We present an implementation of the Maxwell-Bloch formalism for the study of x-ray emission dynamics from periodic multilayer materials whether they are artificial or natural. The treatment is based on a direct finite-difference time-domain solution of Maxwell equations combined with Bloch equations incorporating a random spontaneous emission noise. Besides periodicity of the material, the treatment distinguishes between two kinds of layers, those being active (or resonant) and those being off-resonance. The numerical model is applied to the problem of $K\alpha$ emission in multilayer materials where the population inversion could be created by fast inner-shell photoionization by an x-ray free-electron laser. Specificities of the resulting amplified fluorescence in conditions of Bragg diffraction are illustrated by numerical simulations. The corresponding pulses could be used for specific investigations of nonlinear interaction of x rays with matter.

Hybrid quantum systems involving solid-state spins and superconducting microwave cavities play a crucial role in quantum science and technology, but improving the spin-photon coupling at the single quantum level remains challenging in such systems. Here, we propose a simple technique to strongly couple a single solid-state spin to the microwave photons in a superconducting coplanar waveguide cavity via a magnetic microsphere. We show that strong coupling at the single spin level can be realized by virtual magnonic excitations of a nearby micromagnet. The spin-photon coupling strength can be enhanced up to typically 4 orders of magnitude larger than that without the use of the micromagnet. This work can find applications in quantum information processing with strongly coupled solid-state spin-photonic systems.

In an endeavor to better define the distinction between classical macroscopic and quantum microscopic regimes, the Leggett-Garg inequalities were established as a test of macroscopic-realistic theories, which are commonly thought to be a suitable class of descriptions for classical dynamics. The relationship between their violation and nonclassicality is however not obvious. We show that classical states of light, which in the quantum optical sense are any convex sums of coherent states, may not satisfy the Leggett-Garg inequalities. After introducing a simple Mach-Zehnder setup and showing how to obtain a violation with a single photon using negative measurements, we focus on classical states of light, in particular those of low average photon number. We demonstrate how one can still perform negative measurements with an appropriate assignment of variables, and show that the inequalities are violable with coherent states. Finally, we abandon the initial phase reference and demonstrate that the violation is still possible, in particular with thermal states of light, and we investigate the effect of intermediate dephasing.

In this work, the anisotropic variant of the quantum Rabi model with different coupling strengths of the rotating and counterrotating wave terms is studied by the Bogoliubov operator approach. The anisotropy preserves the parity symmetry of the original model. We derive the corresponding $G$-function, which yields both the regular and exceptional eigenvalues. The exceptional eigenvalues correspond to the crossing points of two energy levels with different parities, and they are doubly degenerate. We find analytically that the ground state and the first excited state can cross several times, indicating multiple first-order phase transitions as a function of the coupling strength. These crossing points are related to manifest parity symmetry of the Hamiltonian, in contrast to the level crossings in the asymmetric quantum Rabi model, which are caused by a hidden symmetry.

In the noisy intermediate-scale quantum era, optimal digitized pulses are requisite for efficient quantum control. This goal is translated into dynamic programming, in which a deep reinforcement learning (DRL) agent is gifted. As a reference, shortcuts to adiabaticity (STA) provide analytical approaches to adiabatic speedup by pulse control. Here, we select the single-component control of qubits, resembling the ubiquitous two-level Landau-Zener problem for gate operation. We aim at obtaining fast and robust digital pulses by combining the STA and DRL algorithm. In particular, we find that DRL leads to robust digital quantum control with the operation time bounded by quantum speed limits dictated by STA. In addition, we demonstrate that robustness against systematic errors can be achieved by DRL without any input from STA. Our results introduce a general framework of digital quantum control, leading to a promising enhancement in quantum information processing.

Studies addressing the question “Can a learner complete the learning securely?” have recently been spurred from the standpoints of fundamental theory and potential applications. In the relevant context of this question, we present a classical-quantum hybrid sampling protocol and define a security condition that allows only legitimate learners to prepare a finite set of samples that guarantees the success of the learning; the security condition excludes intruders. We do this by combining our security concept with the bound of the so-called probably approximately correct (PAC) learning. We show that while the lower bound on the learning samples guarantees PAC learning, an upper bound can be derived to rule out adversarial learners. Such a secure learning condition is appealing, because it is defined only by the size of samples required for the successful learning and is independent of the algorithm employed. Notably, the security stems from the fundamental quantum no-broadcasting principle. No such condition can thus occur in any classical regime, where learning samples can be copied. Owing to the hybrid architecture, our scheme also offers a practical advantage for implementation in noisy intermediate-scale quantum devices.

Motivated by the parton picture of high-energy quantum chromodynamics, we develop a single-particle digitization strategy for the efficient quantum simulation of relativistic scattering processes in a $d+1$-dimensional scalar ${\varphi }^4$ field theory. We work out quantum algorithms for initial state preparation, time evolution, and final state measurements. We outline a nonperturbative renormalization strategy in this single-particle framework.

The promise of quantum computing with imperfect qubits relies on the ability of a quantum computing system to scale cheaply through error correction and fault tolerance. While fault tolerance requires relatively mild assumptions about the nature of qubit errors, the overhead associated with coherent and non-Markovian errors can be orders of magnitude larger than the overhead associated with purely stochastic Markovian errors. One proposal to address this challenge is to randomize the circuits of interest, shaping the errors to be stochastic Pauli errors but leaving the aggregate computation unaffected. The randomization technique can also suppress couplings to slow degrees of freedom associated with non-Markovian evolution. Here, we demonstrate the implementation of Pauli-frame randomization in a superconducting circuit system, exploiting a flexible programming and control infrastructure to achieve this with low effort. We use high-accuracy gate-set tomography to characterize in detail the properties of the circuit error, with and without the randomization procedure, which allows us to make rigorous statements about Markovianity as well as the nature of the observed errors. We demonstrate that randomization suppresses signatures of non-Markovian evolution to statistically insignificant levels, from a Markovian model violation ranging from $43\sigma$ to $1987\sigma$, down to violations between $0.3\sigma$ and $2.7\sigma$ under randomization. Moreover, we demonstrate that, under randomization, the experimental errors are well described by a Pauli error model, with model violations that are similarly insignificant (between $0.8\sigma$ and $2.7\sigma$). Importantly, all these improvements in the model accuracy were obtained without degradation to fidelity, and with some improvements to error rates as quantified by the diamond norm. This demonstrates the ability of Pauli-frame randomization to shape noise into forms that are more benign for quantum error correction and fault tolerance.

Molecular cationic states with two valence holes and one $n$-Rydberg electron can be created after spectator Auger decay. Unraveling these states' dissociation is often very challenging due to the frequent occurrence of conical intersections between cationic potential energy curves. Here, based on an advanced analysis of the experimental multicoincidence data obtained after $\mathrm{O}1s$ core excitation in ${\mathrm{O}}_2$, we achieved an energy-resolution better than we recently exploited in Phys. Rev. A 99, 022511 (2019). We therefore revealed a group of weak channels in the two-dimensional energy-correlation map between the coincident resonant Auger electron and ion in addition to the previously reported strong ones. The fragments in the identified weak channels contain only outmost electrons in valence orbitals; in contrast, the fragments in the strong channels contain an outmost electron in a $n^{\prime }$-Rydberg orbital. Compared with the strong channels, the weak channels preferentially occur at smaller principal quantum number $n$. It indicates that the electron orbital size tends to be conserved during the dissociation process. These weak features are suggested to be created by the Rydberg-valence mixing between the molecular spectator Auger final state and the very dissociative molecular cationic states without the Rydberg electron. A tendency to orbital selection is also suggested in the Rydberg-valence mixing.

A spinless Bose-Hubbard model in a one-dimensional (1D) double-chain tilted lattice is numerically studied at unit filling per cell. When each atom is localized in a two-site cell, the low-energy effective model gives rise to an exact textbook model of the 1D transverse Ising model via superexchange interaction. To validate the effective transverse Ising model, we calculate the energy spectrum and the nearest-neighbor correlation functions of the states in a subspace of the Bose-Hubbard model which is equivalent to the Hilbert space of a spin-1/2 magnetic model. The results show good consistency with the effective transverse Ising model, and we show that it is possible to simulate the dynamical quantum phase transition of the 1D transverse Ising model exactly with such a double-chain Bose-Hubbard model. Our results may provide some inspirations for realizing and exploring an exact 1D transverse Ising model in ultracold neutral atom systems.

Due to their long excited-state lifetimes, rare-earth ions in crystals are widely used in quantum applications. To allow optical readout of the qubit state of individual ions, we propose to dope the crystal with an additional nearby ancilla ion with a shorter radiative lifetime. We show how a Bayesian analysis exhausts the information about the state of the qubit from the optical signal of the ancilla ion. We study the effects of incoherent processes, and we propose ways to reduce their effect on the readout. Finally, we extend the architecture to ions residing in two remote cavities, and we show how continuous monitoring of fluorescence signals from the two ancilla ions leads to entanglement of the qubit ions.

Classical $(1+1)$-dimensional (D) cellular automata, as for instance Domany-Kinzel cellular automata, are paradigmatic systems for the study of nonequilibrium phenomena. Such systems evolve in discrete time steps, and are thus free of time-discretization errors. Moreover, they display nonequilibrium phase transitions which can be studied by simulating the evolution of an initial seed. At any finite time, this has support only on a finite light cone. Thus, essentially numerically exact simulations free of finite-size errors or boundary effects are possible, leading to high-accuracy estimates of critical exponents. Here, we show how similar advantages can be gained in the quantum regime: The many-body critical dynamics occurring in $(1+1)\mathrm{D}$ quantum cellular automata with an absorbing state can be studied directly on an infinite lattice when starting from seed initial conditions. This can be achieved efficiently by simulating the dynamics of an associated one-dimensional, nonunitary quantum cellular automaton using tensor networks. We apply our method to a model introduced recently and find accurate values for universal exponents, suggesting that this approach can be a powerful tool for precisely studying nonequilibrium universal physics in quantum systems.

We study the repulsive Fermi polaron in a two-component, two-dimensional system of fermionic atoms inspired by the results of a recent experiment with ${}^{173}\mathrm{Yb}$ atoms [N. Darkwah Oppong et al., Phys. Rev. Lett. 122, 193604 (2019)]. We use the diffusion Monte Carlo method to report properties such as the polaron energy and the quasiparticle residue that have been measured in that experiment. To provide insight into the quasiparticle character of the problem, we also report results for the effective mass. We show that the effective range, together with the scattering length, is needed in order to reproduce the experimental results. Using different model potentials for the interaction between the Fermi sea and the impurity, we show that it is possible to establish a regime of universality, in terms of these two parameters, that includes the whole experimental regime. This illustrates the relevance of quantum fluctuations and beyond mean-field effects to correctly describe the Fermi polaron problem.

We investigate the shortcuts to adiabaticity (STA) of a quantum harmonic oscillator under time-dependent frictional force, using an invariant-based inverse-engineering method with a class of invariants characterized by a time-dependent frictional coefficient. We discuss the implementation of a shortcut protocol in a generalized framework for any arbitrary time-dependent frictional interaction and study a class of such interaction from which the STA for the quantum harmonic oscillator can be obtained. We also discuss the application of the above protocol for the harmonic oscillator with time-varying mass. For an illustration, we consider the coupled photonic lattice as a harmonic oscillator with time-varying mass and frequency and discuss implementing the above protocol.

We study parity-time ($\mathcal{PT}$) phase transitions in the energy spectra of ladder lattices caused by the interplay between nonorientability and non-Hermitian $\mathcal{PT}$ symmetry. The energy spectra show level crossings in circular ladder lattices with increasing on-site energy gain-loss because of the orientability of a normal strip. However, the energy levels show $\mathcal{PT}$ phase transitions instead of the avoided level crossings of a Hermitian situation in $\mathcal{PT}$-symmetric Möbius ladder lattices due to the nonorientability of a Möbius strip. The latter effectively presents a perturbation that would cause avoided level crossing in a Hermitian situation, but leads, in the presence of $\mathcal{PT}$ symmetry, to locked real energy parts and conjugate values of the imaginary parts. In order to understand the level crossings of $\mathcal{PT}$-symmetric phases, we generalize the rotational transformation using a complex rotation angle. We also study the modification of resonant tunneling induced by a sharply twisted interface in $\mathcal{PT}$-symmetric ladder lattices. Finally, we find that perfect transmissions at the zero energy are recovered at the exceptional points of the $\mathcal{PT}$-symmetric system due to the self-orthogonal states.

Quantum error correction plays a key role for quantum information transmission and quantum computing. In this work, we develop and apply the theory of noncommutative operator graphs to study error correction in the case of a finite-dimensional quantum system coupled to an infinite-dimensional system. We consider as an explicit example a qubit coupled via the Jaynes-Cummings (JC) Hamiltonian with a bosonic coherent field. We extend the theory of noncommutative graphs to this situation and construct, using Gazeau-Klauder coherent states, the corresponding noncommutative graph. As the result, we find the quantum anticlique, which is the projector on the error-correcting subspace, and analyze it as a function of the frequencies of the qubit and the bosonic field. The general treatment is also applied to the analysis of the error-correcting subspace for certain experimental values of the parameters of the Jaynes-Cummings Hamiltonian. The proposed scheme can be applied to any system that possess the same decomposition of spectrum of the Hamiltonian into a direct sum as in JC model, where eigenenergies in the two direct summands form strictly increasing sequences.

Since its key rate can overcome the PLOB bound, sending-or-not-sending twin-field quantum key distribution (SNS-TF QKD) schemes attract more and more attention in the past three years. However, the inflection of statistical fluctuations on key rate was not considered quite comprehensively, which blocks the practical application of SNS-TF QKD. We take into account all the statistical fluctuations on probabilities, propose the finite-key analysis for SNS-TF QKD without any assumption on the type of attacks, and obtain the lower bound of key rates by applying an optimizing model. Then the finite-key rates are simulated under the reasonable values of some observed parameters, which shows that the key rates overcome the PLOB bound when the transmission distance is far from 350 km, if the number of pulses is fixed as $N={10}^{14}$. Compared with other SNS-TF QKD schemes, we provide concise and tight finite-key bounds since the statistical fluctuations of all parameters are considered against general attacks.

Dissociation of $\mathrm{OC}{\mathrm{S}}^{q+}$ $(q=3–7)$ produced by $18\text{−}\mathrm{keV}/\mathrm{u}{\mathrm{Ne}}^{8+}$ ion impact has been investigated using recoil ion momentum spectroscopy. Ten complete three-body Coulomb fragmentation channels were analyzed. Scattered projectile charge state $({\mathrm{Ne}}^{7+} \mathrm{or} {\mathrm{Ne}}^{6+})$ differentiated branching ratios and the kinetic-energy release (KER) distributions were analyzed for different fragmentation channels. Through Dalitz plots, Newton diagrams, and native frames, we have studied the concerted and sequential fragmentation mechanisms for channels leading to ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$, ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{2+}$, ${\mathrm{O}}^{2+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{+}$, and ${\mathrm{O}}^{+}+{\mathrm{C}}^{+}+{\mathrm{S}}^{3+}$. For concerted dissociation mechanisms, the molecular bending fragmentation dominates in these channels. For sequential dissociation mechanisms, the KER distribution for the dissociation of intermediate ions $\mathrm{C}{\mathrm{O}}^{2+}/\mathrm{C}{\mathrm{S}}^{2+}$ were determined and the rotation of the intermediate molecular ion $\mathrm{C}{\mathrm{O}}^{2+}$ before the second step dissociation was discussed. Thereby the relative branching ratios of events coming from sequential versus concerted mechanism were also estimated.