Superconducting qubits and propagating microwave photons

• Juan José García Ripoll (Institute of Fundamental Physics, CSIC)

Room: Aula Renzo Leonardi In this talk I will discuss how it is possible to perform quantum information tasks in superconducting quantum devices using microwave guides and propagating photons. In the first half of the talk I will discuss how superconducting qubits couple to microwave guides, implementing canonical models of quantum optics and condensed matter physics. This includes both strong and ultrastrong coupling regimes, as well as quantum phase transitions, all of which can be studied with analytical and numerical techniques. In the second half of the talk, I will discuss how these setups can be used to implement quantum state transfer between superconducting quantum processors, and how these ideas can be used implementing quantum gates. I will discuss the limits of fidelity and speed, how to improve the transfer correcting for photon dispersion, or how to multiplex the transfer.

Controlling topological phases of matter with quantum light

• Olesia Dmytruk (CNRS, Ecole Polytechnique)

Room: Aula Renzo Leonardi Controlling the topological properties of quantum matter is a major goal of condensed matter physics. A major effort in this direction has been devoted to using classical light in the form of Floquet drives to manipulate and induce states with non-trivial topology. A different route can be achieved with cavity photons. In this talk, I will discuss a prototypical model for topological phase transition, the one-dimensional Su-Schrieffer-Heeger (SSH) model, coupled to a single mode cavity. I will demonstrate that quantum light can affect the topological properties of the system, including the finite-length energy spectrum hosting edge modes and the topological phase diagram. In particular, I will show that depending on the lattice geometry and the strength of light-matter coupling one can either turn a trivial phase into a topological one or vice versa using quantum cavity fields. Furthermore, the polariton spectrum of the coupled electron-photon system contains signatures of the topological phase transition in the SSH model.

Driven-dissipative quantum many-body systems: From instability in cavity-boson systems to enhancement of superconductivity

• Rui Lin (ETH Zurich)

Room: Aula Renzo Leonardi The driven-dissipative nature of quantum optical many-body systems is conventionally captured by the Lindblad form. It leads to substantial distinctions from their static counterparts described by the same effective Hamiltonian, including, for example, the dissipative instability towards high energy states in cavity-boson systems. The combination of the Floquet and Keldysh theories provide a more profound understanding of the underlying mechanism. The developed technique captures the most essential ingredient in the core of all these effects: the relative system-bath rotation, in a way far more comprehensive than the Lindblad form. Particularly, it can be straightforwardly applied to condensed matter systems, and reveals more intriguing and unexplored physics. Specifically in the Hubbard-Stratonovich mean-field description of superconductors, we predict the driven-dissipative enhancement of the superconducting gap at finite temperatures comparable to and beyond the critical temperature.

Towards a many-body atom-photon interface

• Marco Canteri

Room: Aula Renzo Leonardi Quantum networks have already been realized between two remote qubits. Our work aims to develop the ability to entangle remote quantum processors, each consisting of a register of qubits capable of universal quantum processing. On the path towards that, we have developed a small-scale trapped ion quantum processor that allows for each qubit to be entangled with a different propagating photon. Such photons could be used to entangle remote copies of the processor. We demonstrated the capabilities of our system in two different ways: by generating 3 entangled atom-photon pairs and then swapping the quantum state from the matter processor to light; and by scaling our approach to 10 qubits.

Can deep sub-wavelength cavities induce Amperean superconductivity in a 2D material?

• Gian Marcello Andolina (College de France, Paris.)

Room: Aula Renzo Leonardi Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-Tc superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of 1-20 K in two-dimensional, parabolic-band, electron gases embedded inside deep sub-wavelength optical cavities. In this talk, I first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep sub-wavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing.

Enhanced Cavity Optomechanics with Quantum-Well Exciton Polaritons

• Nicola Carlon Zambon (ETH Zürich)

Room: Aula Renzo Leonardi A key figure of merit in optomechanics is the single-photon quantum cooperativity (Cq). Recent works achieved a large cooperativity by engineering resonators with ultra-low mechanical and optical losses [1]. A complementary approach is to enhance optomechanical interactions while working with modest optical and mechanical quality factors. Less stringent bandwidth limitations in optomechanical conversion are thereby imposed [2], while suppressing optical heating and added noise [1]. In this context, GaAs- based resonators engineered to simultaneously confine photons, phonons and QW excitons offer an intriguing opportunity [3]: in the strong exciton-photon coupling regime the system hosts hybrid quasi-particles, or polaritons. These modes are both spectrally separated from the exciton-induced absorption peak, enabling large optical quality factors, while their excitonic component is extremely sensitive to strain fields owing to the large GaAs deformation potential, thus prospecting strong optomechanical interactions. We analytically model the tripartite interaction of light, QW excitons, and sound in three semiconductor microresonators architectures: when considering parameters complying with current GaAs technologies, we show that a near-unity Cq can be obtained for a single polariton excitation. Furthermore, we investigate how polariton nonlinearities modify dynamical back-action via squeezing [4]. [1] H. Ren, M. H. Matheny et al. - Nat. Comm. 11, 3373 (2020) [2] Y.D. Wang and A.A. Clerk, PRL 108, 153603 (2012) [3] G. Rozas, A.E. Bruchhausen et al. - PRB 90, 201302 (2014) [4] N. Carlon Zambon, Z. Denis et al. PRL - 129, 093603 (2022)

Tensor network states for real materials

• Ulrich Schollwöck (University of Munich)

Room: Aula Renzo Leonardi Tensor network states are widely and very successfully used for the simulation of models of strongly correlated systems. These models are often an oversimplification of real materials. In this talk I will show how tensor network methods can be used in the context of combinations of density functional theory for realistic band structures and embedding methods such as the dynamical mean-field theory (DMFT) to describe real materials quantitatively, such as Hund’s metals or materials with important spin-orbit coupling.

Linear dispersion with a tilt: analog black holes, electron lenses and Berry curvature effects

• Andreas Haller (University of Luxembourg)

Room: Aula Renzo Leonardi In this talk, I present our recent study about transport in Weyl semimetals with spatially varying nodal tilt profiles. We discuss two complementary approaches that characterise the electron flow: solutions of the semi-classical equations of motion, in analogy to those encountered in black hole spacetimes, and large-scale microscopic simulations of a scattering region surrounded by semi-infinite leads. We show that the two approaches lead to equivalent results when the wave packet is sufficiently far from the center of the tilt. The two methods are arguably a powerful toolset in the pursuit of tiltronic devices such as e.g. electronic lenses.

Neural Quantum States for Many Body (dissipative) Quantum Dynamics

• Filippo Vicentini (Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL))

Room: Aula Renzo Leonardi Neural network quantum states have delivered state of the art results for the calculation of ground states for systems beyond the reach of more conventional techiniques. Such variational ansatzes have also been applied to the simulation of the dynamics of systems at equilibrium or far from it. In this talk I will discuss recent advancements in the treatment of the dynamics with a particular focus on the dissipative dynamics of Markovian Open Quantum Systems.

Transport in the Asymmetric Simple Inclusion Process and an Unexpected Unification of Bosons and Fermions

• Yuri Minoguchi (TU Wien)

Room: Aula Renzo Leonardi We study the counting statistics of the asymmetric simple inclusion process (ASIP), which describes the dissipative transport of bosons along a one dimensional lattice. By combining exact numerical simulations with a field-theoretical analysis, we evaluate the current fluctuations for this process and determine their asymptotic scaling. Surprisingly, our findings show that the ASIP falls into the KPZ universality class and therefore, despite a drastic difference in the underlying particle statistics, exhibits the same scaling relations as the celebrated asymmetric simple exclusion process (ASEP) for fermions. We observe, however, crucial differences between the two processes in the shape of the distribution function, which we reconcile by mapping both models to the physics of one dimensional interfaces. This unified description shows that bosonic transport corresponds to interface growth, while its fermionic counterpart maps onto an eroding interface instead. Beyond their transport-theoretical interest, these fundamental relations can be probed in various experiments with cold atoms or with long-lived quasi-particles in nano-photonic lattices.

Process Tensor Networks for non-Markovian Many-Body Open Quantum Systems

• Gerald E. Fux (International Center for Theoretical Physics)

Room: Aula Renzo Leonardi There is a range of interesting physical scenarios that include both many-body quantum systems *and* strongly coupled structured environments that lead to a non-Markovian evolution. However, almost all methods for the study of many-body systems only consider closed or Markovian dynamics, while methods for the study of non-Markovian open quantum systems are generally restricted to small system sizes. I will introduce a general numerical method to compute dynamics and multi-time correlations of chains of quantum systems, where each system may couple strongly to a structured environment [1,2]. The method combines the process tensor formalism for general (possibly non-Markovian) open quantum systems with time evolving block decimation (TEBD) for 1D chains. It systematically reduces the numerical complexity originating from system-environment correlations before integrating them into the full many-body problem, making a wide range of applications numerically feasible. Furthermore, on a more conceptional side, I will discuss fundamental connections among the concept of Markovianity, multi-time correlations, and the dynamics of a many-body open quantum system [3]. These connections not only have far reaching consequences in, for example, the field of strong coupling quantum thermodynamics, but also in many-body scenarios that are usually considered to be Markovian in the literature. [1] G. E. Fux, D. Kilda, B. W. Lovett, and J. Keeling, *Thermalization of a spin chain strongly coupled to its environment*, [arXiv:2201.05529](https://arxiv.org/abs/2201.05529) (2022). [2] The TEMPO Collaboration, *OQuPy: A Python 3 package to efficiently compute non-Markovian open quantum systems*, [ReadTheDocs](https://oqupy.readthedocs.io/) (2020). [3] G. E. Fux, *Operationally accessible information backflow in CP-divisible processes*, in preperation.

Non-abelian Berry's phase in photonic waveguides arrays

• Valentina Brosco (CNR, ISC, Università 'La Sapienza' di Roma)

Room: Aula Renzo Leonardi Non-abelian gauge fields emerge naturally in the description of adiabatically evolving quantum systems. In this talk we show that they also play a role in Thouless pumping in the presence of degenerate bands. Specifically, we consider a photonic Lieb lattice and show that when the lattice parameters are slowly modulated, the propagation of the photons bears the fingerprints of the underlying non-abelian gauge structure. The non-dispersive character of the bands enables a high degree of control on photon propagation. Our work paves the way to the generation and detection of non-abelian gauge fields in photonic and optical lattices. The talk includes a review of the physics photonic waveguide arrays as quantum simulators and perspectives on quantum applications of Thouless pumps.

Programmable distribution of multi-qubit entanglement in dual-rail waveguide QED

• Joan Agusti (Walther-Meißner-Institut)

Room: Aula Renzo Leonardi We investigate the autonomous generation of multi-partite entangled states in a dual-rail waveguide QED configuration. Here, qubits arranged along two separated photonic waveguides are illuminated by the output of a nondegenerate parametric amplifier, which drives them into a strongly correlated steady state. We show that in this setup, there exists a large family of pure steady states, for which the connectivity and the degree of multi-qubit entanglement can be selectively adjusted by simply changing the applied pattern of qubit-photon detunings. This offers intriguing new possibilities for distributing ready-to-use multi-partite entangled states across large quantum networks, which do not require any precise pulse control and rely on Gaussian entanglement sources only.

The Quest of Quantum Advantage with a Hybrid Photonics Platform

• Fabio Sciarrino

Room: Aula Renzo Leonardi Boson sampling is a computational problem that has been proposed as a candidate to obtain an unequivocal quantum computational advantage. The problem consists in sampling from the output distribution of indistinguishable bosons in a linear interferometer. There is strong evidence that such an experiment is hard to classically simulate, but it is naturally solved by dedicated photonic quantum hardware, comprising single photons, linear evolution, and photodetection. This prospect has stimulated much effort resulting in the experimental implementation of progressively larger devices. We will review recent advances in photonic boson sampling, describing both the technological improvements achieved and the future challenges. We will discuss recent proposals and implementations of variants of the original problem based on hybrid photonics platform.

High-dimensional optical encodings for integrated error-protected Quantum Computing and Quantum Communication

• Caterina Vigliar (Danmarks Tekniske Universitet)

Room: Aula Renzo Leonardi The control of large photonic integrated devices, processing tailored entangled resources of error-protected qubits, is an important step towards realising an all-photonic quantum computer. Measurement-based encodings, computing tasks and applications, showing improvements in such devices´ computational performance, will be shown. Furthermore, future perspectives on the advantages of the distribution of the above resource entangled states over chip-based quantum networks will also be discussed.

A real-time QRNG-to-QKD stream exploiting FPGA for high security Quantum Communication

• Andrea Stanco (Università di Padova)

Room: Aula Renzo Leonardi Most of the modern Quantum Key Distribution (QKD) and Quantum Random Number Generation (QRNG) systems require the usage of the Field Programmable Gate Array (FPGA) technology as it can guarantee the deterministic behavior necessary for dealing with qubit generation and readout. Nevertheless, the System-on-a-Chip (SoC) technology, which integrates both an FPGA and a CPU and allows for a very high level of flexibility, is not as common as the FPGA. Therefore, we exploited the SoC technology to realize a high performance QKD/QRNG system, implementing what we called “1-random-1-qubit” (QRN2Qubit) encoding. Such encoding grants a higher level of security, as each qubit is encoded with a unique random number. This is possible thanks to a real-time architecture that can continuously stream random data from a high speed QRNG (>300 Mbps) to a QKD transmitter (qubit repetition rate equal to 50 MHz) for BB84 protocol exploiting polarization degree of freedom of single photons. The system was tested for 55 hours and showed no interruptions and correctly delivered the data from the QRNG to the QKD transmitter. Most of the nowadays systems exploit a low-rate QRNG (few Mbit/s) and algorithm expansions to reach the required bitrate but with a major drawback in security as the transmitted qubit sequence is not fully random due to the expansion algorithms. Thus, this system offers a higher level of security for QKD thanks to the true randomness of the qubit sequence. This SoC-based system was used in real scenarios for demonstration of urban QKD networks as well in several QKD/QRNG experiments realized by the QuantumFuture research group. Recently, it was also integrated into the QKD systems provided by ThinkQuantum, a spin-off company from University of Padova.

On-chip genuine three-qubit entanglement from a deterministic source

• Yijian Meng (Niels Bohr Institute, University of Copenhagen)

Room: Aula Renzo Leonardi Multi-photon entangled state is the key ingredient in realizing measurement-based quantum computing. The current proposals for universal quantum computation require simultaneously high generation rates, high fidelity, and low loss, which are beyond the capability of the current experimental systems. In this work, we address this critical problem by demonstrating the on-chip deterministic generation of a three-qubit state. Our work bridges the gap toward an ideal platform where photons are collected with almost unity efficiency on-chip. Using a quantum dot embedded in a photonic crystal waveguide, we charge it deterministically with a single electron spin. We control the electron spin environment by narrowing the nearby nuclear spin distribution, thus improving the T2* time by a factor of 10. Moreover, we demonstrate genuine three-qubit entanglement, which consists of an electron spin and two indistinguishable photons. Our work constitutes a key step toward the next-generation device where the criteria for a fully-fledged photonic quantum computer can be fulfilled.

Integrated photonics for trapped ion quantum computing

• Carmelo Mordini (ETH Zurich - Institute for Quantum Electronics)

Room: Aula Renzo Leonardi Trapped ions are one of the most promising platforms in the field of quantum computing and simulation. Technology nowadays offers incredible tools to trap and manipulate individual particles down to the quantum level, but the current state of the art allows to maintain control of these systems only up to a certain size. One of the most pressing roadblocks to overcome is to make laser beam delivery scalable and efficient On the other hand, integrated photonics is an established and powerful tool for manipulating laser light. Miniaturized optical elements can be precisely manufactured and replicated to scale, allowing control of light that wouldn't be possible with traditional bench-top free-space optics. In this talk, I will introduce the current efforts to bridge these two technologies. Ion trapping experiments can take advantage of photonics for efficient addressing of ions with laser light, shaping light beams in order to tailor atom-light interactions, and integrating photonic structures directly in the trap as a way to scale from lab experiments to the next generation's computers.

TBA

• Tommaso Calarco (Forschungszentrum Jülich GmbH, Universität zu Köln)

Room: Aula Renzo Leonardi

TBA

• Hannes Pichler

Room: Aula Renzo Leonardi

Quantum simulation of SU(2) 1D dynamics with ions qudits

• Giuseppe Calajò

Room: Aula Renzo Leonardi Gauge theories are an ubiquitous concept in physics appearing in different fields of research spanning from high energies to condensed matter. Their resolution using Monte Carlo techniques has been very successful over the years but is unable to tackle many important physical regimes occurring at finite density, especially for nonabelian theories such as quantum chromodynamics. Their alternative Hamiltonian formulation on a lattice has opened new possibilities to tackle these problems via quantum simulations. Along these lines, many proposals have been suggested but experimentally only abelian theories have been simulated so far. In this talk, we present a convenient formulation of a 1D SU(2) nonabelian model which is naturally suitable for implementation on six levels ions qudit recently developed in the lab. By choosing a convenient encoding and performing simultaneous Molmer-Sorensen gates we show that a quite shallow circuit is needed to perform a quantum digital simulation of the model.

Analysing crosstalk with the digital twin of a Rydberg atom QPU

• Alice Pagano (University of Padova)

Room: Aula Renzo Leonardi Decoherence and crosstalk are two adversaries when aiming to parallelize a quantum algorithm: on the one hand, the execution of gates in parallel reduces decoherence due to a shorter runtime, but on the other hand, parallel gates in close proximity are vulnerable to crosstalk. This challenge is visible in Rydberg atom quantum computers where atoms experience strong van der Waals interactions decaying with distance. We demonstrate how the preparation of a 64-qubit GHZ state is affected by crosstalk in the closed system with the help of a tensor network digital twin of a Rydberg atom QPU. Then, we compare the error from crosstalk to the decoherence effects proving the necessity to parallelize algorithms.

Quantum light-matter interaction with a dielectric sphere: theory and applications

• Carlos Gonzalez-Ballestero (Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, Innsbruck, Austria)

Room: Aula Renzo Leonardi A major driving force of the field of levitodynamics — the levitation and control of microobjects in vacuum — is the possibility of generating macroscopic quantum states of the center-of-mass motion of a levitated nanoparticle. Not only can these states help address questions about the interplay between gravity of quantum physics or the nature of wavefunction collapse, but their mere existence would prove the validity of quantum mechanics at regimes of mass 4 orders of magnitude higher than the current record. Recent demonstrations of ground-state motional cooling and quantum control along one motional direction (1D) show that such quantum regime of levitated nanoparticles is within experimental reach. Still, the generation and certification of macroscopic quantum states requires to answer crucial fundamental questions, for instance: can one break the seemingly fundamental limitation which allows to only feedback-cool efficiently one of the three motional degrees of freedom? How to protect motional quantum states from decoherence? and how to generate the strong nonlinearity needed to observe purely quantum (Wigner-negative) states? In my talk, I will discuss our team’s theoretical effort to answer these questions. I will introduce our recently developed theoretical formalism describing the quantum interaction between light and a trapped dielectric sphere of arbitrary size. I will show how we quantitatively predict that (i) 3D ground-state feedback cooling is possible for particles beyond the point-dipole approximation (ii) laser- induced motional decoherence can be fully suppressed by using squeezed light and (iii) shifting from harmonic to double-well potentials allows to generate detectable Wigner negativities within the motional coherence lifetime. Our work sets the theoretical basis of 3D levitated optomechanics and provides the tools to design future macroscopic quantum physics experiments.

Hassle-free Extra Randomness from quantum state’s identicalness with untrusted components

• Hamid Tebyanian (University of York)

Room: Aula Renzo Leonardi This paper investigates a semi-device-independent protocol for quantum randomness generation constructed on the prepare-and-measure scenario based on the on-off-keying encoding scheme and with various detection methods, i.e., homodyne, heterodyne, and single photon detection schemes. The security estimation is based on lower bounding the guessing probability for a general case and is numerically optimized by utilizing semi-definite programming. Additionally, a practical, easy-to-implement optical setup is presented, which can be implemented via commercial off-the-shelf components.

The bosonic skin effect: boundary condensation in asymmetric transport

• Louis Garbe (TU Wien)

Room: Aula Renzo Leonardi We study the incoherent transport of bosonic particles through a one dimensional lattice with different left and right hopping rates, as modelled by the asymmetric simple inclusion process (ASIP). Specifically, we show that as the current passing through this system increases, a transition occurs, which is signified by the appearance of a characteristic zigzag pattern in the stationary density profile near the boundary. In this highly unusual transport phase, the local particle distribution alternates on every site between a thermal distribution and a Bose-condensed state with broken U(1)-symmetry. Furthermore, we show that the onset of this phase is closely related to the so-called non-Hermitian skin effect and coincides with an exceptional point in the spectrum of density fluctuations. Therefore, this effect establishes a direct connection between quantum transport, non-equilibrium condensation phenomena and non-Hermitian topology, which can be probed in cold-atom experiments or in systems with long-lived photonic, polaritonic and plasmonic excitations.

Emergent Pauli blocking in a one-dimensional Bose gas

• Federica Cataldini (TU Wien)

Room: Aula Renzo Leonardi The relationship between many-body interactions and dimensionality is key to emergent quantum phenomena. A striking example is the Bose gas, which upon confinement to one dimension (1D) obeys an infinite set of conservation laws, prohibiting thermalization and steering dynamics. We experimentally demonstrate that the integrable dynamics of a Bose gas can persist deep within the dimensional crossover regime. Starting from a weakly interacting, one-dimensional Bose gas, we perform a quench to instigate dynamics of a single density mode. We find that its relaxation accurately follows predictions of dephasing from the integrable theory, even for temperatures up to three times the conventional limit for one-dimensionality. We attribute our observations to an emergent Pauli blocking of the 3D excitations, caused by the relevant collective excitations of the system assuming fermionic statistics, despite the gas being comprised of weakly interacting bosons. Our experiment demonstrates how the integrable solutions can be employed to establish a direct link between microscopic details of the system and its observed macroscopic behaviour, thus presenting new avenues to investigate emergent quantum many-body phenomena.

Quantum kinetics of quenched two-dimensional Bose superfluids

• Clément Duval (Sorbonne Université)

Room: Aula Renzo Leonardi We study theoretically the non-equilibrium dynamics of a two-dimensional (2D) uniform Bose superfluid following a quantum quench, from its short-time (prethermal) coherent dynamics to its long-time thermalization. Using a quantum hydrodynamic description combined with a Keldysh field formalism, we derive quantum kinetic equations for the low-energy phononic excitations of the system and characterize both their normal and anomalous momentum distributions. We apply this formalism to the interaction quench of a 2D Bose gas and study the ensuing dynamics of its quantum structure factor and coherence function, both recently measured experimentally. Our results indicate that in two dimensions, a description in terms of independent quasi-particles becomes quickly inaccurate and should be systematically questioned when dealing with non-equilibrium scenarios.