QUROPE Quantum Information Processing and Communication in Europe

Magnetic domain walls (DWs) are topological defects that exhibit robust low-energy modes that can be harnessed for classical and neuromorphic computing. However, the quantum nature of these modes has been elusive thus far. Using the language of cavity optomechanics, we show how to exploit a geometric Berry-phase interaction between the localized DWs and the extended magnons in short ferromagnetic insulating wires to efficiently cool the DW to its quantum ground state or to prepare nonclassical states exhibiting a negative Wigner function that can be extracted from the power spectrum of the emitted magnons. Moreover, we demonstrate that magnons can mediate long-range entangling interactions between qubits stored in distant DWs, which could facilitate the implementation of a universal set of quantum gates. Our proposal relies only on the intrinsic degrees of freedom of the ferromagnet, and can be naturally extended to explore the quantum dynamics of DWs in ferrimagnets and antiferromagnets, as well as quantum vortices or skyrmions confined in insulating magnetic nanodisks.

Glassy dynamics have time-reparametrization `softness': glasses fluctuate, and respond to external perturbations, primarily by changing the pace of their evolution. Remarkably, the same situation also appears in toy models of quantum field theory such as the Sachdev-Ye-Kitaev (SYK) model, where the excitations associated to reparametrizations play the role of an emerging `gravity'. I describe here how these two seemingly unrelated systems share common features, arising from a technically very similar origin. This connection is particularly close between glassy dynamics and supersymmetric variants of the SYK model, which I discuss in some detail. Apart from the curiosity that this correspondence naturally arouses, there is also the hope that developments in each field may be useful for the other.

Based on the thermal phase structure of pure SU(2) quantum Yang-Mills theory, we describe the electron at rest as an extended particle, a so-called blob of radius $r_0$ which is comparable to the Bohr radius $a_0$. This blob is of vanishing pressure and traps an electric-magnetic dually interpreted BPS monopole within its bulk at a temperature of $T_0=7.95$ keV. Utilizing a spherical mirror-charge construction, we approximate the blob's charge at a value of the electromagnetic fine-structure constant $\alpha$ of $\alpha^{-1}\sim 134$ for soft external probes. It is shown that the blob does not exhibit an electric dipole or quadrupole moment. We also calculate the mixing angle $\theta_{\rm W}\sim 30^{\circ}$ belonging to the deconfining phases of two SU(2) gauge theories of very distinct Yang-Mills scales ($\Lambda_{\rm e}=3.6 $ keV and $\Lambda_{\rm CMB}\sim 10^{-4} $eV) which establish the blob's stable bulk thermodynamics. The core radius of the monopole is about 1 % of $r_0$.

Neutrino oscillation experiments show that neutrinos have mass; however, the absolute mass scale is exceedingly difficult to measure and is currently unknown. A promising approach is to measure the energies of the electrons released during the radioactive decay of tritium. The energies of interest are within a few eV of the 18.6 keV end point, and so are mildly relativistic. By capturing the electrons in a static magnetic field and measuring the frequency of the cyclotron radiation emitted the initial energy can be determined, but end-point events are infrequent, the observing times short, and the signal to noise ratios low. To achieve a resolution of $<$ 10 meV, single-electron emission spectra need to be recorded over large fields of view with highly sensitive receivers. The principles of Cylotron Radiation Emission Spectroscopy (CRES) have already been demonstrated by Project 8, and now there is considerable interest in increasing the FoV to $>$ 0.1 m$^3$. We consider a range of issues relating to the design and optimisation of inward-looking quantum-noise-limited microwave receivers for single-electron CRES, and present a single framework for understanding signal, noise and system-level behaviour. Whilst there is a great deal of literature relating to the design of outward-looking phased arrays for applications such as radar and telecommunications, there is very little coverage of the new issues that come into play when designing ultra-sensitive inward-looking phased arrays for volumetric spectroscopy and imaging.

The experimentally-observed non-trivial electronic structure of Cr$_2$ dimer has made the calculation of the potential energy curve a theoretical challenge in the last decades. By matching the perturbation theory at small internuclear distances $R$, the multipole expansion at large distances $R$ (supposedly both of asymptotic nature) and by adding a few RKR turning points, extracted from experimental data, the analytic form for the potential energy curve for the ground state $X^1\Sigma^+$ of the Cr$_2$ dimer is found for the whole range of internuclear distances $R$. This has the form of a two-point Pade approximant and provides an accuracy of 3-4 decimal digits in 29 experimental vibrational energies. The resulting ground state $X^1\Sigma^+$ potential curve supports 19694 rovibrational states with a maximal vibrational number $\nu_\text{max}=104$ at zero angular momentum and with a maximal angular momentum $L_\text{max}=312$ with energies $> 10^{-4}$ Hartree, and additionally 218 weakly-bound states (close to the dissociation limit) with energies $< 10^{-4}$ Hartree.

A nearly-integrable isolated quantum many-body system reaches a quasi-stationary prethermal state before a late thermalization. Here, we revisit a particular example in the settings of an open quantum system. We consider a collection of non-interacting atoms coupled to a spatially correlated bosonic bath characterized by a bath correlation length. Our result implies that the integrability of the system depends on such a correlation length. If this length is much larger than the distance between the atoms, such a system behaves as a nearly integrable open quantum system. We study the properties of the emerging prethermal state for this case, i.e., the state's lifetime, the extensive numbers of existing quasi-conserved quantities, the emergence of the generalized Gibbs state, and the scaling of von Neumann entropy, etc. We find that for the prethermal state, the maximum growth of entropy is logarithmic with the number of atoms, whereas such growth is linear for the final steady state, which is the Gibbs state in this case. Finally, we discuss how such prethermal states can have significant applications in quantum entanglement storage devices.

The interaction between Unruh-De Witt spin $1/2$ detectors and a real scalar field is scrutinized by making use of the Tomita-Takesaki modular theory as applied to the Von Neumann algebra of the Weyl operators. The use of the modular theory enables to evaluate in an exact way the trace over the quantum field degrees of freedom. The resulting density matrix is employed to the study of the Bell-CHSH correlator. It turns out that, as a consequence of the interaction with the quantum field, the violation of the Bell-CHSH inequality exhibits a decreasing as compared to the case in which the scalar field is absent.

The emerging technology of quantum reservoir computing (QRC) stands out in the noisy-intermediate scale quantum era (NISQ) for its exceptional efficiency and adaptability. By harnessing the power of quantum computing, it holds a great potential to untangle complex economic markets, as demonstrated here in an application to food price crisis prediction - a critical effort in combating food waste and establishing sustainable food chains. Nevertheless, a pivotal consideration for its success is the optimal design of the quantum reservoirs, ensuring both high performance and compatibility with current devices. In this paper, we provide an efficient criterion for that purpose, based on the complexity of the reservoirs. Our results emphasize the crucial role of optimal design in the algorithm performance, especially in the absence of external regressor variables, showcasing the potential for novel insights and transformative applications in the field of time series prediction using quantum computing.

Multipartite entanglement is a crucial resource for quantum computing, communication, and metrology. However, detecting this resource can be challenging: for genuine multipartite entanglement it may require global measurements that are hard to implement experimentally. In this study, we introduce the concept of entanglement detection length, defined as the minimum length of observables required to detect genuine multipartite entanglement. We characterize the entanglement detection length for various types of genuinely entangled states, including GHZ-like states, Dicke states, and graph states. We also present a classification of genuinely entangled states based on entanglement detection length. Furthermore, we demonstrate that the entanglement detection length differs from the minimum length of observables needed to uniquely determine a genuinely entangled state. Our findings are valuable for minimizing the number of observables that must be measured in entanglement detection.

Time delay in a projectile-target scattering is a fundamental tool in understanding their interactions by probing the temporal domain. The present study focuses on computing and analyzing the Eisenbud-Wigner-Smith (EWS) time delay in low energy elastic e C60 scattering. The investigation is carried out in the framework of a non-relativistic partial wave analysis (PWA) technique. The projectile-target interaction is described in (1) Density Functional Theory (DFT) and (2) Annular Square Well (ASW) static model, and their final results are compared in details. The impact of polarization on resonant and non-resonant time delay is also investigated.

Quantum entanglement plays a pivotal role in various quantum information processing tasks. However, there still lacks a universal and effective way to detecting entanglement structures, especially for high-dimensional and multipartite quantum systems. Noticing the mathematical similarities between the common representations of many-body quantum states and the data structures of images, we are inspired to employ advanced computer vision technologies for data analysis. In this work, we propose a hybrid CNN-Transformer model for both the classification of GHZ and W states and the detection of various entanglement structures. By leveraging the feature extraction capabilities of CNNs and the powerful modeling abilities of Transformers, we can not only effectively reduce the time and computational resources required for the training process but also obtain high detection accuracies. Through numerical simulation and physical verification, it is confirmed that our hybrid model is more effective than traditional techniques and thus offers a powerful tool for independent detection of multipartite entanglement.

Taking into account the importance of the unified theory of quantum mechanics and gravity, and the existence of a minimum length of the order of the Planck scale, we consider a modified Schr\"odinger equation resulting from a generalised uncertainty principle, which finds applications from the realm of quantum information to large-scale physics, with a quantum mechanically corrected gravitational interaction proposed very recently. As the resulting equation cannot be solved by common exact approaches, including Heun or Lie algebraic ones, we propose a Bethe-Ansatz approach, which will be applied and whose results we discuss.

This paper explores the potential contribution of quantum computing, specifically the Variational Quantum Eigensolver (VQE), into atmospheric physics research and application problems using as an example the Lorenz system, a paradigm of chaotic behavior in atmospheric dynamics. Traditionally, the complexity and non-linearity of atmospheric systems have presented significant computational challenges. However, the advent of quantum computing, and in particular the VQE algorithm, offers a novel approach to these problems. The VQE, known for its efficiency in quantum chemistry for determining ground state energies, is adapted in our study to analyze the non-Hermitian Jacobian matrix of the Lorenz system. We employ a method of Hermitianization and dimensionality augmentation to make the Jacobian amenable to quantum computational techniques. This study demonstrates the application of VQE in calculating the eigenvalues of the Lorenz system's Jacobian, thus providing insights into the system's stability at various equilibrium points. Our results reveal the VQE's potential in addressing complex systems in atmospheric physics. Furthermore, we discuss the broader implications of VQE in handling non-Hermitian matrices, extending its utility to operations like diagonalization and Singular Value Decomposition (SVD), thereby highlighting its versatility across various scientific fields. This research extends beyond the realm of chaotic systems in atmospheric physics, underscoring the significant potential of quantum computing to tackle complex, real-world challenges.

Bell's theorem states that quantum mechanical description on physical quantity cannot be fully explained by local realistic theories, and lays solid basis for various quantum information applications. Hardy's paradox is celebrated to be the simplest form of Bell's theorem concerning its "All versus Nothing" way to test local realism. However, due to experimental imperfections, existing tests of Hardy's paradox require additional assumptions of experimental systems, which constitute potential loopholes for faithfully testing local realistic theories. Here, we experimentally demonstrate Hardy's nonlocality through a photonic entanglement source. By achieving a detection efficiency of $82.2\%$, a quantum state fidelity of $99.10\%$ and applying high speed quantum random number generators for measurement setting switching, the experiment is implemented in a loophole-free manner. During $6$ hours of running, a strong violation of $P_{\text{Hardy}}=4.646\times 10^{-4}$ up to $5$ standard deviations is observed with $4.32\times 10^{9}$ trials. A null hypothesis test shows that the results can be explained by local realistic theories with an upper bound probability of $10^{-16348}$. These testing results present affirmative evidence against local realism, and provide an advancing benchmark for quantum information applications based on Hardy's paradox.

We show that a Kirkwood-Dirac type quasiprobability distribution is sufficient to reveal any arbitrary quantum resource. This is achieved by demonstrating that it is always possible to identify a set of incompatible measurements that distinguishes between resourceful states and nonresourceful states. The quasiprobability reveals a resourceful quantum state by having at least one quasiprobabilty outcome with a strictly negative numerical value. We also show that there always exists a quasiprobabilty distribution where the total negativity can be interpreted as the geometric distance between a resourceful quantum state to the closest nonresourceful state. It can also be shown that Kirkwood-Dirac type quasiprobability distributions, like the Wigner distribution, can be made informationally complete, in the sense that it can provide complete information about the quantum state while simultaneously revealing nonclassicality whenever a quasiprobability outcome is negative. Moreover, we demonstrate the existence of sufficiently strong anomalous weak values whenever the quasiprobability distribution is negative, which suggests a means to experimentally test such quasiprobability distributions. Since incompatible measurements are necessary in order for the quasiprobability to be negative, this result suggests that measurement incompatibility may underlie any quantum advantage gained from utilizing a nonclassical quantum resource

In this paper, we consider the real-time parameter estimation problem for a ZZ-coupled system composed of two qubits in the presence of spontaneous emission. To enhance the estimation precision of the coupling coefficient, we first propose two different control schemes, where the first one is feedback control based on quantum-jump detection, and the second one is hybrid control combining Markovian feedback and Hamiltonian control. The simulation results show that compared with free evolution, both control schemes can improve parameter precision and extend system coherence time. Next, on the basis of the two control schemes, we propose a practical single-parameter quantum recovery protocol based on Bayesian estimation theory. In this protocol, by employing batch-style adaptive measurement rules, parameter recovery is conducted to verify the effectiveness of both control schemes.

The need set by a computational industry to increase processing power, while simultaneously reducing the energy consumption of data centers became a challenge for modern computational systems. In this work, we propose an optical communication solution, that could serve as a building block for future computing systems, due to its versatility. The solution arises from Landauer principle and utilizes reversible logic, manifested as an optical logical gate with structured light, here represented as Laguerre-Gaussian modes. We introduced an information encoding technique that employs phase shift as an information carrier and incorporates multi-valued logic in the form of a ternary system. In the experimental validation, the free space communication protocol is implemented to determine the similarity between two images. Obtained results are compared with their binary counterparts, illustrating denser information capacity and enhanced information security, which underscores its capability to transmit and process both quantum and classical information.

Experiments have demonstrated that vibrational strong coupling between molecular vibrations and light modes can significantly change molecular properties, such as ground-state reactivity. Theoretical studies towards the origin of this exciting observation can roughly be divided in two categories, with studies based on Hamiltonians that simply couple a molecule to a cavity mode via its ground-state dipole moment on the one hand, and on the other hand ab initio calculations that self-consistently include the effect of the cavity mode on the electronic ground state within the cavity Born-Oppenheimer (CBO) approximation; these approaches are not equivalent. The CBO approach is more rigorous, but unfortunately it requires the rewriting of electronic-structure code, and gives little physical insight. In this work, we exploit the relation between the two approaches and demonstrate on a real molecule (hydrogen fluoride) that for realistic coupling strengths, we can recover CBO energies and spectra to high accuracy using only out-of-cavity quantities from standard electronic-structure calculations. In doing so, we discover what the physical effects underlying the CBO results are. Our methodology can aid in incorporating more, possibly important features in models, play a pivotal role in demystifying CBO results and provide a practical and efficient alternative to full CBO calculations.

The unique property of tantalum (Ta), particularly its long coherent lifetime in superconducting qubits and its exceptional resistance to both acid and alkali, makes it promising for superconducting quantum processors. It is a notable advantage to achieve high-performance quantum processors with neat and unified fabrication of all circuit elements, including coplanar waveguides (CPW), qubits, and airbridges, on the tantalum film-based platform. Here, we propose a reliable tantalum airbridges with separate or fully-capped structure fabricated via a novel lift-off method, where a barrier layer with aluminium (Al) film is first introduced to separate two layers of photoresist and then etched away before the deposition of tantalum film, followed by cleaning with piranha solution to remove the residual photoresist on the chip. We characterize such tantalum airbridges as the control line jumpers, the ground plane crossovers and even coupling elements. They exhibit excellent connectivity, minimal capacitive loss, effectively suppress microwave and flux crosstalk and offer high freedom of coupling. Besides, by presenting a surface-13 tunable coupling superconducting quantum processor with median $T_1$ reaching above 100 $\mu$s, the overall adaptability of tantalum airbridges is verified. The median single-qubit gate fidelity shows a tiny decrease from about 99.95% for the isolated Randomized Benchmarking to 99.94% for the simultaneous one. This fabrication method, compatible with all known superconducting materials, requires mild conditions of film deposition compared with the commonly used etching and grayscale lithography. Meanwhile, the experimental achievement of non-local coupling with controlled-Z (CZ) gate fidelity exceeding 99.2% may further facilitate qLDPC codes, laying a foundation for scalable quantum computation and quantum error correction with entirely tantalum elements.

In this work we study five Grovers algorithm modifications, where each iteration is constructed by two generalized Householder reflections, against inaccuracies in the phases. By using semi-empirical methods, we investigate various characteristics of the dependence between the probability to find solution and the phase errors. The first of them is the robustness of the probability to errors in the phase. The second one is how quickly the probability falls beyond the stability interval. And finally, the average success rate of the algorithm when the parameters are in the range of the highly robust interval. Two of the modifications require usage of the same Grover operator each iteration and in the other three it differs. Those semi-empirical methods give us the, tool to make prediction of the quantum algorithm modifications overall behavior and compare them for even larger register size