Schedule

(University of Amsterdam)

Quantum Simulation with Trapped Ions in Optical Tweezers

Trapped ions are one of the most advanced platforms for digital and analogue quantum simulation. A suitable ion species such as Yb+ has an internal structure suitable for cooling, initialising, and read-out. These properties, together with long coherence times, have resulted in the highest fidelity two-qubit gates, an essential element for digital quantum simulation. However, analogue quantum simulators may enable more efficient simulation of certain Hamiltonians.
In analogue quantum simulators, qubit interactions are mediated by the Coulomb crystal’s collective vibrations and are thus natively long ranged. However, controlling the interactions remains challenging. We can use optical tweezers, which add a small, tuneable harmonic potential to each ion, to gain more control. I will introduce this proposal and our simulations of experimental effects: micromotion, local stress and intensity noise [1,2]. We find that these effects can be easily circumvented when designing appropriate tweezer patterns.

[1] Arias Espinoza et al., “Engineering spin-spin interactions with optical tweezers in trapped ions”, Phys Rev A 104, 013302 (2021)
[2] Bond et al., “The Effect of Micromotion and Local Stress in Quantum simulation with Trapped Ions in Optical Tweezers“, arXiv:2202.13681 (2022) 

(INSP – Sorbonne Université)

Interlayer Excitons in MoSe2-WSe2 Heterobilayers

Transition Metal Dichalcogenides (TMDs) monolayers are semiconductors hosting strongly bound electron-hole pairs, i.e. excitons, that are remarkably possibly studied up to room temperature. When two distinct monolayers are stacked onto each other, realizing a so-called hetero-bilayer, a periodic bandgap modulation spontaneously forms. Electronic carriers are then confined in an hexagonal lattice potential with nanometric dimensions and tunable depth. This potential is referred to as moiré potential. Here I report experiments probing interlayer excitons, made by the Coulomb attraction between electrons confined in a MoSe2 monolayer and holes in the adjacent WSe2 monolayer, at low temperature (4K). Then, I emphasize the spectroscopic signatures of interlayer excitons, and discuss the role of the moiré potential relying on polarisation resolved spectroscopy. I finally discuss the potential of our studies to explore the physics of the Bose-Hubbard model.

(Heidelberg University)

Quantum Field Simulator for Dynamics in Curved Space times

The observed large-scale structure in our Universe is seen as a result of quantum fluctuations amplified by spacetime evolution. This, and related problems in cosmology, asks for an understanding of the quantum fields of the standard model and dark matter in curved spacetime. Even the reduced problem of a scalar quantum field in an explicitly time-dependent spacetime metric is a theoretical challenge and thus a quantum field simulator can lead to new insights. Here, we demonstrate such a quantum field simulator in a two-dimensional Bose-Einstein condensate with a configurable trap and adjustable interaction strength to implement this model system. We explicitly show the realisation of spacetimes with positive and negative spatial curvature by wave packet propagation and confirm particle pair production in controlled power-law expansion of space. We find quantitative agreement with new analytical predictions for different curvatures in time and space. This benchmarks and thereby establishes a quantum field simulator of a new class. In the future, straightforward upgrades offer the possibility to enter new, so far unexplored, regimes that give further insight into relativistic quantum field dynamics.

(Università degli Studi di Milano Statale – Université de Strasbourg)

Asymmetric power dissipation in electronic transport through a quantum point contact

The understanding of electronic transport through nanostructures is crucial for the development of modern electronic devices. Moreover, the subject is of fundamental interest due to the presence of quantum coherent processes and its position at the frontier between classical and quantum physics. While traditional transport experiments measure the conductance and its dependence on parameters like an applied magnetic field, a recently developed nanothermometry technique is able to detect heat production on the surface of a sample with incredibly high spatial resolution, indicating where dissipation occurs in electronic quantum transport through a two-dimensional electron gas. The aim of this research was to start investigating the power dissipation of an electronic current flowing through a QPC in a 2DEG. Based on the Landauer-Büttiker approach, the power dissipated on the two sides of the narrow constriction is studied as a function of temperature, applied voltage and mean electrochemical potential. We demonstrated that an asymmetry appears in the dissipation, which is most pronounced when the quantum point contact is tuned to a conductance step where the transmission strongly depends on energy. At low temperatures, the asymmetry is enhanced when the temperature increases.

C. Blaas-Anselmi, F. Helluin, R. A. Jalabert, G. Weick, D. Weinmann, Asymmetric power dissipation in electronic transport through a quantum point contact, SciPost Phys. 12, 105 (2022)

(Université de Strasbourg)

Microwave electric field sensing with cold Rydberg atoms

The atom-based traceable standard for microwave electrometry shows promising advantages by enabling stable and uniform measurement. Possessing transitions from gigahertz to terahertz, Rydberg atoms systems have been utilized to develop quantum technology such as microwave electrometry[1,2], microwave to optics conversion[3] and so on. Here we theoretically propose and then experimentally realize an alternative direct International System of Units (SI)–traceable and self-calibrated method for measuring a microwave electric-field strength based on electromagnetically induced absorption (EIA) in cold Rydberg atoms. Comparing with the method of electromagnetically induced transparency, EIA measurement method is more valid and is even more robust against the experimental parameters, which enables us to realize a direct SI-traceable microwave-electric-fifield measurement as small as ∼100 μV/cm[1].

[1] K.-Y. Liao, H.-T. Tu, S.-Z. Yang et al., “Microwave electrometry via electromagnetically induced absorption in cold rydberg atoms,” Physical Review A 101,053432 (2020).
[2] J. A. Sedlacek, A. Schwettmann, H. Küblerwet et al., “Microwave electrometry with rydberg atoms in a vapour cell using bright atomic resonances,” Nature Physics 8, 819–824 (2012).
[3] H.-T. Tu, K.-Y Liao, Z.-X Zhang, X.-H Liu, S.-Y Zheng, S-Z Yang et al., ”High-efficiency coherent microwave-to-optics conversion via off-resonant scattering”, Nature Photonics (2022).

(University of Münster, IBM Research Zürich)

Cool Schottky Contacts

Silicon fin field-effect transistors (FinFETs) are used in classical CMOS electronics but can also provide an attractive platform for the implementation of spin qubits [1,2]. Classical transistors usually have highly doped contacts that determine device polarity (n-type or p-type). Our FinFETs have Schottky contacts formed by a silicide. These contacts can be ambipolar [3] if a midgap silicide is used (NiSi). For a low contact resistance to holes, PtSi is more suitable because it has a lower Schottky barrier height. In my project the goal is to study the physics of these Schottky contacts and develop a low resistance n-type Schottky contact using e.g. ErSi. These contacts will then be used to operate electron spin qubit devices like the ones that have already been demonstrated with hole spins.

[1] R. Maurand et al. Nat Commun (2016)
[2] L. Camenzind et al. ArXiv (2021)
[3] A. Kuhlmann et al. APL (2018)

(Université de Strasbourg)

Disorder-enhanced transport in a chain of lossy dipoles strongly coupled to cavity photons

In one-dimensional systems with short range interaction, it is known from the theory of Anderson localization that an infinitesimal amount of disorder makes all the eigenstates of the system exponentially localized. However, for more complex interactions the effects of disorder become highly nontrivial.
Recently, it has been proposed [1,2] to use the light-matter interaction in order to modify the localization properties of a disordered system, surprisingly leading to an improvement of its transport characteristics. Here, we study the interplay between disorder and light-matter coupling by considering a disordered one-dimensional chain of lossy dipoles coupled to a multimode optical cavity through a microscopically-derived minimal coupling Hamiltonian. Such a system, hosting polaritonic (hybrid light-matter) excitations, may be realized experimentally in a wide range of artificial systems where the dominant coupling mechanism is of dipolar nature, from macroscopic microwave helical antennas to plasmonic and dielectric nanoparticles or ultracold atoms.
By analyzing both the eigenspectrum and the driven-dissipative transport properties of our model, we find that in the strong-coupling regime, increasing disorder leads uncoupled dark states to acquire a photonic part, allowing them to inherit polaritonic long-range transport characteristics. Crucially, we show that this disorder-enhanced transport mechanism is increasingly noticeable when the considered dipoles are lossy.

[1] T. Botzung, D. Hagenmüller, S. Schütz, J. Dubail, G. Pupillo, and J. Schachenmayer, Dark state semilocalization of quantum emitters in a cavity, Phys. Rev. B 102, 144202 (2020)
[2] N. C. Chavez, F. Mattiotti, J. A. Méndez-Bermudez, F. Borgonovi, and G. L. Celardo, Disorder-enhanced and disorder-independent transport with long-range hopping: Application to molecular chains in optical cavities, Phys. Rev. Lett. 126, 153201 (2021)

(University of Geneva)

Implementation of a Self-testing Quantum Random Number Generator

Self-testing quantum random number generators have a unique place in quantum technologies, since they allow for the monitoring of the source’s entropy in real time by measuring the input-output statistics. Our device uses a prepare-and-measure protocol where two states are prepared, a coherent state or the vacuum, and then a detector performs unambiguous state exclusion to discriminate between them. The protocol is semi-device independent, while the randomness is certified only with an assumption on the overlap of the two prepared states. The QRNG is also self-testing, meaning that we can monitor the correct function of the setup in real-time. This implementation is a prototype, implemented in a box using off-the-shelf commercial components. With our simple setup we achieved real-time extraction of truly random numbers with a rate of ~7 MHz.

(ENS Paris-Saclay, Karlsruher Institut für Technologie)

Majorana fermions in interaction with sub- and supraluminal solitons in topological Josephson Junctions

Majorana fermions in condensed matter systems are of great interest for their applications in quantum information processing. One can find them at the edges of two-dimensional time-reversal topological superconductors. Those edge states obey a Dirac equation with an effective speed of light. In a long Josephson junction made from such superconductors, the Dirac field is perturbed by solitons of the superconducting phase difference. One finds bound states localized on the soliton, and more particularly a zero-energy mode, also called Majorana bound state (MBS). By injecting external current, one can control those solitons and make them move slower or faster than the effective speed of light. We study the excitation of all edge states by the passage of a sub- and supraluminal soliton and calculate analytically the induced particle current and energy density. We deduce under which conditions the information encoded in the MBS can be read by a detector.

(TU Delft, European Space Agency)

Towards a Global Satellite Quantum Network: Optimized Key Rate Estimation in a Dynamic Satellite System

Quantum networks will undoubtedly play crucial role in advancing quantum technologies including quantum communication, distributed quantum computing, and quantum metrology. Currently a nascent topic, the fundamental understanding of how to properly and efficiently simulate usable quantum networks will have a profound impact on how to design and implement this technology on a global scale. In one application, such networks are used in quantum communication, wherein provably-secure quantum encryption protocols face important challenges. Reaching high key transmission rates over long distances through fiber or atmosphere is difficult due to exponential photon loss, and can be ameliorated by incorporating satellite links into the quantum communication network. Within this project, an optimized theoretical framework for a global network of satellites will be developed.
The goal of this project is to develop a modular channel model which includes all relevant sources of noise and loss in such a system, and to create an optimization tool which selects the best parameters for a given infrastructure. This will also take into account satellite orbits and their effects on communication windows. Once a single link system has been established, we will be able to simulate a dynamic multi-satellite network with several ground stations, with room for further optimization for parameters such as the minimum number of satellites. Ultimately, we will provide a modular approach for simulating a global network for various cryptographic schemes and satellite constellations, paving the way for a global quantum internet.

(Université Paris-Saclay, Pasqal)

PUSHO, a pulse-level variational quantum algorithm for Rydberg atom platforms

The manipulation of neutral atoms by light is at the heart of countless scientific discoveries in the field of quantum physics in the last three decades. The level of control that has been achieved at the single particle level within arrays of optical traps, while preserving the fundamental properties of quantum matter (coherence, entanglement, superposition), makes these technologies prime candidates to implement disruptive computation paradigms.
The current quantum processors (QPU) are nothing close to universal computers but can already target specific combinatorial problems whose classical complexity scales exponentially with their size, such as finding the Maximum Independent Set (MIS) of a Unit-Disk (UD) graph. Mapping those problems to Rydberg platforms enables to solve them by approximating their cost function with the Hamiltonian of the system. Then the atoms are optimally driven to a solution state which minimises the previous cost with pulses shaped by Bayesian optimisation. Pulse Shaping Optimisation (PUSHO) provides a straightforward but powerful Variational Quantum Algorithm (VQA), taking advantage of our ability to precisely control the shape of the driving fields.

(Friedrich-Alexander-Universität Erlangen-Nürnberg)

Variational Quantum Eigensolver for the J1-J2-model

The ground state properties of the two-dimensional J1-J2-model are very challenging to analyze via classical numerical methods, which makes the model a promising candidate where quantum computers could be helpful and possibly explore regimes that classical computers cannot reach.The J1-J2-model is a quantum spin model composed of Heisenberg interactions along the rectangular lattice edges and along diagonal edges between next-nearest neighbor spins.We propose an ansatz for the Variational Quantum Eigensolver (VQE) to approximate the ground state of an antiferromagnetic J1-J2-Hamiltonian for different lattice sizes and different ratios of J1 and J2.  Moreover, we demonstrate that this ansatz can work without the need for gates along the diagonal next-nearest neighbor interactions. This simplification is of great importance for solid state based hardware with qubits on a rectangular grid, where it eliminates the need for SWAP gates.In addition, we give an outlook on the number of gates and parameters needed for larger lattice sizes.

(Technical University Munich)

Twisted hybrid algorithms for combinatorial optimization

We argue that for certain optimization problems, variational quantum algorithms such as the Quantum Approximate Optimization Algorithm can be further hybridized. Specifically, we consider combining a quantum variational ansatz with a greedy classical post-processing procedure for the MaxCut problem on 3-regular graphs. We show that the average cut-size can be quantified in terms of the energy of a modified problem Hamiltonian. This motivates the consideration of an improved algorithm which variationally optimizes the energy of that Hamiltonian. We call this a twisted hybrid algorithm and give analytic lower bounds on the expected approximation ratios achieved by twisted QAOA. These show that the necessary non-locality of the quantum ansatz can be reduced: For small levels, we show that compared to the original QAOA, the level can be reduced by one while roughly maintaining the expected approximation ratio.

(University of Bristol)

An Introduction to Integrated Quantum Photonics for Quantum Computing

Integrated quantum photonics (IQP) offers a promising platform for quantum computing: circuits are small, reconfigurable and scalable, with the manufacture of IQP chips supported by mature CMOS processes; photons suffer little from decoherence and travel fast enabling long-distance transmission of information along optical fibres; and photons are measured easily allowing fast readout of quantum states. IQP is not without challenges however, with notable examples being photon loss and the deterministic generation of photons. Here we present an introduction to IQP for quantum computing and highlight recent work towards developing the platform, namely creating near unity purity photon sources in silicon [1] and the implementation of reconfigurable graph states with four and eight qubits with the latter using an error protection protocol [2,3].

[1] Paesani, S., Borghi, M., Signorini, S. et al. Near-ideal spontaneous photon sources in silicon quantum photonics. Nat Commun 11, 2505 (2020).

[2] Adcock, J.C., Vigliar, C., Santagati, R. et al. Programmable four-photon graph states on a silicon chip. Nat Commun 10, 3528 (2019).

[3] Vigliar, C., Paesani, S., Ding, Y. et al. Error-protected qubits in a silicon photonic chip. Nat. Phys. 17, 1137–1143 (2021).

(Université de Strasbourg)

Floquet theory for a fast and high fidelity parametric-two-qubit gates

A major challenge in realizing scalable quantum computers is the optimization of two-qubit entangling gates. The realization of high-fidelity quantum gates involves a multitude of parameters governing important properties of the system, such as the gate rate, leakage, spurious interactions etc. One therefore needs to carefully choose these parameters to optimize the gate speed while minimizing unwanted cross-Kerr interaction. Numerically, this can be a resource intensive process as it implies simulating the gate dynamics for each set of device parameters. We present a method based on Floquet theory to extract the interaction rates without having to run full dynamical simulations. We apply this technique to a new two-qubit entangling parametric gate developed in collaboration with Princeton. First experimental results show the realisation of a sqrt(iSWAP) gate in 13 ns with 99.8% fidelity, which is one of the best in the field.