QUROPE Quantum Information Processing and Communication in Europe

Finding a high (or low) energy state of a given quantum Hamiltonian is a potential area to gain a provable and practical quantum advantage. A line of recent studies focuses on Quantum Max Cut, where one is asked to find a high energy state of a given antiferromagnetic Heisenberg Hamiltonian. In this work, we present a classical approximation algorithm for Quantum Max Cut that achieves an approximation ratio of 0.584 given a generic input, and a ratio of 0.595 given a triangle-free input, outperforming the previous best algorithms of Lee \cite{Lee22} (0.562, generic input) and King \cite{King22} (0.582, triangle-free input). The algorithm is based on finding the maximum weighted matching of an input graph and outputs a product of at most 2-qubit states, which is simpler than the fully entangled output states of the previous best algorithms.

Fast and high-fidelity qubit measurement is crucial for achieving quantum error correction, a fundamental element in the development of universal quantum computing. For electron spin qubits, fast readout stands out as a major obstacle in the pursuit of error correction. In this work, we explore the dispersive measurement of an individual spin in a semiconductor double quantum dot coupled to a nonlinear microwave resonator. By utilizing displaced squeezed vacuum states, we achieve rapid and high-fidelity readout for semiconductor spin qubits. Our findings reveal that introducing modest squeezing and mild nonlinearity can significantly improve both the signal-to-noise ratio (SNR) and the fidelity of qubit-state readout. By properly marching the phases of squeezing, the nonlinear strength, and the local oscillator, the optimal readout time can be reduced to the sub-microsecond range. With current technology parameters ($\kappa\approx 2\chi_s$, $\chi_s\approx 2\pi\times 0.15 \:\mbox{MHz}$), utilizing a displaced squeezed vacuum state with $30$ photons and a modest squeezing parameter $r\approx 0.6$, along with a nonlinear microwave resonator charactered by a strength of $\lambda\approx -1.2 \chi_s$, a readout fidelity of $98\%$ can be attained within a readout time of around $0.6\:\mu\mbox{s}$. Intriguing, by using a positive nonlinear strength of $\lambda\approx 1.2\chi_s$, it is possible to achieve an SNR of approximately $6$ and a readout fidelity of $99.99\%$ at a slightly later time, around $0.9\:\mu\mbox{s}$, while maintaining all other parameters at the same settings.

A sender can prepare a quantum state for a remote receiver using preshared entangled pairs, only the sender's single-qubit measurement, and the receiver's simple correction informed by the sender. It provides resource-efficient advantages over quantum teleportation for quantum information. Here, we propose the most efficient approach to detect the remote state preparation (RSP) based on the quantum benefits powered by quantum coherence's static resources of the shared pairs and the dynamic resources both the RSP participants input. It requires only the receiver's minimum of one additional coherence creation operation to verify RSP. Experimentally, we implement the introduced RSP assessment using different photon pair states generated from a high-quality polarization Sagnac interferometer, confirming the necessary and sufficient role played by the static and dynamic quantum coherence resources and demonstrating efficient RSP verification. Our results provide a route to efficiently assess RSP in practical scenarios such as quantum information in quantum networks.

Our main result is a reduction from worst-case lattice problems such as GapSVP and SIVP to a certain learning problem. This learning problem is a natural extension of the `learning from parity with error' problem to higher moduli. It can also be viewed as the problem of decoding from a random linear code. This, we believe, gives a strong indication that these problems are hard. Our reduction, however, is quantum. Hence, an efficient solution to the learning problem implies a quantum algorithm for GapSVP and SIVP. A main open question is whether this reduction can be made classical (i.e., non-quantum).

We also present a (classical) public-key cryptosystem whose security is based on the hardness of the learning problem. By the main result, its security is also based on the worst-case quantum hardness of GapSVP and SIVP. The new cryptosystem is much more efficient than previous lattice-based cryptosystems: the public key is of size $\tilde{O}(n^2)$ and encrypting a message increases its size by a factor of $\tilde{O}(n)$ (in previous cryptosystems these values are $\tilde{O}(n^4)$ and $\tilde{O}(n^2)$, respectively). In fact, under the assumption that all parties share a random bit string of length $\tilde{O}(n^2)$, the size of the public key can be reduced to $\tilde{O}(n)$.

The tunneling dynamics of a magnetic $^{164}$Dy quantum gas in an elongated or circular skewed double-well trap is investigated with a time-dependent extended Gross-Pitaevskii approach. Upon lifting the energy offset, different tunneling regimes can be identified. In the elongated trap and for sufficiently large offset, the different configurations exhibit collective macroscopic tunneling. For smaller offset, partial reflection from and transmission through the barrier lead to density accumulation in both wells, and eventually to tunneling-locking. One can also reach the macroscopic self-trapping regime for increasing relative dipolar interaction strength, while tunneling vanishes for large barrier heights. A richer dynamical behavior is observed for the circular trap. For instance, the supersolid maintains its shape, while the superfluid density gets distorted signifying the emergence of peculiar excitation patterns in the macroscopic tunneling regime. The findings reported here may offer new ways to probe distinctive dynamical features in the supersolid and droplet regimes.

Quantum computing, with its superior computational capabilities compared to classical approaches, holds the potential to revolutionize numerous scientific domains, including pharmaceuticals. However, the application of quantum computing for drug discovery has primarily been limited to proof-of-concept studies, which often fail to capture the intricacies of real-world drug development challenges. In this study, we diverge from conventional investigations by developing an advanced quantum computing pipeline tailored to address genuine drug design problems. Our approach underscores the pragmatic application of quantum computation and propels it towards practical industrial adoption. We specifically construct our versatile quantum computing pipeline to address two critical tasks in drug discovery: the precise determination of Gibbs free energy profiles for prodrug activation involving covalent bond cleavage, and the accurate simulation of covalent bond interactions. This work serves as a pioneering effort in benchmarking quantum computing against veritable scenarios encountered in drug design, especially the covalent bonding issue present in both of the case studies, thereby transitioning from theoretical models to tangible applications. Our results demonstrate the potential of a quantum computing pipeline for integration into real world drug design workflows.

Magnetic nano-skyrmions develop quantized helicity excitations, and the quantum tunneling between nano-skyrmions possessing distinct helicities is indicative of the quantum nature of these particles. Experimental methods capable of non-destructively resolving the quantum aspects of topological spin textures, their local dynamical response, and their functionality now promise practical device architectures for quantum operations. With abilities to measure, engineer, and control matter at the atomic level, nano-skyrmions present opportunities to translate ideas into solid-state technologies. Proof-of-concept devices will offer electrical control over the helicity, opening a promising new pathway towards functionalizing collective spin states for the realization of a quantum computer based on skyrmions. This Perspective aims to discuss developments and challenges in this new research avenue in quantum magnetism and quantum information.

A quantum limit on the measurement of magnetic field has been recently pointed out, stating that the so-called Energy Resolution $E_\mathrm{R}$ is bounded to $E_\mathrm{R} \gtrsim \hbar$. This limit holds indeed true for the vast majority of existing quantum magnetometers, including SQUIDs, solid state spins and optically pumped atomic magnetometers. However, it can be surpassed by highly correlated spin systems, as recently demonstrated with a single-domain spinor Bose-Einstein Condensate. Here we show that similar and potentially much better resolution can be achieved with a hard ferromagnet levitated above a superconductor at cryogenic temperature. We demonstrate $E_\mathrm{R}=\left( 0.064 \pm 0.010 \right) \, \hbar$ and anticipate that $E_\mathrm{R}<10^{-3} \, \hbar$ is within reach with near-future improvements. This finding opens the way to new applications in condensed matter, biophysics and fundamental science. In particular, we propose an experiment to search for axionlike dark matter and project a sensitivity orders of magnitude better than in previous searches.

The machinery of industrial environments was connected to the Internet years ago with the scope of increasing their performance. However, this made such environments vulnerable against cyber-attacks that can compromise their correct functioning resulting in economic or social problems. Lately, an increase of cyberattacks to industrial environments has been experienced. Moreover, implementing cryptosystems in the communications between OT devices is a more challenging task than for IT environments since the OT are generally composed of legacy elements, characterized by low-computational capabilities. Consequently, implementing cryptosystems in industrial communication networks faces a trade-off between the security of the communications and the amortization of the industrial infrastructure. Critical Infrastructure (CI) refers to the industries which provide key resources for the daily social and economical development, e.g. electricity or water, and their communications are a very exposed target to cyberattacks. Furthermore, a new threat to cybersecurity has arisen with the theoretical proposal of quantum computers, due to their potential ability of breaking state-of-the-art cryptography protocols, such as RSA or ECC. The chase of functional quantum computers has resulted in a technological race involving many global agents. Those agents have become aware that transitioning their secure communications to a quantum secure paradigm is a priority that should be established before the arrival of fault-tolerance. In this sense, two main cryptographic solutions have been proposed: QKD and PQC. Nevertheless, quantum secure solutions have been mainly centered from the perspective of IT environments. In this paper, we provide a perspective of the problem of applying PQC solutions to CI and analyze which could be the most suitable cryptography schemes for these scenarios.

Extreme learning machines explore nonlinear random projections to perform computing tasks on high-dimensional output spaces. Since training only occurs at the output layer, the approach has the potential to speed up the training process and the capacity to turn any physical system into a computing platform. Yet, requiring strong nonlinear dynamics, optical solutions operating at fast processing rates and low power can be hard to achieve with conventional nonlinear optical materials. In this context, this manuscript explores the possibility of using atomic gases in near-resonant conditions to implement an optical extreme learning machine leveraging their enhanced nonlinear optical properties. Our results suggest that these systems have the potential not only to work as an optical extreme learning machine but also to perform these computations at the few-photon level, paving opportunities for energy-efficient computing solutions.

We address interconnection challenges in limited-qubit superconducting quantum computers (SQC), which often face crosstalk errors due to expanded qubit interactions during operations. Existing mitigation methods carry trade-offs, like hardware couplers or software-based gate scheduling. Our innovation, the Context-Aware COupler REconfiguration (CA-CORE) compilation method, aligns with application-specific design principles. It optimizes the qubit connections for improved SQC performance, leveraging tunable couplers. Through contextual analysis of qubit correlations, we configure an efficient coupling map considering SQC constraints. Our method reduces depth and SWAP operations by up to 18.84% and 42.47%, respectively. It also enhances circuit fidelity by 40% compared to IBM and Google's topologies. Notably, our method compiles a 33-qubit circuit in less than 1 second.

Driven classical self-sustained oscillators have been studied extensively in the context of synchronization. Using the master equation, this work considers the classically driven generalized quantum Rayleigh-van der Pol oscillator, which is characterized by linear dissipative gain and loss terms as well as three non-linear dissipative terms. Since two of the non-linear terms break the rotational phase space symmetry, the Wigner distribution of the quantum mechanical limit cycle state of the undriven system is, in general, not rotationally symmetric. The impact of the symmetry-breaking dissipators on the long-time dynamics of the driven system are analyzed as functions of the drive strength and detuning, covering the deep quantum to near-classical regimes. Phase localization and frequency entrainment, which are required for synchronization, are discussed in detail. We identify a large parameter space where the oscillators exhibit appreciable phase localization but only weak or no entrainment, indicating the absence of synchronization. Several observables are found to exhibit the analog of the celebrated classical Arnold tongue; in some cases, the Arnold tongue is found to be asymmetric with respect to vanishing detuning between the external drive and the natural oscillator frequency.

We use an erbium doped CaWO$_4$ crystal as a resonant transducer between the RF and optical domains at 12 GHz and 1532 nm respectively. We employ a RF resonator to enhance the spin coupling but keep a single-pass (non-resonant) setup in optics. The overall efficiency is low but we carefully characterize the transduction process and show that the performance can be described by two different metrics that we define and distinguish: the electro-optics and the quantum efficiencies. We reach an electro-optics efficiency of -84 dB for 15.7 dBm RF power. The corresponding quantum efficiency is -142 dB for 0.4 dBm optical power. We develop the Schr\"odinger-Maxwell formalism, well-known to describe light-matter interactions in atomic systems, in order to model the conversion process. We explicitly make the connection with the cavity quantum electrodynamics (cavity QED) approach that are generally used to describe quantum transduction.

Fusing photon pairs creates an arena where indistinguishability can exist between two two-photon amplitudes contributing to the same joint photodetection event. This two-photon interference has been extensively utilized in creating multiphoton entanglement, from passive to scalable generation, from bulk-optical to chip-scale implementations. While significant, no experimental evidence exists that the full capability of photon fusion can be utterly quantified like a quantum entity. Herein, we demonstrate the first complete capability quantification of experimental photon fusion. Our characterization faithfully measures the whole abilities of photon fusion in the experiment to create and preserve entangled photon pairs. With the created four- and six-photon entangled states using spontaneous parametric down-conversion entanglement sources, we show that capability quantification provides a faithful assessment of interferometry for generating genuine multiphoton entanglement and Einstein-Podolsky-Rosen steering. These results reveal a practical diagnostic method to benchmark photon fusion underlying the primitive operations in general quantum photonics devices and networks.

Understanding the phenomenon of quantum superposition of gravitational fields induced by massive quantum particles is an important starting point for quantum gravity. The purpose of this study is to deepen our understanding of the phenomenon of quantum superposition of gravitational fields. To this end, we consider a trade-off relation of entanglement (monogamy relation) in a tripartite system consisting of two massive particles and a gravitational field that may be entangled with each other. Consequently, if two particles cannot exchange information mutually, they are in a separable state, and the particle and gravitational field are always entangled. Furthermore, even when two particles can send information to each other, there is a trade-off between the two particles and the gravitational field. We also investigate the behavior of the quantum superposition of the gravitational field using quantum discord. We find that quantum discord increases depending on the length scale of the particle superposition. Our results may help understand the relationship between the quantization of the gravitational field and the meaning of the quantum superposition of the gravitational field.

The time evolution of a wave packet is a tool to detect topological phase transitions in two-dimensional Dirac materials, such as graphene and silicene. Here we extend the analysis to HgTe/CdTe quantum wells and study the evolution of their electron current wave packet, using 2D effective Dirac Hamiltonians and different layer thicknesses. We show that the two different periodicities that appear in this temporal evolution reach a minimum near the critical thickness, where the system goes from normal to inverted regime. Moreover, the maximum of the electron current amplitude changes with the layer thickness, identifying that current maxima reach their higher value at the critical thickness. Thus, we can characterize the topological phase transitions in terms of the periodicity and amplitude of the electron currents.

We report on the controlled creation and destruction of antiferromagnetic dimers using two-tone Floquet engineering. We consider a one-dimensional spin-1/2 lattice with periodically modulated bonds using parametric resonances. The stroboscopic dynamics generated from distributed bond modulations lead to pair correlation between spins. Consequently, subharmonic response in local observables breaks discrete time translational symmetry and leads to the emergence of Floquet dynamical dimerisation. We present a protocol allowing the control of local spin-correlated pairs driven by one-period evolution operators, providing significant insight into new nonequilibrium states of matter that can be feasibly implemented in current quantum simulator platforms.

As is well known, an external magnetic field in configuration space coupled to a quantum dynamics induces noncommutativity in its velocity momentum space. By phase space duality, an external vector potential in the conjugate momentum sector of the system induces noncommutativity in its configuration space. Such a rationale for noncommutativity is explored herein for an arbitrary configuration space of Euclidean geometry. Ordinary quantum mechanics with a commutative configuration space is revisited first. Through the introduction of an arbitrary positive definite $*$-product, a one-to-one correspondence between the Hilbert space of abstract quantum states and that of the enveloping algebra of the position quantum operators is identified. A parallel discussion is then presented when configuration space is noncommutative, and thoroughly analysed when the conjugate magnetic field is momentum independent and nondegenerate. Once again the space of quantum states may be identified with the enveloping algebra of the noncommutative position quantum operators. Furthermore when the positive definite $*$-product is chosen in accordance with the value of the conjugate magnetic field which determines the commutator algebra of the coordinate operators, these operators span a Fock algebra of which the canonical coherent states are the localised noncommutative quantum analogues of the sharp and structureless local points of the associated commutative configuration space geometry. These results generalise and justify a posteriori within the context of ordinary canonical quantisation the heuristic approach to quantum mechanics in the noncommutative Euclidean plane as constructed and developed by F. G. Scholtz and his collaborators.

We propose a method for the estimation of the initial polarization of spinful nuclei of the ${}^{13}C$ isotope in diamond via a measurement of the evolution of the coherence of an NV center spin qubit. Existing polarization measurement methods are difficult to implement experimentally, because they require direct interference in the environment of the qubit. Here, in order to obtain the information, it is necessary to measure the qubit coherence at certain points of time, which are unambiguously determined by the applied magnetic field. For sufficiently high magnetic fields, the minimum value of the measured coherence constitutes an upper bound on the product of the initial polarizations of each environmental spin. The most significant advantage of the method, which allows to infer initial values of nuclear polarizations without any direct access to the environment, lies in its simplicity and the small amount of experimental resources that it requires. We exemplify the operation of the scheme on a realistic, randomly generated environment of eight nuclear spins, obtaining a reasonably accurate estimation of the initial polarization.

We report on the coherent excitation of the ultranarrow $^{1}\mathrm{S}_0$-$^{3}\mathrm{P}_2$ magnetic quadrupole transition in $^{88}\mathrm{Sr}$. By confining atoms in a state insensitive optical lattice, we achieve excitation fractions of 97(1)% and observe linewidths as narrow as 58(1) Hz. With Ramsey spectroscopy, we find coherence times of 14(1) ms, which can be extended to 266(36) ms using a spin-echo sequence. We determine the linewidth of the M2 transition to 24(7) $\mu$Hz, confirming longstanding theoretical predictions. These results establish an additional clock transition in strontium and pave the way for applications of the metastable $^{3}\mathrm{P}_2$ state in quantum computing and quantum simulations.