We study spin and density excitations of one-dimensional self-bound Bose-Bose droplets within Bogoliubov theory and show that spin excitations come alive, especially as the interspecies coupling is made less attractive. We argue that spin excitations are particularly relevant in the one-dimensional droplet regime, where droplets are realized within the mean-field stability regime, as has been confirmed by the quantum Monte Carlo simulations. As the interspecies coupling strength increases within the mean-field stability regime, spin modes ultimately fall below the particle-emission threshold, thus becoming observable in the droplet spectrum. We analyze the Bogoliubov model for both pseudospinor and population-imbalanced scalar mixtures, encompassing both the density and spin sectors. We corroborate our findings through variational analysis of density- and spin-breathing modes, which offers physical insight into the mode structure and independently validates the spectrum, as well as through real-time dynamics. Additionally, we compare our results with both Petrov's original theory, which considers the Lee-Huang-Yang (LHY) correction at the attractive edge of the mean-field stability regime, and a beyond-LHY description of Bose-Bose mixtures.

Nonequilibrium transport remains a central theme in modern physics, spanning from condensed matter to synthetic systems. Here, we investigate particle transport in an extended Aubry-André model with system-scale hopping, namely, nonlocal hopping with a range proportional to the system size, and uncover a metal-insulator coexistence regime in real space, where metallic and insulating spatial domains coexist within the same system and are separated by sharp spatial boundaries. In the insulating region, particles exhibit flat-band-like localization in the absence of quasiperiodic potentials, while a quasiperiodic potential induces distinct multipoint localization, different from conventional exponential localization. Meanwhile, particles can freely propagate and tunnel across spatially disconnected metallic domains separated by the insulating region. Beyond this coexistence phase, we identify unconventional gap-crossing domain-wall modes with comblike spatial profiles that mediate nonlocal, multipoint transport across separated metallic domains. Our findings reveal a rich interplay between localization, nonlocality, and transport in systems with nonlocal hopping.

We investigate the variability of the quantum coherences of an arbitrary density matrix, in a given basis. An easily computable quantity that captures this variability notion is proposed and a dual relationship with an appropriate entropy is observed, with coherence dispersion (CD) being maximized for intermediate values of disorder and vanishing for extremal entropy values. This is a prevalent signature of complexity quantifiers across different fields, from evolutionary biology to information science. In addition, the physical significance of CD is investigated in the context of entropy production in open systems. We also study a specific family of nonequilibrium systems for which coherence dispersion presents a peaked maximum for finite temperatures. The final part of the manuscript is dedicated to the application of CD as a key ingredient to establish quite general quantum speed limits in the Hilbert space.

We find that the seemingly disparate control approaches for classical and quantum continuous-variable systems can be unified via differential manifolds of the ancillary representations. For classical systems, the ancillary representation is defined by the time-dependent ancillary canonical variables resulting from symplectic transformation over the original canonical variables. Under the Hamilton-Jacobi theory, the ancillary canonical variables act as dynamical invariants to guide the system nonadiabatically through the entire phase space. The second quantization of the Liouville equation for the dynamical invariants leads to the Heisenberg equation for the relevant ancillary operators, which is found to be a sufficient condition to activate nonadiabatic passages toward arbitrary target states in both Hermitian and non-Hermitian systems and yield constrained exact solutions of the time-dependent Schrödinger equation. Using the non-Hermitian Hamiltonian rigorously derived from the Lindblad master equation, our theory is exemplified by the generation of single-mode squeezed states with a squeezing level about 29.3 dB and double-mode squeezed states with 20.6 dB, respectively.

We consider the problem of identifying the quantum spin states that are the optimal sensors of a given transformation, averaged over all possible orientations of the spin system. Our geometric approach to the problem is based on a fidelity criterion and is entirely general, encompassing any unitary transformation. This formalism leads to a universality result: For any value of the spin, there exists a zero-measure subset of states that can be the optimal sensors for certain transformations and the worst sensors for others, and this set does not depend on the transformation under consideration. In other words, some spin states are simply the best (or worst) sensors, regardless of what they detect.

We investigate the performance of the charger-mediated quantum battery modeled by a two-qubit system. One of the qubits acts as the battery and the other acts as the charger, which is subjected to a reservoir. We derive the time-local master equation in Lindblad form with a time-dependent dephasing rate. The dephasing rate may be negative in the early stage of the charging process and thus indicate the presence of the memory effect. We find that such early-stage memory effect could increase the maximal ergotropy of the battery compared with the one under Markovian approximation with the corresponding asymptotic dephasing rate. The enhancement of the performance is explained by means of the non-Markovian quantum jumps. Moreover, a discrete-time scheme of the measurement-enhanced quantum battery is proposed in a quantum circuit with global and random local operations.

Dynamic quantum circuits generate states that depend on the measurement results obtained during circuit execution. To date such a quantum computing model has mainly been implemented with qubit-based superconducting hardware utilizing reset operations and classical logic. Here we develop a model of optical reset by using time-bin self-looped interferometers demonstrated in recent experiments. Synchronizing the optical reset with a simple classical device storing history of measurement results allows one to decrease uncertainty of future measurements, which suggests new possibilities for constructing dynamical circuits on optical platforms. Information flow with significant multitime correlations and memory depth is identified through distinct information-theoretic measures. We discuss potential applications of the proposed reset model, including the realization of boson sampling and experimental tests of the quantum-mechanical formulation of Landauer's principle.

Simulating the dynamics of non-Hermitian quantum systems is essential for probing the frontiers of modern physics. However, existing quantum algorithms are fundamentally constrained by the probability loss inherent to probabilistic postselection, extensive quantum circuit depths, and the substantial overhead of ancillary qubits. Herein, we introduce a hybrid quantum-classical algorithm grounded in quantum circuit Monte Carlo (QCMC) to concurrently address both limitations. By reformulating nonunitary evolution as statistical sampling of Hamiltonian dynamics, our framework alleviates the probability loss issue induced by postselection, achieves a reduction in quantum circuit depth, and requires at most one additional ancillary qubit. This general and flexible approach is capable of simulating arbitrary, time-dependent, and even nondiagonalizable Hamiltonians, providing a unified solution for diverse problems including parity-time ($\mathcal{PT}$)-symmetric systems, open quantum systems usually governed by completely positive trace-preserving (CPTP) maps, as well as non-CPTP maps. We demonstrate our method on a many-body open system, achieving excellent agreement with exact solutions. This work provides a resource-efficient pathway for exploring different phenomena in complex non-Hermitian systems on near-term quantum devices.

We present the simultaneous trapping and cooling of ${}^{85}\mathrm{Rb}$, ${}^{87}\mathrm{Rb}$, and ${}^{133}\mathrm{Cs}$ in a triple-species magneto-optical trap (MOT). This demonstration is obtained using an all-fiber compact and robust laser system based on telecom 1.5- and 2-$\mu \mathrm{m}$ technologies. We characterize the two-body interspecies losses in the double- and triple-species MOT. We calculate the two-body interspecies trap-loss coefficient for the ${}^{85}\mathrm{Rb}\text{−}{}^{133}\mathrm{Cs}$ and the ${}^{87}\mathrm{Rb}\text{−}{}^{133}\mathrm{Cs}$ pairs, representing a variation of less than 7% in atom number. No losses are observed between ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ in our experimental conditions. We find that interspecies interactions inside the triple-species MOT do not prevent trapping and cooling ${10}^8$ atoms for each species, making our system suitable for future multispecies cold-atom interferometry applications.

The nonadiabatic wavepacket dynamics is used to calculate the excitation and quenching cross sections in the ${\mathrm{N}}^{+}\left({}^3\mathrm{P}\right)–\mathrm{He}\left({}^1\mathrm{S}\right)$ and ${\mathrm{N}}^{+}({}^1\mathrm{D}$)–He(${}^1\mathrm{S}$) collisions using potential energy curves and spin-orbit coupling elements calculated ab initio. The results are verified by comparison with cross sections calculated by solving the time-independent coupled Schrödinger equation with the logarithmic derivative method, as well as with cross sections calculated semiclassically using the Landau-Zener method. The comparison shows that the wavepacket dynamics can be used for both the excitation and the quenching processes at energies ranging from 0.01 to 0.02 Hartree above the activation energy up to the energies of the order of 10 Hartree.

Programable photonic meshes provide a promising platform for analog matrix-vector multiplication, but their scalability is often limited by the large phase-shift ranges required in universal interferometer circuits. We introduce a perturbative programing method that operates the circuit near a fixed reference configuration and realizes the target transformation through interferometric subtraction, thereby reducing the required programable phase excursion. We develop this approach for photonic matrix-vector multiplication architectures based on universal unitary meshes and for low-depth nonunitary constructions based on sums of unitaries. We identify favorable reference configurations through a local conditioning criterion, analyze the phase statistics obtained for random target matrices, and show that perturbative programing produces phase distribution shrinking as the matrix size increases. Identified reference point allows for the subtraction to be carried out electronically after detection, avoiding the interferometric subtraction loss. We further quantify the trade-off between the reduced phase range and the intrinsic overhead introduced by the subtraction architecture and show that for sufficiently lossy phase shifters the reduced phase range can compensate for this penalty. These results identify perturbative programing as a conditional but potentially useful route toward more scalable programable photonic matrix processors.

Integrated single-photon filters capable of spectral purification and mode selection are essential components for scalable quantum photonics. However, achieving on-chip filters with sharp roll-offs and high reconfigurablity remains challenging in quantum photonics, due to the requirement of a large number of accurately coupled high-Q stages. We propose a Bragg grating chain of quantum emitters as an on-chip spectral filter with a near-rectangular response, enabling efficient suppression of sideband emission from solid-state single-photon sources without loss of brightness. The proposed architecture allows continuous tuning of both the central frequency and bandwidth. Crucially, spectral filtering of a weak coherent-state pulse by the atomic chain significantly reduces the two-photon component, demonstrating strong nonlinearity at the single-photon level. This nonlinear filter offers a promising route to simultaneously enhance the indistinguishability and purity of quantum resources.

Coherent quantum noise cancellation (CQNC) suppresses radiation-pressure backaction noise by introducing an auxiliary oscillator with an effective negative mass. In this context, once shot noise is further reduced by increasing the intracavity field intensity, the noise associated with the auxiliary system ultimately limits further improvements in sensitivity. To overcome this limitation, we propose and investigate a hybrid cavity-optomechanical weak-force sensing scheme that combines CQNC and mechanical-mode softening. We show that mechanical-mode softening exponentially increases the mechanical zero-point displacement scale, thereby enhancing the mechanical response to an external force and further amplifying the corresponding weak-force-induced optical output signal. As a result, the signal-to-noise ratio is substantially improved, enabling the limitation imposed by the auxiliary-system noise on the sensing sensitivity to be overcome. Meanwhile, mechanical-mode softening strengthens the effective optomechanical coupling, and the synergistic interaction between the enhanced optomechanical coupling and the signal-amplification effect leads to a dual suppression of optical shot noise. Therefore, unlike conventional CQNC schemes that rely on strong driving to reduce shot noise, our scheme can efficiently suppress shot noise over a broad frequency range even at relatively low intracavity field intensities. As a result, it achieves force sensitivities far exceeding the standard quantum limit (SQL) over a broad bandwidth. These results establish a practical route toward broadband optomechanical weak-force sensing beyond the SQL.

Quantum reference frame transformations have been proposed to provide a means by which to translate descriptions of quantum systems relative to each other. At present, there are several differing frameworks for describing quantum reference frames, with concomitantly different transformation rules. Here we investigate a simple example of three qubit systems with ${\mathbb{Z}}_2$ symmetry in order to analyze physical and conceptual distinctions between three contemporary approaches to quantum reference frames—dubbed perspective-neutral, extra particle, and operational. By constructing two three-qubit states that are indistinguishable by observables relative to one frame but are distinguishable by observables relative to another, we show that each of the three approaches provides a distinct transformation rule, which may be understood to reflect differing attitudes towards the global state and the information that can be accessed by each frame. This helps us to shed light on the physical meaning of each approach and the contexts in which they may be most naturally applied.

We address the task of converting $n$ uses of an unknown unitary transformation into a quantum state (i.e., storage) and later retrieval of the transformation. Specifically, we consider the case where the unknown unitary is selected with equal prior probability from two options. First, we prove that the optimal storage strategy involves the sequential application of the $n$ uses of the unknown unitary, and it produces the optimal state for discrimination between the two possible unitaries. Next, we show that incoherent “measure-and-prepare” retrieval achieves the maximum fidelity between the retrieved operation and the original (qubit) unitary. We then identify the retrieval strategy that maximizes the probability of successfully and perfectly retrieving the unknown transformation. In the regime in which the fidelity between the two possible unitaries is large the probability of success scales as $P_{\text{succ}}=1-O\left(n^{-2}\right)$, which is a quadratic improvement with respect to the case in which the unitaries are drawn from the entire unitary group $U\left(d\right)$ with uniform prior probability. Finally, we present an optical experiment for this approach and assess the storage and retrieval quality using quantum tomography of states and processes. The results are discussed in relation to nonoptimal measure-and-prepare strategy, highlighting the advantages of our protocol.

We study a modified fixed-point version of the quantum approximate optimization algorithm (fpQAOA), where parameters are trained classically on small instances and then transferred to larger problems. Our scheme combines three ingredients: (i) targeting approximate solutions via a prescribed approximation ratio (AR), (ii) scaling the circuit depth linearly with the problem size using a two-parameter sin-cos angle encoding, and (iii) normalizing quadratic unconstrained binary optimization (QUBO) Hamiltonians by their Frobenius norm. Noiseless numerical simulations (for system sizes up to 30 qubits) across a variety of random QUBO ensembles show that with these modifications the median number of quantum circuit runs (“shots”) required to achieve AR = 0.95 counterintuitively decreases towards a nearly constant value as the problem size increases, while the per-shot time remains polynomial. Extrapolation of this finite-size behavior is consistent with an effectively constant sampling complexity. Moreover, removing any single component of the scheme restores rapid growth of the required number of shots, highlighting the synergistic nature of the three modifications. These empirical findings suggest that fpQAOA, equipped with the proposed protocol, may achieve scalable approximate performance with polynomial-depth circuits for the considered problem classes.

The Casimir-Polder force acts on polarizable particles due to quantum fluctuations of the electromagnetic field that are modified by the presence of material bodies. We investigate the Casimir-Polder interaction for atoms near cylindrical fibers with hollow cores. This geometry represents one of the archetypal configurations encountered in numerous experimental setups designed to control and manipulate atoms in fundamental and quantum technological applications. Specifically, we analyze how the interplay of both geometrical and material-related length scales characterize the interaction, emphasizing the impact of the shell thickness. We develop a flexible and fast-converging numerical scheme for evaluating the interaction over a wide range of atom-cylinder separations at both zero and finite temperature. Furthermore, we provide a detailed analytical investigation of how various material properties modify the Casimir-Polder potential. Finally, we analyze and discuss a number of limiting cases and compare numerical computations with corresponding analytical asymptotic expressions. In particular, in this geometry the Casimir-Polder potential is able to distinguish between an Ohmic and non-Ohmic description of conductors. One of the most significant outcomes of our work is that the shell thickness emerges as a useful parameter for controlling the interaction, opening avenues for both fundamental physics and applications in quantum technologies.

The energy correction associated with the self-energy diagram is the leading (in magnitude) and fundamental (in significance) contribution to the Lamb shift in few-electron atoms and ions. Conventional approaches to this correction rely on partial-wave expansion, which is a major limitation on the achievable accuracy. To elucidate the issue, we perform a comprehensive comparative analysis of partial-wave-expansion convergence between the Feynman and Coulomb gauges. We also implement and discuss several techniques for improving the convergence. Both the Green's function and finite basis set are employed in the calculations.

We theoretically investigate the physical principles underlying the attoclock method in strong-field ionization, with particular emphasis on the interpretation of the offset angle and the role of the Coulomb potential. Our analysis is based on the numerical solution of the time-dependent Schrödinger equation (TDSE) and several variants of the strong-field approximation (SFA) combined with quantum-orbit theory. Among the investigated models, the best agreement with the TDSE is obtained when the ionization probability and the initial conditions for the electron trajectory are determined using the SFA together with the saddle-point method. The corresponding final electron momentum is then calculated by classical propagation in the combined laser and Coulomb fields.