QUROPE Quantum Information Processing and Communication in Europe

As quantum computing develops, the problem of implementing entangling and disentangling quantum gates in a controllable manner reemerges in multiple contexts. One of the newest applications of such disentangling channels are quantum convolutional neural networks, where the core idea lies in the systematic decrease of qudit numbers without loss of information encoded in entangled states.

In this work, we focus on quantum analogues of convolution and pooling - basic building block for convolutional networks - and construct and characterize parametrizable ``quantum convolution'' channels as coherifications of permutation tensors. Operations constructed in this manner generically provide high (dis)entangling power. In particular, we identify conditions necessary for the convolution channels constructed using our method to possess maximal entangling power. Based on this, we establish new, continuous classes of bipartite 2-unitary matrices of dimension $d^2$ for $d = 7$ and $d = 9$, with $2$ and $4$ free nonlocal parameters, corresponding to perfect tensors of rank $4$ or $4$-partite absolutely maximally entangled states. The newly established families may serve as the prototype for trainable convolution/pooling layers in quantum convolutional neural networks.

Understanding the dynamics of bound state formation is one of the fundamental questions in confining quantum field theories such as Quantum Chromodynamics (QCD). One hadronization mechanism that has garnered significant attention is the breaking of a string initially connecting a fermion and an anti-fermion. Deepening our understanding of real-time string-breaking dynamics with simpler, lower dimensional models like the Schwinger model can improve our understanding of the hadronization process in QCD and other confining systems found in condensed matter and statistical systems. In this paper, we consider the string-breaking dynamics within the Schwinger model and investigate its modification inside a thermal medium, treating the Schwinger model as an open quantum system coupled to a thermal environment. Within the regime of weak coupling between the system and environment, the real-time evolution of the system can be described by a Lindblad evolution equation. We analyze the Liouvillian gaps of this Lindblad equation and the time dependence of the system's von Neumann entropy. We observe that the late-time relaxation rate decreases as the environment correlation length increases. Moreover, when the environment correlation length is infinite, the system exhibits two steady states, one in each of the sectors with definite charge-conjugation-parity (CP) quantum numbers. For parameter regimes where an initial string breaks in vacuum, we observe a delay of the string breaking in the medium, due to kinetic dissipation effects. Conversely, in regimes where an initial string remains intact in vacuum time evolution, we observe string breaking (melting) in the thermal medium. We further discuss how the Liouvillian dynamics of the open Schwinger model can be simulated on quantum computers and provide an estimate of the associated Trotter errors.

Diversity of interpretations of quantum mechanics is often considered as a sign of foundational crisis. In this note we proceed towards unification the relational quantum mechanics of Rovelli, Bohmian mechanics, and many worlds interpretation on the basis so called Dendrogramic Holographic Theory (DHT). DHT is based on the representation of observed events by dendrograms (finite trees) presenting observers subjective image of universe. Dendrograms encode the relational hierarchy between events, in applications they are generated by clustering algorithms; an algorithm with the branching index p >1 generate p-adic trees. The infinite p-adic tree represents the ontic event universe. We consider an ensemble of observers performing observations on each other and representing them by p-adic trees. In such observers universe we introduce a kind of Minkowski space structure, which is statistical by its nature. This model unites the observer/system discrepancy. Measurements are performed by observers on observers. Such observers universe is dynamically changing and is background independent since the space itself is emergent. And within this model, we unify the aforementioned interpretations.

We apply the concepts of classical and quantum channels to the modeling of opinion dynamics and propose a stochastic method for representing the temporal variation of individual and group opinions. In particular, we use quantum Galois noise channels to couple quantum information theory with social interaction to construct a new model of opinion dynamics that accounts for error rates and noise effects. This model captures more complex opinion propagation and interaction by incorporating the concepts of partial traces and entanglement. We also consider the role of superspreaders in the propagation of noisy information and their suppression mechanisms, and represent these dynamics in a mathematical model. We model the influence of superspreaders on interactions between individuals using unitary transformations and propose a new approach to measure social trustworthiness. In addition, we elaborate on the modeling of opinion propagation and suppression using Holevo channels. These models provide a new framework for a better understanding of social interactions and expand the potential applications of quantum information theory.

Defect detection is one of the most important yet challenging tasks in the quality control stage in the manufacturing sector. In this work, we introduce a Tensor Convolutional Neural Network (T-CNN) and examine its performance on a real defect detection application in one of the components of the ultrasonic sensors produced at Robert Bosch's manufacturing plants. Our quantum-inspired T-CNN operates on a reduced model parameter space to substantially improve the training speed and performance of an equivalent CNN model without sacrificing accuracy. More specifically, we demonstrate how T-CNNs are able to reach the same performance as classical CNNs as measured by quality metrics, with up to fifteen times fewer parameters and 4% to 19% faster training times. Our results demonstrate that the T-CNN greatly outperforms the results of traditional human visual inspection, providing value in a current real application in manufacturing.

Dempster-Shafer Theory (DST) as an effective and robust framework for handling uncertain information is applied in decision-making and pattern classification. Unfortunately, its real-time application is limited by the exponential computational complexity. People attempt to address the issue by taking advantage of its mathematical consistency with quantum computing to implement DST operations on quantum circuits and realize speedup. However, the progress so far is still impractical for supporting large-scale DST applications. In this paper, we find that Boolean algebra as an essential mathematical tool bridges the definition of DST and quantum computing. Based on the discovery, we establish a flexible framework mapping any set-theoretically defined DST operations to corresponding quantum circuits for implementation. More critically, this new framework is not only uniform but also enables exponential acceleration for computation and is capable of handling complex applications. Focusing on tasks of classification, we based on a classical attribute fusion algorithm putting forward a quantum evidential classifier, where quantum mass functions for attributes are generated with a simple method and the proposed framework is applied for fusing the attribute evidence. Compared to previous methods, the proposed quantum classifier exponentially reduces the computational complexity to linear. Tests on real datasets validate the feasibility.

In this work, we use the term ``quantum chaos'' to refer to spectral correlations similar to those found in random matrix theory. Quantum chaos can be diagnosed through the analysis of level statistics using the spectral form factor, which detects both short- and long-range level correlations. The spectral form factor corresponds to the Fourier transform of the two-point spectral correlation function and exhibits a typical slope-dip-ramp-plateau structure (aka correlation hole) when the system is chaotic. We discuss how this structure could be detected through the dynamics of two physical quantities accessible to experimental many-body quantum systems: the survival probability and the spin autocorrelation function. When the system is small, the dip reaches values that are large enough at times which are short enough to be detected with current experimental platforms and commercially available quantum computers.

We introduce the entangled quantum polynomial hierarchy $\mathsf{QEPH}$ as the class of problems that are efficiently verifiable given alternating quantum proofs that may be entangled with each other. We prove $\mathsf{QEPH}$ collapses to its second level. In fact, we show that a polynomial number of alternations collapses to just two. As a consequence, $\mathsf{QEPH} = \mathsf{QRG(1)}$, the class of problems having one-turn quantum refereed games, which is known to be contained in $\mathsf{PSPACE}$. This is in contrast to the unentangled quantum polynomial hierarchy $\mathsf{QPH}$, which contains $\mathsf{QMA(2)}$.

We also introduce a generalization of the quantum-classical polynomial hierarchy $\mathsf{QCPH}$ where the provers send probability distributions over strings (instead of strings) and denote it by $\mathsf{DistributionQCPH}$. Conceptually, this class is intermediate between $\mathsf{QCPH}$ and $\mathsf{QPH}$. We prove $\mathsf{DistributionQCPH} = \mathsf{QCPH}$, suggesting that only quantum superposition (not classical probability) increases the computational power of these hierarchies. To prove this equality, we generalize a game-theoretic result of Lipton and Young (1994) which says that the provers can send distributions that are uniform over a polynomial-size support. We also prove the analogous result for the polynomial hierarchy, i.e., $\mathsf{DistributionPH} = \mathsf{PH}$. These results also rule out certain approaches for showing $\mathsf{QPH}$ collapses.

Finally, we show that $\mathsf{PH}$ and $\mathsf{QCPH}$ are contained in $\mathsf{QPH}$, resolving an open question of Gharibian et al. (2022).

Characterizing the indistinguishability of photons is a key task in quantum photonics, underpinning the tuning and stabilization of the photon sources and thereby increasing the accuracy of quantum operations. The protocols for measuring the degree of indistinguishability conventionally require photon-coincidence measurements at several different time or phase delays, which is a fundamental bottleneck towards the fast measurements and real-time monitoring of indistinguishability. Here, we develop a static dielectric metasurface grating without any reconfigurable elements that enables single-shot characterization of the indistinguishability between two photons in multiple degrees of freedom including time, spectrum, spatial modes, and polarization. Topology optimization is employed to design a silicon metasurface with polarization independence, high transmission, and high tolerance to measurement noise. We fabricate the metasurface and experimentally quantify the indistinguishability of photons in the time domain with fidelity over 98.4%. We anticipate that the developed framework based on ultrathin metasurfaces can be further extended for multi-photon states and additional degrees of freedom associated with spatial modalities.

Optical quantum routers which play a crucial role in quantum networks, have been extensively studied in both theory and experiment, resulting in significant advancements in their performance. However, these routers impose stringent requirements for achieving optimal routing performance, where the incident photon frequency must be in strict resonance with one or several specific frequencies. To address this challenge, we have designed an efficient quantum router capable of stable output with 100\% transfer rate over the entire energy band of coupled-resonator waveguide (CRW) by coupling a giant atom to two or more semi-infinite CRWs. We also explain and prove the fundamental physical mechanism behind this distinctive phenomenon as the result of destructive interference between two waves composing the final reflected wave. We hope that quantum router with output results unaffected by the energy of the incoming information carriers present a more reliable solution for the implementation of quantum networks.

Thouless pumping, a dynamical version of the integer quantum Hall effect, represents the quantized charge pumped during an adiabatic cyclic evolution. Here we report experimental observations of nontrivial topological pumping that is induced by disorder even during a topologically trivial pumping trajectory. With a 41-qubit superconducting quantum processor, we develop a Floquet engineering technique to realize cycles of adiabatic pumping by simultaneously varying the on-site potentials and the hopping couplings. We demonstrate Thouless pumping in the presence of disorder and show its breakdown as the strength of disorder increases. Moreover, we observe two types of topological pumping that are induced by on-site potential disorder and hopping disorder, respectively. Especially, an intrinsic topological pump that is induced by quasi-periodic hopping disorder has never been experimentally realized before. Our highly controllable system provides a valuable quantum simulating platform for studying various aspects of topological physics in the presence of disorder.

The goal of this Article is to perform a systematic study the global entanglement and coherence length dynamics in a natural light-harvesting system Fenna-Matthews-Olson (FMO) complex across various parameters of a dissipative environment from low to high temperatures, weak to strong system-environment coupling, and non-Markovian environments. The non-perturbative numerically exact hierarchical equations of motions method is employed to generate the dynamics of the system. We found that entanglement is driven primarily by the strength of interaction between the system and environment, and it is modulated by the interplay between temperature and non-Markovianity. In contrast, coherence length is found not to be sensitive to non-Markovianity. Our results do not show the direct correlation between global entanglement and the efficiency of the excitation energy transfer.

The quantum SearchRank algorithm is a promising tool for a future quantum search engine based on PageRank quantization. However, this algorithm loses its functionality when the $N/M$ ratio between the network size $N$ and the number of marked nodes $M$ is sufficiently large. We propose a modification of the algorithm, replacing the underlying Szegedy quantum walk with a semiclassical walk. To maintain the same time complexity as the quantum SearchRank algorithm we propose a simplification of the algorithm. This new algorithm is called Randomized SearchRank, since it corresponds to a quantum walk over a randomized mixed state. The performance of the SearchRank algorithms is first analyzed on an example network, and then statistically on a set of different networks of increasing size and different number of marked nodes. On the one hand, to test the search ability of the algorithms, it is computed how the probability of measuring the marked nodes decreases with $N/M$ for the quantum SearchRank, but remarkably it remains at a high value around $0.9$ for our semiclassical algorithms, solving the quantum SearchRank problem. The time complexity of the algorithms is also analyzed, obtaining a quadratic speedup with respect to the classical ones. On the other hand, the ranking functionality of the algorithms has been investigated, obtaining a good agreement with the classical PageRank distribution. Finally, the dependence of these algorithms on the intrinsic PageRank damping parameter has been clarified. Our results suggest that this parameter should be below a threshold so that the execution time does not increase drastically.

Identifying a reasonably small Hilbert space that completely describes an unknown quantum state is crucial for efficient quantum information processing. We introduce a general dimension-certification protocol for both discrete and continuous variables that is fully evidence-based, relying solely on the experimental data collected and no other assumptions whatsoever. Using the Bayesian concept of relative belief, we take the effective dimension of the state as the smallest one such that the posterior probability is larger than the prior, as dictated by the data. The posterior probabilities associated with the relative-belief ratios measure the strength of the evidence provide by these ratios so that we can assess whether there is weak or strong evidence in favor or against a particular dimension. Using experimental data from spectral-temporal and polarimetry measurements, we demonstrate how to correctly assign Bayesian plausible error bars for the obtained effective dimensions. This makes relative belief a conservative and easy-to-use model-selection method for any experiment.

Bayesian networks are powerful tools for probabilistic analysis and have been widely used in machine learning and data science. Unlike the parameters learning mode of neural networks, Bayes classifiers only use sample features to determine the classification results without a time-consuming training process. We study the construction of quantum Bayes classifiers (QBCs) and design a naive QBC and three semi-naive QBCs (SN-QBCs). These QBCs are applied to image classification. A local features sampling method is employed to extract a limited number of features from images to reduce the computational complexity. These features are then used to construct Bayesian networks and generate QBCs. We simulate these QBCs on the MindQuantum quantum platform and test them on the MNIST and Fashion-MNIST datasets. Results show that these QBCs based on a limited number of features exhibit good classification accuracies. The classification accuracies of QBCs on the MNIST dataset surpass that of the classical Bayesian network and quantum neural networks that utilize all feature points.

Optimized blockade is an efficient tool in generating a single-magnon state, that is fundamental to manipulate the magnonic systems at the quantum level. In this study, we consider a hybrid system in which a qubit is strongly coupled to $N$ magnons via the exchange interaction. The qubit and the magnon modes are subject to the probing field and driving fields, respectively. It is interesting to find the scalable conditions in minimizing the equal-time second-order correlation function $g^{(2)}(0)$ for each magnon with respect to $N$. In particular, the simultaneous blockade is optimized when (i) the detuning between the qubit (magnon) and the probing (driving field) field is $\sqrt{N}$ times the magnon-qubit coupling strength, (ii) the probing intensity is $3\sqrt{N}$ times the driving intensity, and (iii) the relative phase between probing and driving fields is $2/(3\sqrt{N}$) times the ratio of the system decay rate to the magnon-qubit coupling strength. More than a high-degree blockade, we can generate a significant population on the single-magnon state. With experimental-relevant driving intensity and decay rate, the correlation function can achieve about $g^{(2)}(0)\sim10^{-7}$ in company with a large single-magnon population $P_1\sim0.24$ when $N=1$ and $g^{(2)}(0)\sim10^{-7}$ with $P_1\sim0.12$ when $N=2$.

We study single-photon scattering spectra of a giant atom chirally coupled to a one-dimensional waveguide at multiple connection points, and examine chirality induced effects in the scattering spectra by engineering the chirality of the coupling strengths. We show that the transmission spectra typically possess an anti-Lorentzian lineshape with a nonzero minimum, but when the chirality satisfies some specific conditions independent of the number of coupling points, the transmission spectrum of an incident photon can undergo a transition from complete transmission to total reflection at multiple frequency ``windows'', the width of which can be flexibly tuned in situ by engineering the coupling strengths of a certain disordered coupling point. Moreover, we show that a perfect nonreciprocal photon scattering can be achieved due to the interplay between internal atomic spontaneous emission and the chirally external decay to the waveguide, in contrast to that induced by the non-Markovian retardation effect. We also consider the non-Markovian retardation effect on the scattering spectra, which allows for a photonic band gap even with only two chiral coupling points. The giant-atom-waveguide system with chiral coupling is a promising candidate for realizing single-photon routers with multiple channels.

In the context of measurement-induced entanglement phase transitions, the influence of quantum noises, which are inherent in real physical systems, is of great importance and experimental relevance. In this Letter, we present a comprehensive theoretical analysis of the effects of both temporally uncorrelated and correlated quantum noises on entanglement generation and information protection. This investigation reveals that entanglement within the system follows $q^{-1/3}$ scaling for both types of quantum noises, where $q$ represents the noise probability. The scaling arises from the Kardar-Parisi-Zhang fluctuation with effective length scale $L_{\text{eff}} \sim q^{-1}$. Moreover, the timescales of information protection are explored and shown to follow $q^{-1/2}$ and $q^{-2/3}$ scaling for temporally uncorrelated and correlated noises, respectively. The former scaling can be interpreted as a Hayden-Preskill protocol, while the latter is a direct consequence of Kardar-Parisi-Zhang fluctuations. We conduct extensive numerical simulations using stabilizer formalism to support the theoretical understanding. This Letter not only contributes to a deeper understanding of the interplay between quantum noises and measurement-induced phase transition but also provides a new perspective to understand the effects of Markovian and non-Markovian noises on quantum computation.

Sensors play a crucial role in advanced apparatuses and it is persistently pursued to improve their sensitivities. Recently, the singularity of a non-Hermitian system, known as the exceptional point (EP), has drawn much attention for this goal. Response of the eigenfrequency shift to a perturbation $\epsilon$ follows the $\epsilon^{1/n}$-dependence at an $n$th-order EP, leading to significantly enhanced sensitivity via a high-order EP. However, due to the requirement of increasingly complicated systems, great difficulties will occur along the path of increasing the EP order to enhance the sensitivity. Here we report that by utilizing the spectral anomaly of the coherent perfect absorption (CPA), the sensitivity at a third-order EP can be further enhanced owing to the cooperative effects of both CPA and EP. We realize this synthetically enhanced sensor using a pseudo-Hermitian cavity magnonic system composed of two yttrium iron garnet spheres and a microwave cavity. The detectable minimum change of the magnetic field reaches $4.2\times10^{-21}$T. It opens a new avenue to design novel sensors using hybrid non-Hermitian quantum systems.

A method is proposed to produce a classical optical state that is `intersystem nonseparable' and a close analog of the $\phi^+$ Bell state. A derivation of the CHSH-Bell inequality is sketched within the framework of classical polarization optics using {\em noncontextuality} for factorizable states as an axiom rather than any hidden variable theory, and it is shown that the classical state violates this inequality.