We consider a spatiotemporal differentiator of the first order based on a three-layer metal-dielectric-metal (MDM) structure. We show theoretically and numerically that, in contrast to all-dielectric layered structures, the differentiator based on this MDM structure enables generating an optical pulse possessing an optical vortex in the spatiotemporal domain. High-quality edge detection of the incident spatiotemporal optical signal using the MDM differentiator is also demonstrated. Moreover, we investigate a “double” MDM structure consisting of two single (three-layer) MDM structures which implements the operation of spatiotemporal differentiation of the second order. By considering the generation of an optical pulse with several phase singularities and studying the edge-detection operation, we demonstrate that the operation of second-order spatiotemporal differentiation is implemented with high accuracy. The obtained results may find application in the creation of analog optical computing and optical information processing systems.

Floquet engineering can induce complex collective behavior and an interesting synthetic gauge field in quantum systems through temporal modulation of the system parameters by periodic drives. Using a Floquet drive on frequencies of the magnon modes, we realize a chiral state transfer in a cavity-magnonic system. The time-reversal symmetry is broken in such a promising platform for coherent information processing. In particular, the photon mode is adiabatically eliminated in the large-detuning regime and the magnon modes under conditional longitudinal drives can be indirectly coupled to each other with a phase-modulated interaction. The effective Hamiltonian is then used to generate chiral currents in a circular loop, whose dynamics is evaluated to measure the symmetry of the system Hamiltonian. Beyond the dynamics limited in the manifold with a fixed number of excitations, our protocol applies to continuous-variable systems with arbitrary states. Also, it is found to be robust against systematic errors in the photon-magnon coupling strength and Kerr nonlinearity.

We use a numerical method combining the cluster mean-field theory and the matrix product state to study the ground-state phase diagram of a one-dimensional two-qubit Dicke-Hubbard model with $XY$ qubit-qubit interaction. When counterrotating wave terms (CRTs) in the qubit-cavity coupling are neglected, we observe a rich phase diagram including a quantum phase transition between the Mott-insulating phase and the superfluid phase. This phase transition can be either first-order or the second-order type depending on whether the total angular momentum changes across the phase boundary. The finite qubit-qubit interaction gives rise to the emergence of two quantum triple points at which three different phases coexist. The structure of this phase diagram remains unchanged even when CRTs are considered.

Implementation of high-speed quantum key distribution (QKD) has become one of the major focuses in the field, which produces a high key-generation rate for applications. To achieve high-speed QKD, tailored techniques are developed and employed to quickly generate and detect quantum states. However, these techniques may introduce unique loopholes to compromise the security of QKD systems. In this paper, we investigate the loopholes of the self-differencing (SD) avalanche photodiode (APD) detector, typically used for high-speed detection in a QKD system, and demonstrate experimental testing of the SD APD detector under strong-pulse-illumination attack. This attack presents blinding stability and helps an eavesdropper to learn the secret key without introducing extra quantum bit rate error. Based on this testing, we propose a set of criteria for protecting SD APD detectors from the strong-pulse-illumination attack.

In the work by Rebouças and Brandão [Phys. Rev. A 104, 063514 (2021)] the authors compute the scattering amplitude for a $\mathcal{P}\mathcal{T}$-symmetric double-$\delta$-function potential in three dimensions by invoking the far-zone approximation and summing the resulting Born series. We show that the analysis of this paper suffers from a basic error. Therefore, its results are inconclusive. We give an exact closed-form expression for the scattering amplitude of this potential.

We investigate the Loschmidt echo in a one-dimensional spin chain having Kitaev-type interaction in constant and kicked magnetic fields. The Loschmidt echo for the initial states having different magnon excitations shows long-time revivals for smaller chains and has short-time revival peaks for the longer chains. The system near the critical point shows peculiarly long-time revival peaks of the Loschmidt echo for relatively larger chains. The presence of a magnon in the initial state affects the Loschmidt echo revival peaks. The momentum distribution function exhibits maxima for a few momenta that are associated with the momentum of the magnon excitation present in the initial states. The probability maxima decay as $O(1/N)$ with the system size. For the Hamiltonian with kicked magnetic fields, the Loschmidt echo depends on the kick period. For a special kick period, the Loschmidt echo shows no evolution at all irrespective of the system size.

In this paper we provide a genuine relativistic quantum teleportation protocol the classical communication component of which makes use of relativistic causal propagation of a quantum field. Consequently, the quantum teleportation is fully relativistic by construction. Our scheme is based on the Unruh-DeWitt qubit detector model, where the quantum state being teleported is associated to an actual qubit rather than the field mode considered by Alsing and Milburn [P. M. Alsing and G. J. Milburn, Phys. Rev. Lett. 91, 180404 (2003)]. We show that the existing works in (relativistic) quantum information, including good definitions of one-shot and asymptotic channel capacities, as well as algebraic formulation of quantum field theory, already provide us with all the necessary ingredients to construct a fundamentally relativistic teleportation protocol in a relatively straightforward manner.

Random quantum circuit sampling, a task to sample bit strings from a random quantum circuit, is considered a suitable benchmark task to demonstrate the outperformance of quantum computers even with noisy qubits. Recently, random quantum circuit sampling was performed on the Sycamore quantum processor with 53 qubits [Nature (London) 574, 505 (2019)] and on the Zuchongzhi quantum processor with 56 qubits [Phys. Rev. Lett. 127, 180501 (2021)]. Here, we analyze and compare the statistical properties of the outputs of the random quantum circuit sampling by the Sycamore and Zuchongzhi processors. Using the Marchenko-Pastur law of random matrices of bit strings and the Wasssertein distances between bit strings, we find that the statistical properties of Sycamore bit strings are quite different from those of Zuchongzhi bit strings, while both processors score similar values of linear cross-entropy fidelity for random circuit sampling. Some bit strings sampled by the Zuchongzhi processor pass the NIST random number tests while both Sycamore and Zuchongzhi processors show similar patterns in the heat maps of bit strings. Zuchongzhi bit strings are much closer to classical uniform random bits than those of Sycamore. It is shown that the statistical properties of bit strings of both random quantum circuits change little as the depth of the random quantum circuits increases. Our findings raise a question about the computational reliability of noisy quantum processors because two quantum processors with similar noise levels and similar qubit structures produced statistically different outputs for the same random quantum circuit sampling.

Demonstrations of quantum advantage have largely focused on computational speedups and on quantum simulation of many-body physics, limited by fidelity and the capability of current devices. Discriminating laser-pulse-modulated classical-communication code words at the minimum allowable probability of error using universal-quantum processing presents a promising parallel direction, one that is of both fundamental importance in quantum state discrimination and technological relevance in deep-space laser communications. Here we present an experimental realization of a quantum joint detection receiver for binary phase shift keying modulated code words of a 3-bit linear tree code using a recently proposed quantum algorithm: belief propagation with quantum messages. The receiver, translated to a quantum circuit, was experimentally implemented on a trapped-ion device—the recently released Honeywell LT-1.0 system using ${}^{171}\mathrm{Yb}^{+}$ ions, which possesses all-to-all connectivity and midcircuit measurement capabilities that are essential to this demonstration. We conclusively realize a previously postulated but hitherto not demonstrated joint quantum detection scheme and provide an experimental framework that surpasses the quantum limit on the minimum average decoding error probability associated with pulse-by-pulse detection in the low-mean-photon-number limit. The full joint detection scheme bridges across photonic and trapped-ion-based quantum information science, mapping the photonic coherent states of the modulation alphabet onto inner product-preserving states of single-ion qubits. Looking ahead, our work opens new avenues in hybrid realizations of quantum-enhanced receivers with applications in astronomy and emerging space-based platforms.

Coulomb effects in the high-energy part of photoelectron spectra produced through the above-threshold ionization of an atom subject to intense short laser pulses are analyzed. The analysis is carried out for linearly polarized and tailored bicircular laser pulses consisting of the fundamental and the second-harmonic components. We include the Coulomb effects in the analytic semiclassical description of the photoelectron rescattering plateau using the recently developed adiabatic approach [Phys. Rev. A 104, 033109 (2021)] and the expression for the Coulomb phase derived for description of the high harmonic generation [J. Phys. B: At., Mol. Opt. Phys. 51, 144006 (2018)]. Comparisons between predictions of the analytic approach and numerical solutions of the time-dependent Schrödinger equation are presented. They allow the identification of the contribution of the Coulomb interaction to photoelectron spectra for different laser pulse forms. We show that the Coulomb effects appear more pronounced for bicircular fields, where they demonstrate considerable sensitivity to the photoelectron emission angle. Instead, for linear polarization, the Coulomb effect in the high-energy plateau is generally insignificant.

We theoretically explore a promising route to achieve the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a spin-imbalanced ultracold Fermi gas. In the current stage of cold atom physics, search for this exotic Fermi superfluid is facing two serious difficulties: One is the desperate destruction of the FFLO long-range order by FFLO pairing fluctuations, which precludes entering the phase through a second-order transition, even in three dimension. The other is the fierce competition with the phase separation into the BCS (Bardeen-Cooper-Schrieffer) state and the spin-polarized normal state. By including strong FFLO pairing fluctuations within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that the anisotropy of Fermi surface introduced by an optical lattice makes the FFLO state stable against the paring fluctuations. This stabilized FFLO state is also found to be able to overcome the competition with the phase separation under a certain condition. Since the realization of unconventional Fermi superfluids is one of the most exciting challenges in cold atom physics, our results would contribute to the further development of this field.

We present an approach to the Bose polaron based on the notion of quantum groups, also known as $q$-deformed Lie algebras. In this approach, a mobile impurity can be depicted as a deformation of the Lie algebra of the bosonic creation and annihilation operators of the bath, in which the impurity is immersed. Accordingly, the Bose polaron can be described as a bath of noninteracting $q$-deformed bosons, which allows us to provide an analytical formulation of the Bose polaron at arbitrary couplings. We derive its ground state energy in the phonon branch of the Bogoliubov dispersion and demonstrate that the previously observed transition from a repulsive to an attractive polaron occurs at the vicinity where the quantum group symmetry is broken. Furthermore, our approach has the potential to open up avenues in polaron physics by connecting it with seemingly unrelated research topics where quantum groups play an essential role, such as anyons.

Coherent quantum-noise cancellation (CQNC) can be used in optomechanical sensors to surpass the standard quantum limit (SQL). In this paper, we investigate an optomechanical force sensor that uses the CQNC strategy by cascading the optomechanical system with an all-optical effective negative-mass oscillator. Specifically, we analyze matching conditions and losses and compare the two possible arrangements in which either the optomechanical or negative-mass system couples first to light. While both of these orderings yield a sub-SQL performance, we find that placing the effective negative-mass oscillator before the optomechanical sensor will always be advantageous for realistic parameters. The modular design of the cascaded scheme allows for better control of the subsystems by avoiding undesirable coupling between system components while maintaining a performance similar to the integrated configuration proposed earlier. We conclude our work with a case study of a micro-optomechanical implementation.

The Hilbert hotel is an old mathematical paradox about sets of infinite numbers. This paradox deals with the accommodation of a new guest in a hotel with an infinite number of occupied rooms. Over the past decade, there have been many attempts to implement these ideas in photonic systems. In addition to the fundamental interest that this paradox has attracted, this research is motivated by the implications that the Hilbert hotel has for quantum communication and sensing. In this paper, we experimentally demonstrate the fractional-vortex Hilbert hotel scheme proposed by Gbur [Optica 3, 222 (2016)]. More specifically, we performed an interference experiment using the fractional orbital angular momentum of light to verify the Hilbert infinite hotel paradox. In our implementation, the reallocation of a guest in new rooms is mapped to interference fringes that are controlled through the topological charge of an optical beam.

The Markovian and non-Markovian dynamics of quantum coherence, including single-spin coherence, two-spin coherence, and its distribution (collective and localized coherence), are investigated in the $XX$ spin chain with three-spin interaction. We find that the single-spin and localized coherence do not decrease but increase with time, which makes the dynamic evolution of the two-spin coherence more robust than entanglement and quantum discord. The three-spin interaction can lower the values of quantum coherence in Markovian evolution and heighten the oscillation frequency of quantum coherence in non-Markovian evolution because it can speed up the flow of coherence information. Finally, the trade-off relation between the collective and the localized coherence is clearly shown in their dynamic evolution. Thanks to the coherence trade-off, we find the substitution of localized coherence for collective coherence in dominating the dynamics of the two-spin coherence.

Almost a century ago, it was proposed that solid hydrogen could become metallic at sufficiently high pressure. Five years back, scientists observed an insulator to metal transition in liquid deuterium experimentally, at around 300 $\mathrm{GPa}$. The present work discusses and demonstrates the metal-like behavior in elemental hydrogen under various plasma environments. At the onset, the Herzfeld criterion is invoked to examine such characteristics in such plasmas under multimegabar pressure. However, a thorough study using this condition can only explain the metal-like pattern in $s$-wave states under a shell-confined condition (the system is trapped inside two concentric spheres with inner and outer radii $R_a,R_b$). Moreover, using this criterion, it is not possible to explain such phenomena in (i) confined systems (involving all $\ell$) and (ii) for $\ell \ne 0$ states in a shell-confined environment. Here this criterion is modified to incorporate the environmental conditions (nuclear charge, screening constant, boundary conditions) by utilizing several independent and generalized scaling concepts. The present condition can interpret a metallic pattern in confined and shell-confined plasmas connecting $\ell \ge 0$ states. Further, a different descriptor is proposed therefrom. The role of pressure in defining such a descriptor is also examined. Pilot calculations are performed using Debye Hückel, exponential screen Coulomb potential, and ion-sphere plasmas. The relevance of the shell-confined model in the context of plasmas is elaborated. Additionally, an attempt is made to investigate the metallic character in H-like systems embedded in fullerene under high pressure.

We provide a framework to solve generic models describing the dissociation of multiple molecular Bose-Einstein condensates in a nonadiabatic regime. The competition between individual chemical reactions can lead to nontrivial dependence on critical components such as path interference and symmetries, thus, affecting the final distribution of atomic population. We find an analytical solution for an illustrative example model involving four atomic modes. When the system parameters satisfy $CPT$ symmetry, where $C$ is the charge conjugation, $P$ is parity, and $T$ is time-reversal symmetry, our solution predicts a population imbalance between atomic modes that is exponentially sensitive to system parameters. However, a weakly broken symmetry alters the population in each atomic mode and can reverse the population imbalance. Our solution also demonstrates a strong quantum correlation between atomic modes that leads to the spontaneous production of atoms in a multimode squeezed state. Moreover, in our framework, a time-dependent non-Hermitian quantum mechanics naturally manifests, which can alternatively be realized experimentally in photonic systems.

We experimentally and numerically demonstrate a method to generate multiply quantized superfluid circulation about an obstacle in highly oblate Bose-Einstein condensates (BECs). We experimentally achieve pinned superflow with winding numbers as high as 11, which persists for at least 4 s. Our method conceptually involves spiraling a blue-detuned laser beam, around and towards the center of the BEC, and is experimentally implemented by moving the BEC in a spiral trajectory around a stationary laser beam. This optical potential serves first as a repulsive stirrer to initiate superflow, and then as a pinning potential to transport the superfluid circulation within the BEC. The spiral technique can be used either to generate a high-winding-number persistent current, or for controlled placement of a cluster of singly quantized vortices of the same circulation. Thus, the technique may serve as a building block in experimental architectures to create on-demand vortex distributions in BECs.

Higher-order exceptional points (EPs) in non-Hermitian systems have attracted great interest due to their advantages in sensitive enhancement and distinct topological features. However, realization of such EPs is still a challenge because more fine-tuning parameters are generically required in quantum systems, compared to the second-order EP (EP2). Here, we propose a non-Hermitian three-mode optomechanical system in the blue-sideband regime for predicting the third-order EP (EP3). By deriving the pseudo-Hermitian condition for the proposed system, one cavity with loss and the other with gain must be required. Then we show that EP3 or EP2 can be observed when the mechanical resonator (MR) is neutral, loss, or gain. For the neutral MR, we find that two degenerate or two nondegenerate EP3s can be predicted by tuning system parameters in the parameter space, while four nondegenerate EP2s can be observed when the system parameters deviate from EP3s, which is distinguished from the previous study in the red-detuned optomechanical system. For the gain (loss) MR, we find that only two degenerate EP3s or EP2s can be predicted by tuning enhanced coupling strength. Our proposal provides a potential way to predict higher-order EPs or multiple EP2s and study multimode quantum squeezing around EPs using blue-detuned non-Hermitian optomechanical systems.