Realizing nonreciprocal scattering and absorption from subwavelength elements has several applications across the electromagnetic frequency spectrum. Here we show that gyromagnetic cylindrical rods with subwavelength dimensions exhibit strong absorption with diminished scattering in a narrow frequency band for an incident plane wave. We show that the strong narrowband absorption depends on the photonic spin of the incident wave. Further, it is shown that this spin-dependent absorption leads to nonreciprocal reflection and absorption characteristics of an array of gyromagnetic cylinders suspended over a reflecting plane. The theoretical claims of strong spin-dependent absorption in subwavelength elements is validated by experimentally demonstrating nonreciprocal absorption and transmission from a single gyromagnetic rod placed in a substrate integrated waveguide.

By numerically solving the nonlinear Schrödinger equation, we theoretically study the nonlinear propagation dynamics and self-healing properties of elliptical Airy beams (EABs) propagating in water under Kerr nonlinearity. Compared with linear propagation, EABs exhibit extended propagation distances and enhanced stability in nonlinear media. Furthermore, particular emphasis is placed on the impact of Kerr nonlinearity strength on the propagation and self-healing properties of EABs. By varying the input power, it is found that EABs within a moderate power range can propagate longer distances while maintaining higher intensity and exhibit improved robustness after being blocked, indicating better self-healing performance. Based on this analysis, an explanation of the physical mechanism related to the enhanced self-healing capability by Kerr nonlinearity is proposed. Finally, we propose an optimal input power for EABs through a quantitative analysis of the impact of Kerr nonlinearity, enabling them to achieve the greatest propagation distance and maintain the highest stability. Our work provides new insights into the interaction between EABs and Kerr nonlinearity, with potential applications for long-distance laser transmission.

We propose the Caldeira-Leggett quantum time crystal in which the interaction between a quantum system and a viscoelastic sub-Ohmic environment leads to periodic revivals of quantum coherence. Using the Caldeira-Leggett model of quantum decoherence with a sub-Ohmic environment characterized by the spectral function $J\left(\omega \right)\propto {\omega }^s$ ($0\le s<1$), we discover that decreasing the sub-Ohmic parameter $s$ enhances the coherence lifetime of Gaussian wave packets, with the purity (i.e. coherence) decaying as $t^{-s/2}$ throughout the regime $0\le s<1$. A revival oscillation of the purity with lifetime $e^{2/s}$ occurs in the deep sub-Ohmic regime ($s<0.1$), which we call the Caldeira-Leggett quantum time crystal. We reveal that such oscillations originate from the fact that an environment in the deep sub-Ohmic regime behaves as a gel-like fractal medium with viscosity and elasticity, suggesting the significance of fractal gel-like environments in vivo and in information processing.

The thermodynamic uncertainty relation (TUR) is a fundamental principle in nonequilibrium thermodynamics that relates entropy production to fluctuations in a system, establishing a trade-off between the precision of an observable and the thermodynamic cost. Investigating TUR violations challenges classical thermodynamic limits, offering the potential for improved precision-entropy trade-offs, which is crucial for enhancing performance and optimization in quantum technologies. In this work, we investigate the thermodynamic uncertainty relation within a quantum collisional model, which offers the advantage of discretizing interactions into successive collisions with auxiliaries, allowing for precise tracking of dynamics and the incorporation of memory effects and non-Markovian behavior. We consider three types of dynamics in the collisional model: one is Markovian evolution, achieved by taking the continuous-time limit and imposing the stability condition, while the other two are non-Markovian dynamics—one arising from increasing the collision time between the system and the auxiliaries, and the other from incorporating interactions between the auxiliaries. For the Markovian dynamics, we examine the classical and quantum TUR bounds in the nonequilibrium steady-state regime, and also the finite-time TUR bound. We identify two distinct regimes of classical TUR violation: in some cases, the maximum violation occurs in the steady state, while in others, it is necessarily transient—appearing only at early times and vanishing with further evolution. For the two non-Markovian approaches, we find that both the degree and type of non-Markovianity crucially affect TUR violations. The second approach shows more pronounced violations during transient times, while the first approach has much stronger violations in the steady-state regime for a certain parameter window.

The absolutely separable (resp. positive partial transpose) states remain separable (resp. PPT) under any global unitary operation. We present a compact form of the extreme points in the sets of absolutely separable states and PPT states in two-qubit and qubit-qudit systems. The results imply that each extreme point has at most three distinct eigenvalues. For a two-qubit system, we also quantify geometric probabilities of absolute separability. We establish a necessary and sufficient condition for determining extreme points of the set of absolutely PPT states in two-qutrit and qutrit-qudit systems, expressed as solvable linear equations. We also demonstrate that any extreme point in a qutrit-qudit system has at most seven distinct eigenvalues. We introduce the concept of robustness of nonabsolute separability. It quantifies the minimal amount by which a state needs to mix with other states such that the overall state is absolutely separable. We show that the robustness satisfies positivity, invariance under unitary transformation, monotonicity, and convexity, so it is a good measure within the resource theory of nonabsolute separability. Analytical expressions for this measure are given for pure states in an arbitrary system and rank-two mixed states in a two-qubit system. Furthermore, we extend the study of absolutely separable and PPT states to multipartite systems, proving that three-qubit absolutely separable states form a proper subset of three-qubit absolutely PPT states, with symmetry under subsystem permutations and stricter spectral constraints.

Adiabatic control is a fundamental technique for manipulating quantum systems, guided by the quantum adiabatic theorem, which ensures suppressed nonadiabatic transitions under slow parameter variations. Quantum annealing, a heuristic algorithm leveraging adiabatic control, seeks the ground states of Ising spin-glass models and has drawn attention for addressing combinatorial optimization problems. However, exponentially small energy gaps in such models often necessitate impractically long runtime to satisfy the adiabatic condition. Despite this limitation, improving the quality of approximate solutions remains crucial for practical applications. The quantum adiabatic brachistochrone provides a method to enhance adiabaticity by minimizing an action representing nonadiabaticity via the variational principle. While effective, its implementation requires detailed energy spectra, complicating its use in quantum annealing. Shortcuts to adiabaticity by counterdiabatic driving offer alternative approaches for accelerating adiabatic processes. However, the theory of shortcuts to adiabaticity often faces challenges such as nonlocal control requirements, high computational cost, and trade-offs between speed and energy efficiency. In this work we propose a quantum adiabatic brachistochrone protocol tailored for quantum annealing that eliminates the need for energy spectrum information. Our approach builds on advancements in counterdiabatic driving to design efficient parameter schedules. We demonstrate the effectiveness of our method through numerical simulations on the transverse-field Ising chain and axial next-nearest-neighbor Ising models.

In this paper, we correct a mistake we made in [Phys. Rev. Lett. 122, 190402 (2019)] and [Phys. Rev. A 103, 012213 (2021)] regarding the Wigner function of the so-called smoothed Weak-Valued state (SWV state). Here smoothing refers to estimation of properties at time $t$ using information obtained in measurements both before and after $t$. The SWV state is a pseudostate (Hermitian but not necessarily positive) that gives, by the usual trace formula, the correct value for a weak measurement preformed at time $t$, i.e., its weak value. The Wigner function is a pseudo probability distribution (real but not necessarily positive) over phase space. A smoothed (in this estimation sense) Wigner distribution at time $t$ can also be defined by applying classical smoothing for probability distributions to the Wigner functions. The smoothed Wigner distribution gives identical means for the canonical phase-space variables as does the SWV state. However, contrary to the assumption in the above references, the Wigner function of the SWV state is not the smoothed Wigner distribution.

In a previous work [Avritzer and Brun, Phys. Rev. A 109, 032401 (2024)] we discussed a theoretical grounding for classical steganography using quantum Fock and coherent states in an optical channel, building on previous work by Wu et al. [Opt. Express 14, 3738 (2006); Opt. Express 21, 2065 (2013)]. In that work, we discussed protocols that disguise communications to mimic the thermal state of a harmonic oscillator. In this work we extend this to transmission of quantum information and demonstrate the utility of steganographic entanglement sharing in practical contexts like nonclassical state teleportation. We show that non-Gaussian entangled resources can be transmitted covertly while mimicking a Gaussian thermal state and still retain sufficient entanglement to violate a qubit Bell inequality, even with an eavesdropper present. We also examine the performance of other quantum protocols that utilize entanglement, such as dense coding and teleportation of unentangled but nonclassical states, to determine the effectiveness of the steganographic entanglement sharing under a variety of active and passive eavesdropper conditions.

Recent precision isotope shift spectroscopy experiments have reported King plot nonlinearities that may uncover previously hidden nuclear properties or hint at new force-carrying bosons. Interpreting the data requires many isotope pairs, but sufficient stable spin-zero nuclei are not always available. In this paper we consider the use of isotopes with nonzero nuclear spin for these studies. We confirm that observed King plot nonlinearity in strontium is explained by a second-order hyperfine shift in the fermionic nucleus, and we discuss the limitations of using Yb isotopes with nonzero spin in King plot studies. By introducing a specific difference of two isotopes with nuclear spin, we allow for a promising addition to future experimental schemes.

We present the results of theoretical and experimental investigation of optical linear dichroism signals caused by alignment, and arising under the action of pumping with linearly polarized light in a magnetic field transverse to the pumping in an ensemble of hydrogenlike (including alkali) atoms. Several simple models implying mixing of Zeeman sublevels of the excited state are considered, and common patterns among them are identified. It is shown that in the general case the properties of the medium causing dichroism can be characterized by only two parameters of the longitudinal and transverse linear polarizability, while in the case of complete mixing, by a single linear polarizability parameter. Expressions that relate these parameters to the transition probabilities are provided, as is an extension of the theory for the case of incomplete mixing of sublevels of the excited state. The single-beam scheme for recording linear dichroism signals, which is of greatest interest from a practical point of view, is investigated. It is shown that incomplete mixing can lead to inversion of dichroism signals in systems including more than one optical transition. A comparison with the results of an experimental study of dichroism in alkali-metal (cesium) vapors is made.

We theoretically investigate the spin-mixing dynamics of a spinor BEC subject to a time-dependent confining potential. Our study provides a theory framework for the experimental results reported in Phys. Rev. A 109, 043309 (2024). We exploit the disparity of energy scales associated with the spatial and internal (spin) degrees of freedom under typical experimental conditions to develop an effective few-mode description of the spin dynamics. Our model demonstrates how the details of the potential, such as driving frequency and amplitude, can be used to independently control spin-changing and spin-preserving collision processes as well as the effective Zeeman energy of the internal states. We obtain the dynamical phase diagram of the effective model and discuss how its structure is altered relative to a spinor BEC with frozen spatial degrees of freedom. The applicability of our effective model is verified through Gross-Pitaevskii simulations that capture the interplay of spin and spatial degrees of freedom, and we identify parameter regimes that can be feasibly explored by future experiments. Our findings highlight the utility of dynamical confining potentials for the control of nonequilibrium spin-mixing dynamics in spinor BECs.

Engineering artificial gauge fields in synthetic lattices offers an efficient avenue for exploring topological states in cold atomic physics and photonics in recent years. Interactions between the system constituents in synthetic dimensions become long ranged, and can induce more striking features. Here, we report the experimental observation of asymmetric chiral currents induced by interactions in a synthetic zigzag ladder with tunable staggered flux, based on the laser-driven coupling of discrete momentum states of ultracold ${}^{133}\mathrm{Cs}$ atoms. We observe that the staggered flux induces the symmetric chiral dynamics along two legs of the zigzag ladder in a noninteracting regime. However, the flux-dependent chiral currents along the two legs become asymmetric with increasing interactions, and the chiral trajectory is changed dramatically under strong interactions. Our finding opens the way for future exploration of exotic dynamics dictated by an interplay of inhomogeneous gauge fields and interactions in synthetic lattices.

The paradigmatic Lugiato-Lefever model describes the electric field envelope in a ring cavity filled with a Kerr medium and driven by a coherent injected laser beam. This model is applied to the formation of frequency combs associated with localized structures in micro and macroresonators. Including temporal filtering, we derive a generalized Lugiato-Lefever equation. This equation includes diffusion, linear, and nonlinear convection, and third-order dispersion with purely imaginary coefficients. Multiscale analysis enables us to disregard higher-order terms, such as nonlinear convection and third-order dispersion. We investigate the formation of periodic and localized structures resulting from the combined action of temporal spectral filtering effect together with Kerr nonlinearity, pumping, dissipation, and frequency detuning. We show that spectral filtering reduces the intensity of the output field and increases the period of traveling solutions. Similarly, the maximum intensity of moving localized structures, often called dissipative solitons, is reduced. In addition, we show that the threshold associated with breathers is shifted toward large input intensities and that the associated domain of existence is significatively reduced. We show that, when the drift is absent, dissipative solitons exhibit a homoclinic snaking bifurcation. Increasing the strength of the temporal filter reduces the pinning range. The presence of the drift breaks the homoclinic snaking and transforms it into isolas of solutions.

An entangled state is called $m$ resistant if it remains entangled after losing an arbitrary subset of $m$ particles but becomes fully separable after losing any number of particles larger than $m$. Quinta et al. [Phys. Rev. A 100, 062329 (2019)] conjectured that for any $N$-particle systems, there always exists an $m$-resistant pure state of some local dimension $d$. In this paper, we give two general constructions of $m$-resistant pure states. One is from the mixtures of Dicke states, which provides strong $(N-k)$-resistant pure $N$-qubit states with $k=4$ or 5. The other is from classical error-correcting codes, which provides new $m$-resistant pure qudit states for certain $m<N/2$.

Quantum simulation, the study of strongly correlated quantum matter using synthetic quantum systems, has been the most successful application of quantum computers to date. It often requires determining observables with high precision, for example when studying critical phenomena near quantum phase transitions. Thus, readout errors must be carefully characterized and mitigated in data postprocessing, using scalable and noise-model agnostic protocols. We present a readout error-mitigation protocol that uses only single-qubit Pauli measurements and avoids experimentally challenging randomized measurements. The proposed approach captures a very broad class of correlated noise models and is scalable to large qubit systems. It is based on a complete and efficient characterization of few-qubit correlated positive operator-valued measures, using overlapping detector tomography. To assess the effectiveness of the protocol, observables are extracted from simulations involving up to 100 qubits employing readout errors obtained from experiments with superconducting qubits.

We propose a scalable protocol in the Rydberg atomic system to generate the Greenberger-Horne-Zeilinger (GHZ) states, where the qubits are encoded on the hyperfine ground levels. Our system is featured with off-resonant driving fields rather than strong Rydberg interaction sufficient for blockade. Closely it can follow the desired nonadiabatic passage during the time evolution and avoid the unwanted transition by imposing the passage-dependent and fast-varying global phase, that serves as an error correction to the universal quantum control. In our protocol, an $N$-qubit GHZ state is prepared in $N-1$ steps, which is found to be robust against both environmental noise and systematic deviation. Our protocol therefore provides an avenue toward large-scale entanglement, which is essential for quantum information processing and computation based on neutral atoms.

In this paper, we propose a nonlinear interferometer with feedback loops and explore its efficiency for phase estimation. We analyze two feedback schemes, one where both modes of the interferometer are fed back into the device and another where only one mode is fed back. The quantum Fisher information (QFI) for phase estimation in each feedback scheme increases with each feedback loop, and similar to the standard SU(1,1) nonlinear interferometer, phase estimation in this scheme is sensitive to photon loss when the inputs are vacuum state. In terms of resources, we show that, in the low-loss regime, our scheme performs better than the standard nonlinear interferometer. The feedback scheme provides the minimum phase variance when the unknown phase is small. We also provide a special case where the feedback scheme provided enhanced QFI even for large phase values. This is achieved by switching between squeezing and antisqueezing operators after every few loops.