qubits Using a Phase-Stable Standing Wave. Phys. Rev. Lett. 131, 220601 (2023).
2. Sutherland, R. T. & Srinivas, R. Universal hybrid quantum computing in trapped ions. Phys. Rev. A 104, 032609 (2021).
3. Meekhof, D. M., Monroe, C., King, B. E., Itano, W. M. & Wineland, D. J. Generation of Nonclassical Motional States of a Trapped Atom. Phys. Rev. Lett. 76, 1796–1799 (1996).
4. Băzăvan, O. et al. Squeezing, trisqueezing, and quadsqueezing in a spin-oscillator system. Preprint at http://arxiv.org/abs/2403.05471 (2024).

Speaker: Donovan Webb (University of Oxford)

16:20 → 16:45 Coffee break

16:45 → 18:15 Skill session: Colloquium by Rainer Blatt

18:15 → 21:15 Conference dinner

Friday, 12 July

09:00 → 10:30 Antimatter

09:00 Adaptable platform for trapped cold electrons, hydrogen and lithium anions and cations

Cold-charged particles play an essential role in interstellar molecular formation, are present in many high-precision experiments, antimatter physics, and chemistry, and are also relevant for studies on the origin of biological homochirality. In this contribution, I will describe a system based on the Matrix Isolation Sublimation (MISu) technique [1],[2] to generate and trap these species in the laboratory. After growing a thin film of Neon upon a cold (4 K) sapphire subtract, we implant different species inside this film via laser ablation of a solid target. With a heat pulse to the sapphire surface, we sublimate the solid neon at low temperatures, and the inert gas carries the particles that were confined inside the solid, producing a beam at low energies. We guide the charged particles using the magnetic field produced by two perpendicular coils and trap the particles in a Penning-Malmberg trap using low voltages (~1 V) and weak magnetic fields (~0.1 T).
We have measured energy distribution for positive and negative trapped charge particles whose peak was below 25 meV. Using an on-trap-time-of-flight scheme, we demonstrate the presence of electrons, hydrogen anions, protons, lithium cations and anions, and light molecular ions.
The hydrogen anions can be used to produce a cold sample of neutral trappable hydrogen by near-threshold photodetachment (0.754 eV). For example, a laser at 1575 nm will leave 0.2 K of recoil energy, less than the ion sample's typical temperature or energy dispersion, to the neutral H. The fraction of resulting atoms with energy below 0.5 K can remain trapped in a 1 T trap depth superposed magnetic trap and could be detected using the sensitive technique [3]. These cold H can loaded into the ALPHA [4] antihydrogen trap at CERN toward direct spectroscopic comparison of both conjugated species beyond 13 significant figures. The production is scalable and adaptable to different species, including deuterium and tritium, which is relevant for neutrino mass and fusion research.

[1] - Azevedo, L.O.A., Costa, R.J.S., Wolff, W. et al. Adaptable platform for trapped cold electrons, hydrogen and lithium anions and cations. Commun Phys6, 112 (2023).
[2] - Sacramento, R. L. et al. Matrix Isolation Sublimation: an apparatus for producing cryogenic beams of atoms and molecules. Rev. Sci. Instrum.86, 073109 (2015).
[3] - Cesar, C. L. A sensitive detection method for high resolution spectroscopy of trapped antihydrogen, hydrogen and other trapped species. J. Phys. B49, 074001 (2016).
[4] - Ahmadi, M. et al. Characterization of the 1S-2S transition in antihydrogen. Nature557, 71 (2018).

Speaker: Levi Oliveira De Araujo Azevedo (Federal University of Rio de Janeiro (BR))

09:27 Gravity experiments with magnetically confined antihydrogen in ALPHAg.

The hydrogen atom has been studied extensively throughout history and provides the most precisely measured systems in physics. Antihydrogen has a significantly shorter history of study but the same potential for precision physics measurements. Comparisons between hydrogen and antihydrogen then offer the possibility to test fundamental symmetries such as charge, parity, and time reversal (CPT) symmetry at high precision.

The antihydrogen laser physics apparatus (ALPHA) at CERN produces and traps antihydrogen atoms in a magnetic minimum and studies its atomic spectrum. The latest venture for the ALPHA collaboration has been a new experiment, ALPHAg, aiming to observe the motion of antimatter in Earth’s gravitational field for the first time. As CPT makes no assertion about the motion of antimatter in Earth’s gravitational field this is a test of the equivalence principle.

Antihydrogen atoms are confined in a vertical magnetic minimum trap, the trapping potential is then different between the top and bottom of the trap by -mgΔh, where m is the antihydrogen mass, g is the gravitational acceleration and h is the height. When the vertical confining field is then removed during a slow magnetic release, antihydrogen escape in a direction favouring the gravitational acceleration. The difference in trapping potential is equivalent to a magnetic field difference of approximately $4× 10^{-4}$ T. It follows then that by intentionally adding a magnetic bias to the trap, one can find a bias that balances the effect of gravity. As the magnetic field is changed from 1.7 T to 1 T over 20 seconds during the release, it is necessary to control and measure the magnetic fields at each end of the magnetic trap to a higher precision than the gravitational potential difference.

I will discuss the systematic studies of these magnetic fields using electron plasmas in a Penning-Malmberg trap [1] and the magnetic release experiment results that enabled the first determination of the gravitational acceleration of antihydrogen, $a_\bar{g}$ = (0,75 ± 0,13 (stat. + syst.) ± 0,16 (simulation))g where g = 9.81 $\mathrm{m/s^2}$ [2].

[1] Electron cyclotron resonance (ECR) magnetometry with a plasma reservoir, E. D. Hunter ; A. Christensen ; J. Fajans ; T. Friesen ; E. Kur ; J. S. Wurtele Phys. Plasmas 27, 032106 (2020)

[2] Observation of the effect of gravity on the motion of antimatter, The ALPHA collaboration, Nature volume 621, pages716–722 (2023)

Speaker: Adam Powell (Dep. of Phys. and Astronomy University of Calgary (CA))

09:54 Hyperfine spectroscopy of antihydrogen with microwaves

For CPT symmetry to be conserved, the energy spectrum of both matter and antimatter atoms should be identical. The ALPHA collaboration uses antihydrogen, the antimatter counterpart of hydrogen, to perform CPT symmetry tests.

Microwave spectroscopy techniques were applied in the ALPHA experiment to observe, for the first time, the transition between hyperfine energy levels of antihydrogen. It is known as the positron spin resonant (PSR) transition and is induced by flipping the spin of the positron.
However, the transition produced by flipping the spin of the antiproton, known as nuclear magnetic resonant (NMR) transition, has only been measured in hydrogen, not on antihydrogen.

I will present the recent progress made to improve the PSR measurement and to enable a precise measurement of the NMR transition.

Speaker: Alberto Jesus Uribe Jimenez (Dep. of Phys. and Astronomy University of Calgary (CA))

10:12 Towards an ion trap source of cold atomic hydrogen

Understanding the origins of the cosmos has been a much sought after pursuit. One of the greatest mysteries is the composition of the universe itself, which deviates from the Standard Model predictions, since observations indicate that it is made almost entirely out of matter [1]. These observations paved the way for experiments that directly compare matter and antimatter, with the latest one being on the effect of gravity on antimatter [2].

A novel scheme for producing cold, magnetically trappable atomic hydrogen has been proposed [3]. The whole process relies on the production of Ba+/BaH+ ions through laser ablation of a solid BaH2 target, trapping these ions, laser cooling Ba+ and thus sympathetically cooling BaH+ ions, and finally photodissociating BaH+ ions to produce atomic hydrogen. We designed a new experimental setup, with a Paul trap as the main feature, as a proof of concept of the proposed scheme.
Here, I present our first results on the trapping and laser cooling of Ba+ ions extracted from a Ba target, as well as removing BaO, that was formed during handling the target, from its surface. Next phase of the experiment will use a BaH2 target and will result in the production of hydrogen. These techniques are compatible with the current generation antihydrogen experiments [4], since they all use ion traps to form antihydrogen, and make the whole scheme suitable for loading hydrogen into an antihydrogen experiment, for the direct comparison of the two species.

[1] G. Steigman. Observational tests of antimatter cosmologies. Annual review of astronomy and astrophysics 14 (1), 339 – 372 (1976).
[2] E. K. Anderson, C. J. Baker, W. Bertsche, et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023).
[3] S. A. Jones. An ion trap source of cold atomic hydrogen via photodissociation of the BaH+ molecular ion. New J. Phys. 24, 023016 (2022).
[4] G. Andresen, M. Ashkezari, M. Baquero-Ruiz, et al. Trapped antihydrogen. Nature 468, 673–676 (2010).

Speaker: Nikolaos Efthymiadis (PhD student, University of Groningen, Netherlands)

10:30 → 11:00 Coffee break

11:00 → 12:30 Quantum Information & Computing

11:00 Demonstration of fault-tolerant Steane quantum error correction

Encoding information redundantly in quantum error correcting (QEC) codes is a way – perhaps the only way – to protect quantum information processors from the harmful effects of noise that impede large-scale computation. However, the execution of QEC itself is subject to faults which can transform and spread uncontrollably unless fault-tolerant design principles are applied as well. The consequence of this is that device capabilities, noise profile, QEC code, and correction scheme are all influencing each other.
In my talk I will present the first experimental demonstration of Steane QEC [1] which in combination with the transversal CNOT and full qubit connectivity minimizes the necessary coupling between data and auxiliary qubit register. We demonstrate the benefits of Steane error correction over previously demonstrated universal, fault-tolerant gate implementations [2] using traditional flag-based syndrome readout with three different types of codes of increasing code distances, establishing experimental Steane QEC as a competitive paradigm for fault-tolerant quantum computing.
[1] Postler, Lukas, et al. "Demonstration of fault-tolerant Steane quantum error correction." arXiv preprint arXiv:2312.09745 (2023).
[2] Postler, Lukas, et al. "Demonstration of fault-tolerant universal quantum gate operations." Nature 605.7911 (2022): 675-680.

Speaker: Dr Christian Marciniak (Universität Innsbruck)

11:23 Cycle Error Reconstruction on a trapped ion quantum computer

The presence of noise in quantum system makes the precise and efficient characterization of errors necessary. A myriad of benchmarking and tomography routines have been developed over the years to address this challenge. However, most of these suffer from scalability problems in implementation and the information extracted is frequently lacking in predictive or diagnostic utility. A major challenge towards practically useful error characterization techniques is to determine which errors of the exponentially many possible are relevant. The cyclic error reconstruction (CER) protocol tackles this issue by producing error marginals successively, giving the experimenter the choice of how much knowledge is extracted. CER is an extension of the cycle benchmarking protocol expanding its diagnostic utility – giving insight not only in how large the overall error is but also of its origin. In contrast to randomized benchmarking CER uses only single-qubit Pauli twirling and therefore is amenable to characterize multi-qubit processes. Here we apply the CER protocol to a trapped ion quantum computer learning error rates and crosstalk of gates in their natural context scaling from a single qubit gate to logical gadgets.

Speaker: Robert Freund

11:45 Focusing of microwave-driven gate interactions using dynamical decoupling

In trapped-ion quantum computing, quantum logic gates are often performed using lasers. Alternatively, gates can also be driven by microwave fields for which the technology is cheaper and more reliable, making it simpler to scale up. However, due to their centimetre wavelength, microwaves cannot be focused to a small spot size, making it difficult to address an individual ion within a cluster of ions confined by the same potential well.

We have proposed and demonstrated a method to enable microwave-driven entangling gate operations only in micron-sized zones, corresponding to $10^{-5}$ microwave wavelengths, whilst suppressing this interaction everywhere else [1]. This is done by utilising the variation in phase of a microwave-field across a surface trap. We find that the required interaction introduces $3.7(4) \times 10^{-4}$ error per emulated gate in a single-qubit benchmarking sequence. We then model the scheme for a 17-qubit ion crystal, and find that any pair of ions should be addressable with an average crosstalk error of $\sim 10^{-5}$.

[1] M. C. Smith et al., arXiv:2309.02125 (2023).

Speaker: Molly Smith (University of Oxford)

12:08 Towards High-Fidelity Microwave Gates on Microfabricated Ion-Traps

Trapped ions have proved to be the leading quantum computing platform, due to their long coherence times and simple reproducibility. The design of modular architectures is also facilitated, which is crucial for a scalable, universal quantum computer. Our blueprint for a trapped-ion based quantum computer outlines operating with global microwave (MW) fields to dress the ground-state hyperfine manifold of $^{171}Yb^+$ ions [1]. By applying individually controlled static (DC) voltages, ions can be effectively shuttled around and between modules [2], while modulated radio-frequency (RF) signals are utilised to implement quantum logic gate operations [3].

We have further developed microfabricated surface traps featuring X-junction arrays with embedded current-carrying wires (CCWs) able to provide a controllable magnetic field gradient [4]. The imminent way forward is the characterisation of these novel chips which serve as the modules of our scalable architecture. In addition, demonstration of high-fidelity single and two-qubit gate operations will be enabled by quantum control techniques designed for more robust entanglement gates [5].

[1] B. Lekitsch, S. Weidt, A. G. Fowler, K. Mølmer, S. J. Devitt, C. Wunderlich, and W. K.Hensinger, “Blueprint for a microwave trapped ion quantum computer”, Science Advances 3 (2017).

[2] M. Akhtar, F. Bonus, F. R. Lebrun-Gallagher et al., “A high-fidelity quantum matter-link between ion-trap microchip modules”, Nature Communications 14, 531 (2023).

[3] S. Weidt, J. Randall, S. C. Webster et al., “Trapped-ion quantum logic with global radiation fields”, Physical Review Letters 117 (2016).

[4] Z. D. Romaszko, S. Hong, M. Siegele et al. “Engineering of microfabricated ion traps and integration of advanced on-chip features”, Nat Rev Phys 2, 285–299 (2020).

[5] C. H. Valahu, I. Apostolatos, S. Weidt and W. K. Hensinger, “Quantum control methods for robust entanglement of trapped ions”, J. Phys. B: At. Mol. Opt. Phys. 55 204003 (2022).

Speaker: Petros Zantis (University of Sussex - Ion Quantum Technology group)

12:30 → 14:30 Lunch break

13:45 → 14:15 Lab tours

14:30 → 19:00 Social Activity (optional)