Employing a nonperturbative approach based on an instantaneous interaction between a two-level Unruh-DeWitt detector and a massive scalar field, we investigate the ability of the latter to generate or destroy coherence in the detector by deriving the cohering and decohering power of the induced quantum evolution channel. For a field in a coherent state a previously unobserved effect is reported in which the amount of coherence that the field generates displays a revival pattern with respect to the size of the detector. Extending previous results into the nonperturbative regime of an arbitrary coupling strength, it is demonstrated that in the case of a thermal field with a positive mass a detector initialized in a maximally coherent state experiences a smaller degree of decoherence after its interaction with the field, compared to the massless case. In both examples for a suitable choice of detector radius, field energy, and coupling strength it is possible to infer the mass of the field by either measuring the coherence present in the detector in the case of an interaction with a coherent field or the corresponding decoherence of a maximally coherent state in the case of a thermal field. In view of recent advances in the study of Proca metamaterials, these results suggest the possibility of utilizing the theory of massive electromagnetism for the construction of novel applications for use in quantum technologies.

Random circuit sampling, the task of sampling bit strings from a random unitary operator, has been implemented to demonstrate quantum advantage on the Sycamore quantum processor with 53 qubits and on the Zuchongzhi quantum processor with 56 and 61 qubits. Recently, it was claimed that classical computers using tensor network simulation could catch on to current noisy quantum processors for random circuit sampling. While the linear cross-entropy benchmark fidelity was used to certify all these claims, it may not capture statistical properties of outputs in detail. Here, we compare the bit strings sampled from classical computers using tensor network simulation by Pan et al. [F. Pan, K. Chen, and P. Zhang, Phys. Rev. Lett. 129, 090502 (2022)] and by Kalachev et al. [G. Kalachev, P. Panteleev, P. Zhou, and M.-H. Yung, arXiv:2112.15083] with the bit strings from the Sycamore quantum processor. It is shown that all of Kalachev et al.'s samples passed the NIST random number tests. The heat maps of bit strings show that Pan et al.'s and Kalachev et al.'s samples are quite different from the Sycamore or Zuchongzhi samples. The analysis with the Marchenko-Pastur distribution and the Wasssertein distances demonstrates that Kalachev et al.'s samples are statistically closer to the Sycamore samples than Pan et al.'s while the three datasets have similar values for the linear cross-entropy fidelity. Our finding implies that further study is needed to certify or beat the claims of quantum advantage using random circuit sampling.

The ability of quantum walks to evolve in a superposition of distinct quantum states has been used as a resource in quantum communication protocols. Under certain settings, the $k$-cycle discrete-time quantum walks (DTQW) are known to recur to its initial state after every $t_r$ steps. We first present a scheme to optically realize any $k$-cycle DTQW using $J$-plate, orbital angular momentum (OAM) sorters, optical switch, and optical delay line. This entangles the polarization and OAM degrees of freedom (DoF) of a single photon. Making use of this recurrence phenomena of $k$-cycle DTQW and the entanglement generated during the evolution, we present an alternate quantum direct communication protocol. The recurrence and entanglement in the $k$-cycle walk are effectively used to retrieve and secure the information, respectively, in the proposed protocol. We investigate the security of the protocol against intercept and the resend attack. We also quantify the effect of amplitude damping and depolarizing noises on recurrence and mutual information between polarization and the OAM DoF of a single photon. Finally, we indicated an optimal attack strategy by which an Eavesdropper can tamper part of the message without revealing her presence. However, when the quantum communication channel is less noisy, any attempt by the Eavesdropper to tamper the message would end up in exposing her to the receiver.

We have computed the cross sections of the mutual neutralization reaction between ${\mathrm{I}}^{+}$ and ${\mathrm{I}}^{-}$ for a collision energy varying from 0.001 eV to 50 eV. These cross sections were obtained using the adiabatic potential energy curves of the ${\mathrm{I}}_2$ system computed with a direct relativistic multireference configuration interaction method and a semiclassical approach (i.e., Landau-Zener surface hopping). We report the cross sections towards the following neutral states: $\text{I}\left({}^{2}P_{3/2}\right)+\text{I}{(}^2{P}_{3/2})$, $\text{I}{(}^2{P}_{3/2})+\text{I}{(}^2{P}_{1/2})$, $\text{I}{(}^2{P}_{1/2})+\text{I}{(}^2{P}_{1/2})$, and $\text{I}\left(5p^{4}6s\right)+\text{I}{(}^2{P}_{3/2})$. We also discuss the cross sections towards the following two excited ionic states: ${\text{I}}^{-}{(}^1{S}_0)+{\text{I}}^{+}{(}^3{P}_0)$ and ${\text{I}}^{-}{(}^1{S}_0)+{\text{I}}^{+}{(}^3{D}_2)$. The results of these calculations are in qualitative accordance with recent experimental measurements conducted in the Double ElectroStatic Ion Ring ExpEriment (DESIREE) in Stockholm. These results can be used to model iodine plasma kinetics and thus to improve our understanding of the latter.

We present a theoretical study of nondipole excitation of a single trapped atom by twisted light. Special emphasis is placed on effects that arise from the interplay between internal (electronic) and vibrational (center-of-mass) degrees of freedom of an atom. In order to provide a fully quantum mechanical understanding of the excitation, we used the density-matrix approach based on the Liouville–von Neumann equation. The developed theory has been applied to the particular case of the $4s{}^{2}S_{1/2}\to 3d{}^{2}D_{5/2}$ electric quadrupole ($E2$) transition in a ${}^{40}\mathrm{Ca}^{+}$ ion induced by Laguerre-Gaussian modes. It was found that the Rabi oscillations can show unconventional anharmonic behavior that is attributed to the strong coupling between vibrational levels of the trap. This effect is accompanied by the transfer of angular momentum to the center-of-mass motion and becomes most pronounced when the Rabi frequency is comparable to the trapping frequency.

Various superpositions of Bessel-Gaussian beams and modified Bessel-Gaussian beams are considered. Two selected parameters characterizing these beams, with respect to which the superpositions are constructed, are the topological index $n$ associated with the orbital angular momentum carried by the beam and $\chi$ related to the dilation of the beam. It is shown that, from these modes, by choosing appropriate weighting factors, it is possible to create a number of well- and less-known solutions of the paraxial equation: a Gaussian (shifted and nonshifted) beam, a $\gamma$ beam, Kummer-Gaussian beam, a special hyperbolic Bessel-Gaussian beam, a certain special Laguerre-Gaussian beam, and generalized paraxial beams in hyperbolic and regular versions.

The exploration of the constructive effects of noise is an interesting and important research topic. In this work, we investigate the influence of noise on a linear system using the model of a driven and dissipative linear-optical cavity. Results show that solely white noise can induce quasiregular dynamical oscillations, which would otherwise be in a steady state, owing to dissipation in the absence of noise. The regularity of noise-induced oscillations can be significantly enhanced by a linear coherent feedback loop. This is because the dissipation of the system is suppressed owing to the interference between the cavity field and feedback field. Our findings may provide a way to efficiently control white-noise-induced oscillations in a linear system and lead to further investigation of the interaction between noise and linear systems.

The hybrid system between the transmon and the magnon plays an important role in quantum information science. Additionally, the cat states of the magnon are a valuable quantum resource and represent macroscopic quantum superpositions of large numbers of spins. In this paper, we propose an approach to generate magnonic Schrödinger cat states (SCSs) in a hybrid ferromagnet-superconductor system. Moreover, high-fidelity magnonic SCSs can be generated when the dissipation of the system is considered. The coherent amplitude and the fidelities of the cat states can be adjusted by controlling the parameters of the system.

Anomalous weak values and the Wigner function's negativity are well-known witnesses of quantum contextuality. We show that these effects occur when analyzing the energetics of a single-qubit gate generated by a resonant coherent field traveling in a waveguide. The buildup of correlations between the qubit and the field is responsible for bounds on the gate fidelity, but also for a nontrivial energy balance recently observed in a superconducting setup. In the experimental scheme, the field is continuously monitored through heterodyne detection and then postselected over the outcomes of a final qubit's measurement. The postselected data can be interpreted as the field's weak values and can show anomalous values in the variation of the field's energy. We model the joint system dynamics with a collision model, gaining access to the qubit-field entangled state at any time. We find an analytical expression of the quasiprobability distribution of the postselected heterodyne signal, i.e., the conditional Husimi-Q function. The latter grants access to all the field's weak values: we use it to obtain that of the field's energy change and display its anomalous behavior. Finally, we derive the field's conditional Wigner function and show that anomalous weak values and Wigner function negativities arise for the same values of the gate's angle.

The Markovianity of quantum dynamics is an important property of open quantum systems determined by various ingredients of the system and bath. Apart from the system-bath interaction, the initial state of the bath, etc., the dimension of the bath plays a critical role in determining the Markovianity of quantum dynamics, as a strict decay of the bath correlations requires an infinite dimension for the bath. In this work, we investigate the role of finite bath dimension in the Markovianity of quantum dynamics by considering a simple but nontrivial model in which a central spin is isotropically coupled to a finite number of bath spins, and show how the dynamics of the central spin transits from non-Markovian to Markovian as the number of the bath spins increases. The non-Markovianity is characterized by the information backflow from the bath to the system in terms of the trace distance of the system states. We derive the time evolution of the trace distance analytically, and find periodic collapse-revival patterns in the information flow. The mechanism underlying this phenomenon is investigated in detail, and it shows that the period of the collapse-revival pattern is determined by the competition between the number of the bath spins, the system-bath coupling strength, and the frequency detuning. When the number of bath spins is sufficiently large, the period of the collapse-revival structure as well as the respective collapse and revival times increase in proportion to the number of the bath spins, which characterizes how the information backflow decays with a large dimension of the bath. We also analyze the effect of the system-bath interaction strength and frequency detuning on the collapse-revival patterns of the information flow, and obtain the condition for the existence of the collapse-revival structure. The results are illustrated by numerical computation.

We establish one-body reduced density matrix (1RDM) functional theory for the canonical ensemble in a finite basis set at an elevated temperature. Including temperature guarantees the differentiability of the universal functional by occupying all states and additionally not fully occupying the states in a fermionic system. We use the convexity of the universal functional and invertibility of the potential-to-1RDM map to show that the subgradient contains only one element which is equivalent to differentiability. This allows us to show that all 1RDMs with a purely fractional occupation number spectrum ($0<n_i<1{\forall }_i$) are uniquely $v$-representable up to a constant.

Assessing the quality of an ensemble of noisy entangled states is a central task in quantum information processing. Usually, this is done by measuring and hence destroying multiple copies, from which state tomography or fidelity estimation can be employed to characterize states. Here we propose several methods to directly distinguish between two different sets of states, e.g., if their fidelity is above or below a certain threshold value. This turns out to be significantly more efficient and importantly keeps the verified states intact. We make use of auxiliary entanglement or an ensemble of larger size, where we operate on the whole ensemble, but measure only a small fraction where information has been concentrated. For certain state families, we demonstrate that such an approach can even outperform optimal methods that collectively measure directly a fixed fraction of the ensemble.

In previous important examinations by Aleiner et al. [Nat. Phys. 6, 900 (2010)] and Michal et al. [Proc. Natl. Acad. Sci. (U.S.A.) 113, E4455 (2016)], the fluid insulator transition has been explored in one-dimensional (1D) disordered bosons by analytical methods. However, the nature of this “fluid” is still not quite understood. Here we shed more light on this “fluid” by studying its properties under changes in the average particle number $\langle N\rangle$ and temperature $T$ in the absence of superfluidity. For this purpose, the worm algorithm path-integral Monte Carlo method is utilized in the grand-canonical ensemble. A measure for the width of path fluctuations is introduced that we call ${\sigma }_J$. It is found that by increasing $\langle N\rangle$, ${\sigma }_J$ rises as the bosons become less repulsive and gain more mobility. The compressibility $\chi \left(\mu \right)$, $\mu$ being the chemical potential, reveals fluid behavior at the lower $\gamma$ and a proposed Bose-glass phase at the larger $\gamma$ (the Lieb-Liniger interaction parameter). The kinetic energy $E_K$ versus $\gamma$ at various $T$ signals a transition by its convergence towards a common value. $E_K$ versus $\mu$ reveals a peculiar inversion as one goes from small to larger values of the speckle strength $\langle V\rangle$ demonstrating a complex influence of the disorder. A tipover of $E_K$ versus $\gamma$ from an increasing to a decreasing trend at the lower $\gamma$ also signals a transition. Further, Luttinger's liquid theory cannot be applied to describe 1D bosons in strong disorder at finite $T$ because of the absence of superfluidity. Moreover, $\langle V\rangle$ largely controls the behavior of $\chi \left(\mu \right)$, the pair-correlation function at the origin $g_2\left(0\right)$, and $E_K$. Surprisingly, the diffusion constant $D_0$ does not obey the classical isomorphism as it rises with $T$. We also emphasize the importance of this work via its applicability to quantum wires.

The general Hardy-like quantum pigeonhole paradoxes for n-qubit states were presented by Tang [W. Tang, Phys. Rev. A 105, 032457 (2022)]. It has been shown that each of such paradoxes can be associated with an uncolorable solution of a specific vertex-coloring problem induced from the projected-coloring graph (a kind of unconventional graph). The Hardy-like quantum pigeonhole paradoxes can even give rise to a higher suc- cess probability in demonstrating the conflict between quantum mechanics and local or noncontextual realism than the previous Hardy's paradoxes. Moreover, multiqubit states and high-dimensional states can exhibit paradoxes. In contrast to only one type of contradiction presented in the original quantum pigeonhole paradox, we discussed two kinds of three-qubit projected-coloring graph states as the minimal illustration in our work. An optical experiment to verify such a stronger paradox is performed. This quantum paradox provides innovative thoughts and methods for exploring new types of stronger multiparty quantum nonlocality. It may have potential applications in untrusted multiparty communications and device-independent random number generation.

Quantum mechanics describes seemingly paradoxical relations between the outcomes of measurements that cannot be performed jointly. In Hilbert space, the outcomes of such incompatible measurements are represented by nonorthogonal states. In this paper we investigate how the relation between outcomes represented by nonorthogonal quantum states differs from the relations suggested by a joint assignment of measurement outcomes that do not depend on the actual measurement context. The analysis is based on a well-known scenario where three statements about the impossibilities of certain outcomes would seem to make a specific fourth outcome impossible as well, yet quantum theory allows the observation of that outcome with a nonvanishing probability. We show that the Hilbert space formalism modifies the relation between the four measurement outcomes by defining a lower bound of the fourth probability that increases as the total probability of the first three outcomes drops to zero. Quantum theory thus not only makes the violation of noncontextual consistency between the measurement outcomes possible, but actually requires it as a necessary consequence of the Hilbert space inner products that describe the contextual relation between the outcomes of different measurements.

In Phys. Rev. A 100, 062317 (2019), the authors reported an algorithm to implement, in a circuit-based quantum computer, a general quantum measurement (GQM) of a two-level quantum system, a qubit. Even though their algorithm seems right, its application involves the solution of an intricate nonlinear system of equations to obtain the angles determining the quantum circuit to be implemented for the simulation. In this article, we identify and discuss a simple way to circumvent this issue and implement GQMs on any $d$-level quantum system through quantum state preparation algorithms. Using some examples for one qubit, one qutrit, and two qubits, we illustrate the easy of application of our protocol. In addition, we show how one can utilize our protocol for simulating quantum instruments, for which we also give an example. All our examples are demonstrated using IBM's quantum processors.

When three or more particles are considered, quantum correlations can be stronger than the correlations generated by so-called hybrid local hidden variable models, where some of the particles are considered as a single block inside which communication and signaling is allowed. We provide an exhaustive classification of Bell inequalities to characterize various hybrid scenarios in four- and five-particle systems. In quantum mechanics, these inequalities provide device-independent witnesses for the entanglement depth. In addition, we construct a family of inequalities to detect a nonlocality depth of $(n-1)$ in $n$-particle systems. Moreover, we present two generalizations of the original Svetlichny inequality, which was the first Bell inequality designed for hybrid models. Our results are based on the cone-projection technique, which can be used to completely characterize Bell inequalities under affine constraints; even for many parties, measurements, and outcomes.