QUROPE Quantum Information Processing and Communication in Europe

Optical quantum memories are essential for quantum communications and photonic quantum technologies. Ensemble optical memories based on 3-level interactions are a popular basis for implementing these memories. However, ensemble optical memories based on an off-resonant 3-level interaction, such as the Raman gradient echo memory (GEM), suffer loss due to scattering from the intermediate state. This scattering is normally reduced by a large detuning from the intermediate state. In this work we show that loss is reduced in GEM due to electromagnetically induced transparency adjacent to the Raman absorption line, and the highest efficiency is instead achieved at a moderate detuning. Furthermore, the effectiveness of the transparency, and therefore the efficiency of GEM, depends on the order in which gradients are applied to store and recall the light. We provide a theoretical analysis and show experimentally how the efficiency depends on gradient order and detuning.

The concept of entanglement is at the core of the theory of quantum information. In this paper a criterion for unentanglement of quantum states is proposed and proved. This criterion is natural, practical and easy to check.

Two interacting Rydberg atoms coupled to a waveguide realize a giant-atom platform that exhibits the controllable (phase-dependent) chirality where the direction of nonreciprocal photon scattering can be switched on demand, e.g., by the geometrical tuning of an external driving field. At variance with previous chiral setups, the simplified approach of our proposed platform arises from an optical implementation of the local phase difference between two coupling points of the Rydberg giant atom. Furthermore, employing two or more driving fields, this platform could also be used as a frequency converter with its efficiency exhibiting a strong asymmetry and being significantly enhanced via the chiral couplings. Our results suggest an extendable giant-atom platform that is both innovative and promising for chiral quantum optics and tunable frequency conversion in the optical domain.

The pairing ratio, a crucial metric assessing a biphoton source's ability to generate correlated photon pairs, remains underexplored despite theoretical predictions. This study presents experimental findings on the pairing ratio, utilizing a double-$\Lambda$ spontaneous four-wave mixing biphoton source in cold atoms. At an optical depth (OD) of 20, we achieved an ultrahigh biphoton generation rate of up to $1.3\times10^7$ per second, with a successful pairing ratio of $61\%$. Increasing the OD to 120 significantly improved the pairing ratio to $89\%$, while maintaining a consistent biphoton generation rate. This achievement, marked by high generation rates and robust biphoton pairing, holds great promise for advancing efficiency in quantum communication and information processing. Additionally, in a scenario with a lower biphoton generation rate of $5.0 \times 10^4$ per second, we attained an impressive signal-to-background ratio of 241 for the biphoton wavepacket, surpassing the Cauchy-Schwarz criterion by approximately $1.5\times10^4$ times.

The variance of quantum channels involving a mixed state gives a hybrid of classical and quantum uncertainties. We seek certain decomposition of variance into classical and quantum parts in terms of the Wigner-Yanase skew information. Generalizing the uncertainty relations for quantum observables to quantum channels, we introduce a new quantity with better quantum mechanical nature to describe the uncertainty relations for quantum channels. We derive several uncertainty relations for quantum channels via variances and the Wigner-Yanase skew information.

We present a theoretical model of matter-wave diffraction through a material nanostructure. This model is based on the numerical solution of the time-dependent Schr{\"o}dinger equation, which goes beyond the standard semi-classical approach. In particular, we consider the dispersion force interaction between the atoms and the material, which is responsible for high energy variations. The effect of such forces on the quantum model is investigated, along with a comparison with the semi-classical model. In particular, for atoms at low velocity and close to the material surface, the semi-classical approach fails, while the quantum model accurately describes the expected diffraction pattern. This description is thus relevant for slow and cold atom experiments where increased precision is required, e.g. for metrological applications.

We explore the first-order phase transition in the lattice Schwinger model in the presence of a topological $\theta$-term by means of the variational quantum eigensolver (VQE). Using two different fermion discretizations, Wilson and staggered fermions, we develop parametric ansatz circuits suitable for both discretizations, and compare their performance by simulating classically an ideal VQE optimization in the absence of noise. The states obtained by the classical simulation are then prepared on the IBM's superconducting quantum hardware. Applying state-of-the art error-mitigation methods, we show that the electric field density and particle number, observables which reveal the phase structure of the model, can be reliably obtained from the quantum hardware. To investigate the minimum system sizes required for a continuum extrapolation, we study the continuum limit using matrix product states, and compare our results to continuum mass perturbation theory. We demonstrate that taking the additive mass renormalization into account is vital for enhancing the precision that can be obtained with smaller system sizes. Furthermore, for the observables we investigate we observe universality, and both fermion discretizations produce the same continuum limit.

Computer Vision (CV) labelling algorithms play a pivotal role in the domain of low-level vision. For decades, it has been known that these problems can be elegantly formulated as discrete energy minimization problems derived from probabilistic graphical models (such as Markov Random Fields). Despite recent advances in inference algorithms (such as graph-cut and message-passing algorithms), the resulting energy minimization problems are generally viewed as intractable. The emergence of quantum computations, which offer the potential for faster solutions to certain problems than classical methods, has led to an increased interest in utilizing quantum properties to overcome intractable problems. Recently, there has also been a growing interest in Quantum Computer Vision (QCV), with the hope of providing a credible alternative or assistant to deep learning solutions in the field. This study investigates a new Quantum Annealing based inference algorithm for CV discrete energy minimization problems. Our contribution is focused on Stereo Matching as a significant CV labeling problem. As a proof of concept, we also use a hybrid quantum-classical solver provided by D-Wave System to compare our results with the best classical inference algorithms in the literature.

We show that classical states can emerge as pure ground state solutions of a quantum many-body system. We use a simple Hubbard model in 1D with strong short-range interactions and a second nearest neighbor hopping with N particles arranged among M sites. We show that the ground state of this Hubbard chain for M=2N-1 consists of a single many-body state where the strongly interacting particles arrange in a classical state with crystalline order. The ground state is separated by an energy gap from the first excited state, and survives in the thermodynamic limit for large N. The energy gap increases linearly with the strength of the interaction between the particles making the classical ground state robust to external perturbations like disorder. Our result is an example of how a quantum system can converge to a classical state, like a crystal, without requiring decoherence, wavefunction collapse or other external mechanisms.

Charge noise has been one of the main issues in realizing high fidelity two-qubit quantum gates in semiconductor based qubits. Here, we study the influence of quasistatic noise in quantum dot detuning on the controlled phase gate for spin qubits that defined on a double quantum dot. Analytical expressions for the noise averaged Hamiltonian, exchange interaction, as well as the gate fidelity are derived for weak noise covering experimental relevant regime. We also perform interleaved two-qubit randomized benchmarking analysis for the controlled phase gate and show that an exponential decay of the sequential fidelity is still valid for the weak noise.

The measurement process of observables in a quantum system comes out to be an unsovable problem which started in the early times of the development of the theory. In the present note we consider the measured system part of an open system interacting with the measuring device and show under which ideal conditions the measure process may ideally work. Our procedure leads to the conclusion that there is no hope that any experimental procedure will be able to lead to a clean solution of the process, except maybe in very specific cases. The reasons for this situation are deeply rooted in the fundamental properties of quantum theory.

We demonstrate that geometric frustration and long-range interactions both promote order-by-disorder in the antiferromagnetic transverse-field Ising model on the ruby lattice. To this end we investigate the quantum phase diagram for truncated $J_1$-$J_2$-$J_3$ Ising interactions. In the low-field limit we derive an effective quantum dimer model, analyzing how the extensive ground-state degeneracy at zero field is lifted by two distinct order-by-disorder scenarios. We support our analysis by studying the gap-closing of the high-field phase using series expansions. For $J_2>J_3$, we find an emergent clock-ordered phase at low fields, stabilized by resonating plaquettes, and a 3d-XY quantum phase transition to the polarized high-field phase. For $J_3>J_2$, an order-by-disorder mechanism stabilizes a distinct $k=(0,0)$ order and a quantum phase transition in the 3d-Ising universality class is observed. In contrast to the triangular lattice, on the ruby lattice algebraically decaying long-range interactions favor the clock-ordered low-field phase and therefore allow a robust implementation in existing Rydberg atom quantum simulators.

Quantum information with many-body quantum spin systems has, from time to time, given intriguing and intuitive outcomes to our understanding of multiport quantum communications. We identify that in an anisotropic many-body quantum spin system with two- and three-body interactions, when its two-spin subsystems are all negative under partial transpose, one can restrict this system for realizing only the multiport quantum dense coding protocol which has $(N-1)$ senders and a single receiver. All other single and multi channel dense coding protocols will have quantum dense coding capacities less than that of their respective classical capacities. We characterize the multiport quantum dense coding capacity with $(N-1)$ senders and a single receiver for this system with respect to its system parameters. We also define a magnetic field averaged dense coding capacity for this system, which allows us to comprehensively capture the influence of the entire range of external applied magnetic field and characterize its variation with respect to other system parameters.

Matter-wave interferometry with increasingly larger masses could pave the way to understanding the nature of wavefunction collapse, the quantum to classical transition or even how an object in a spatial superposition interacts with its gravitational field. In order to improve upon the current mass record, it is necessary to move into the nano-particle regime. In this paper we provide a design for a nano-particle Talbot-Lau matter-wave interferometer that circumvents the practical challenges of previously proposed designs. We present simulations of the expected fringe patterns that such an interferometer would produce, considering all major sources of decoherence. We discuss the practical challenges involved in building such an experiment as well as some preliminary experimental results to illustrate the proposed measurement scheme. We show that such a design is suitable for seeing interference fringes with $10^6$amu SiO$_2$ particles, and that this design can be extended to even $10^8$amu particles by using flight times below the typical Talbot time of the system.

Generating macroscopic non-classical quantum states is a long-standing challenge in physics. Anharmonic dynamics is an essential ingredient to generate these states, but for large mechanical systems, the effect of the anharmonicity tends to become negligible compared to decoherence. As a possible solution to this challenge, we propose to use a motional squeezed state as a resource to effectively enhance the anharmonicity. We analyze the production of negativity in the Wigner distribution of a quantum anharmonic resonator initially in a squeezed state. We find that initial squeezing enhances the rate at which negativity is generated. We also analyze the effect of two common sources of decoherence, namely energy damping and dephasing, and find that the detrimental effects of energy damping are suppressed by strong squeezing. In the limit of large squeezing, which is needed for state-of-the-art systems, we find good approximations for the Wigner function. Our analysis is significant for current experiments attempting to prepare macroscopic mechanical systems in genuine quantum states. We provide an overview of several experimental platforms featuring nonlinear behaviors and low levels of decoherence. In particular, we discuss the feasibility of our proposal with carbon nanotubes and levitated nanoparticles.

For convex and sequential effect algebras, we study spectrality in the sense of Foulis. We show that under additional conditions (strong archimedeanity, closedness in norm and a certain monotonicity property of the sequential product), such effect algebra is spectral if and only if every maximal commutative subalgebra is monotone $\sigma$-complete. Two previous results on existence of spectral resolutions in this setting are shown to require stronger assumptions.

A source assumed to prepare a specified reference state sometimes prepares an anomalous one. We address the task of identifying these anomalous states in a series of $n$ preparations with $k$ anomalies. We analyse the minimum-error protocol and the zero-error (unambiguous) protocol and obtain closed expressions for the success probability when both reference and anomalous states are known to the observer and anomalies can appear equally likely in any position of the preparation series. We find the solution using results from association schemes theory. In particular we use the Johnson association scheme which arises naturally from the Gram matrix of this problem. We also study the regime of large $n$ and obtain the expression of the success probability that is non-vanishing. Finally, we address the case in which the observer is blind to the reference and the anomalous states. This scenario requires an universal protocol for which we prove that in the asymptotic limit the success probability correspond to average of the known state scenario.

Quantum state readout serves as the cornerstone of quantum information processing, exerting profound influence on quantum communication, computation, and metrology. In this study, we introduce an innovative readout architecture called Compression Shadow (CompShadow), which transforms the conventional readout paradigm by compressing multi-qubit states into single-qubit shadows before measurement. Compared to direct measurements of the initial quantum states, CompShadow achieves comparable accuracy in amplitude and observable expectation estimation while consuming similar measurement resources. Furthermore, its implementation on near-term quantum hardware with nearest-neighbor coupling architectures is straightforward. Significantly, CompShadow brings forth novel features, including the complete suppression of correlated readout noise, fundamentally reducing the quantum hardware demands for readout. It also facilitates the exploration of multi-body system properties through single-qubit probes and opens the door to designing quantum communication protocols with exponential loss suppression. Our findings mark the emergence of a new era in quantum state readout, setting the stage for a revolutionary leap in quantum information processing capabilities.

While it has become widely appreciated that defining (higher) gauge theories requires, in addition to ordinary phase space data, also "flux quantization" laws in generalized differential cohomology, there has been little discussion of the general rules, if any, for lifting Poisson-brackets of (flux-)observables and their quantization from traditional phase spaces to the resulting higher moduli stacks of flux-quantized gauge fields.

In this short note, we present a systematic analysis of (i) the canonical quantization of flux observables in Yang-Mills theory and (ii) of valid flux quantization laws in abelian Yang-Mills, observing (iii) that the resulting topological quantum observables form the homology Pontrjagin algebra of the loop space of the moduli space of flux-quantized gauge fields.

This is remarkable because the homology Ponrjagin algebra on loops of moduli makes immediate sense in broad generality for higher and non-abelian (non-linearly coupled) gauge fields, such as for the C-field in 11d supergravity, where it recovers the quantum effects previously discussed in the context of "Hypothesis H".

Cavity quantum electrodynamics (CQED) and its extensions are widely used for the description of exciton-polariton systems. However, the exciton-polariton models based on CQED vary greatly within different contexts. One of the most significant discrepancies among these CQED models is whether one should include direct intermolecular interactions in the CQED Hamiltonian. To answer this question, in this article, we derive an effective dissipative CQED model including free-space dipole-dipole interactions (CQED-DDI) from a microscopic Hamiltonian based on macroscopic quantum electrodynamics. Dissipative CQED-DDI successfully captures the nature of vacuum fluctuations in dielectric media and separates it into the free-space effects and the dielectric-induced effects. The former include spontaneous emissions, dephasings and dipole-dipole interactions in free space; the latter include exciton-polariton interactions and photonic losses due to dielectric media. We apply dissipative CQED-DDI to investigate the exciton-polariton dynamics (the population dynamics of molecules above a plasmonic surface) and compare the results with those based on the methods proposed by several previous studies. We find that direct intermolecular interactions are a crucial element when employing CQED-like models to study exciton-polariton systems involving multiple molecules.