Future quantum networks will enable the interconnection of multiple users distributed across vast geographic distances. Due to these large separations and limited physical resources, communication will often rely on multipath routing strategies, where physical resources are distributed across channels of varying lengths and delivered to end users. Efficient long-distance quantum communication therefore requires optimizing the allocation of these resources across available paths. In this work we introduce a performance-oriented approach to quantum network routing by extending the classical concept of quality of service (QOS) to the context of multipath quantum resource distribution. Unlike prior models that consider entanglement generation or quantum memory coherence in isolation, we investigate the interplay between path assignment strategies, coherence time constraints, and quantum error correction (QEC) and how they jointly impact end-to-end communication fidelity. We analyze both unencoded and encoded transmissions over aggregated network paths, quantifying the effects of resource allocation on transmission success. Our findings show that fidelity cannot be optimized independently of memory lifetimes and that while QEC can enhance performance under specific conditions, it also imposes additional constraints depending on network topology and path-length asymmetries. This work provides a foundation for developing QOS-aware quantum routing protocols that balance fidelity, throughput, and memory utilization, key considerations for near-term quantum repeater networks.

This paper develops and implements the stochastic projected Gross-Pitaevskii equation for spin-1 Bose gases, addressing key considerations for numerical simulations. As an application of the theory we explore equilibrium phases in a two-dimensional spin-1 gas, where quasi-long-range order emerges via a Berezinskii-Kosterlitz-Thouless transition. Our analysis includes the definition of superfluid densities for both mass and spin degrees of freedom in a manner suitable for implementation within a stochastic projected Gross-Pitaevskii equation simulation. We present a finite-temperature phase diagram for the ferromagnetic spin-1 Bose gas and identify three distinct superfluid phases: two exhibiting conventional Berezinskii-Kosterlitz-Thouless-like behavior and an additional phase that simultaneously supports independent mass and spin superflows. As temperature increases, the stability region of this additional phase shrinks. We provide a characterization of the phase transitions through consideration of the spin-component densities and the unbinding of multiple types of vortices. This work provides a foundation for further studies of nonequilibrium and finite-temperature phenomena in spinor Bose gases.

Metasurfaces have become a powerful platform for strengthening light-matter interactions. Recent research has shown that all-dielectric metasurfaces supporting high-quality ($Q$) factor resonances, such as anapole modes and quasibound states in the continuum (BIC), can significantly enhance third harmonic generation (THG) efficiency. Nevertheless, a key challenge remains: precisely controlling the $Q$ factor of these resonances is difficult, and maintaining a high-$Q$ factor across a wide range of incident angles is even more demanding. Here, we achieve substantial enhancement of THG by introducing perturbations within the metasurface. The guided modes below the light line can be folded into the light cone, resulting in high-$Q$ resonant modes. Compared with previous BICs, this folded resonance exhibits a persistent high-$Q$ characteristic in momentum space, which makes it possible to achieve angular insensitive nonlinear enhancement. The angular robustness of the proposed metasurface ensures consistent high nonlinear performance, making it a viable candidate for practical applications in nonlinear optics, such as frequency conversion and photonic signal processing.

We develop a comprehensive model for the effective two-photon density matrix produced by a parametric source of entangled-photon pairs under a variety of detector configurations commonly seen in a laboratory setting: two and four photon-number-resolving (PNR) and threshold detectors. We derive the probability of obtaining a single coincidence assuming Poisson-distributed photon pairs, nonunit detection efficiency, and dark counts; obtain the effective density matrix; and use this quantity to compute the fidelity of this state with respect to the maximally entangled ideal. The four-detector PNR case admits an analytic result valid for any combination of parameters, while all other cases leverage low-efficiency approximations to arrive at closed-form expressions. Interestingly, our model reveals appreciable fidelity improvements from four detectors as opposed to two but minimal advantages for PNR over threshold detectors in the regimes explored. Overall, our work provides a valuable tool for the quantitative design of two-photon experiments under realistic nonidealities.

We investigate the coherent single-photon scattering process in a topological waveguide coupled to a driven three-level atom in the $\mathrm{\Lambda }$ configuration, where two ground states are optically connected to a common excited state. We derive an analytical expression for transmittance by using the scattering formalism for three distinct sublattice sites (A, B, and AB) of the waveguide, which couple to the $\mathrm{\Lambda }$ system. We have demonstrated that the system's response is topology independent for A and B sublattice-site coupling and becomes topology dependent for AB sublattice-site coupling. In a weak-control-field regime, the system behaves as a perfect mirror in all of these configurations. Upon control field strength enhancement, the transmission spectrum evolves from Electromagnetically Induced Transparency to Autler-Townes splitting in A and B sublattice-site coupling. The manipulation of transmission from opaque to transparent holds the key mechanism of a single-photon switching. Furthermore, the topology-dependent AB sublattice configuration allows multiple-path-induced quantum interference, resulting in the sharper Fano line shape. This topological Fano resonance is not only tunable via the control field and coupling strength but also robust against structural imperfections due to the topological nature of the waveguide. These combined features such as tunability and topological protection make our system a promising platform for next-generation quantum technologies, including single-photon switches, high-resolution sensors, and communication devices.

Statistical quantifiers are generically required to contract under physical maps, following the intuition that information should be lost under noisy transformations. This principle is so important in statistics that it even allows one to derive uniqueness results based on it: the Chentsov-Petz theorem identifies the Fisher information metrics as the only family, on the space of probability distributions (or density matrices), that contracts under physical maps. This construction could suggest that statistical quantifiers are a derived concept, while the only fundamental objects are the physical evolutions. The aim of this work is to disprove this belief. To this end, we prove a statement dual to the Chentsov-Petz theorem, showing that among all possible linear maps, the only ones that contract the Fisher information (including, in the quantum case, any idle ancilla) are exactly the physical ones. This result proves that, contrary to the common opinion, there is no fundamental hierarchy between physical maps and canonical statistical quantifiers, as either of them can be defined in terms of the other.

We investigate the global and local performances of a quantum battery exposed to the depolarizing and bit-flip channels, respectively, using the correlation matrix method. The battery is initially prepared as the highest-energy state of a two-qubit Ising model with open boundary conditions and is discharged via interaction with local noise channels. We show that compared to the independent noise channels, the correlated noise channels provide varying degrees of protection for the global and local performances of the Ising battery. The correlated depolarizing channel not only protects the global energy, ergotropy, and capacity of the Ising battery, but also protects the local ergotropy and capacity, whereas the correlated bit-flip channel provides protection only for the global ergotropy and capacity of the Ising battery. In both the correlated depolarizing and bit-flip channels, the greater the correlation strength, the greater the protective effect. Furthermore, we also use the same method to investigate the protective effect of correlated channels on the performances of the multiqubit battery and general Heisenberg battery in noise channels.

Reservoir engineering makes it possible to adjust the dissipation of quantum systems by driving the reservoir. This inspires investigations on how to slow a system's damping via squeezing reservoirs. In this paper we characterize damping by the quantum reliability of quantum states. We analyze the reliability loss of the harmonic oscillators in a squeezed thermal reservoir, where the quantum states under consideration have garnered significant interest in recent experiments, such as the coherent state, the squeezed coherent state, and the cat state. We explore the optimal protocol to extend the lifetime of these quantum states by adjusting the squeezed parameters of the squeezed thermal reservoir in both the high-damping and low-damping regimes. In the low-damping regime, our findings suggest that the unique optimal protocol follows the phase-matching condition and is insensitive to the reservoir's temperature. We also evaluate the optimal condition for the large damping regime. Furthermore, we show that when the state parameters satisfy a specific condition, there exists an interval for the damping rate, within which a bifurcation of the optimal protocol occurs.

We present an application of the affine covariant integral quantization (ACIQ) [Adv. Oper. Theory 5, 901 (2020); Adv. Oper. Theory 7, 22 (2022)] to quantum mechanics on the punctured plane. The associated four-dimensional phase space is identified with the similitude group SIM(2), which comprises translations, rotations, and dilations of the plane. Due to the topology of the punctured plane, our quantization procedure gives rise to an affine vector potential. This potential can be interpreted as the Aharonov-Bohm (AB) gauge field produced by an infinite solenoid. This observation supports a reinterpretation of the AB effect: it emerges from the topological constraint imposed by the impenetrable coil rather than from an externally applied classical gauge field. In addition to this gauge structure, ACIQ also generates a repulsive, centrifugal-like scalar potential, a feature already encountered when applying ACIQ to motion on the half-line, whose phase space is the open half-plane. These results provide a new perspective on the AB effect, highlighting the central roles of topology and symmetry in quantum mechanics.

In this paper, we show how random fluctuations in the magnetic field gradient jitter the paths of a matter-wave interferometer randomly, hence, decohere the quantum superposition. To create a large spatial superposition with nanoparticles, we envisage embedding a spin in a nanoparticle as a defect and applying an inhomogeneous magnetic field as in a Stern-Gerlach-type experiment to create a macroscopic quantum superposition. Such matter-wave interferometers are the cornerstone for many new fundamental advancements in physics; particularly, adjacent matter-wave interferometers can use entanglement features to test physics beyond the standard model, test the equivalence principle, improve quantum sensors, and test the quantum nature of space-time in a laboratory. In particular, we study how white and flicker noise induces decoherence for a setup involving superconducting wires embedded in a chip. We show that to obtain a tiny spatial superposition of a nanometer separation, $\mathrm{\Delta }x\approx O\left({10}^{-9}\right)\mathrm{m}$ and to minimize decoherence, $\mathrm{\Gamma }\le O\left({\omega }_0/2\pi \right)$, where $\mathrm{\Gamma }$ is the dephasing rate and ${\omega }_0$ is the frequency of the oscillator, we will need current fluctuations to be $\delta I/I\le O\left({10}^{-8}\right)$, which is not impossible to obtain in superconducting wire arrangements. For such tiny fluctuations, we demonstrate that the Humpty-Dumpty problem in a matter-wave interferometer arising from a mismatch in position and momentum does not cause a loss in contrast.

Achieving high-precision detection of weak alternating signals in noisy environments is a fundamental challenge in metrology. Given that the lock-in amplification technique has been successfully applied to facilitate sensitive detection of various unknown weak signals, in this study, we investigate the extension of this technique, termed quantum lock-in amplification, for precise detection of an unknown alternating signal. With a generalized spin-oscillator hybrid quantum system, we show that the target signal driving the oscillator can be precisely locked in by modulating the $\pi$ pulses that drive the spin, while the amplification of the signal is simultaneously achieved by alternatively driving the probe (i.e., oscillator), similar to its classical counterpart. Unlike previous methods that utilize $\pi$ pulses solely for signal accumulation and amplification, the quantum lock-in amplification scheme proposed here simultaneously harnesses both the spin-oscillator entanglement resources and the quantum squeezing of the oscillator. Notably, the simultaneous use of entanglement and squeezing can achieve greater noise compression compared to the case using entanglement only. Consequently, the spin state, characterized by large phase accumulation, can be measured with significantly high sensitivity even under the noisy environment, allowing for precise locking and amplification of the unknown weak signal.

While abstract models of quantum computation assume a closed system of two-level states, practical quantum devices inevitably couple to the environment in some way, creating sources of noise. Understanding the tolerance to noise of specific quantum algorithms run on specific devices is important for determining the feasibility of quantum computing in the current noisy intermediate-scale quantum era. Of particular interest is understanding the noise sensitivity of these devices as more qubits are added to the system. Classical simulations are a useful tool to understand the effects of this noise, but direct classical simulations of open quantum systems are burdened by an exponentially growing cost in the number of qubits and a large local Hilbert space dimension. For one-dimensional, shallow circuits, using tensor networks can replace this exponential cost with a linear one and simulate far wider systems than what would normally be available. In this paper, we describe a tensor network simulation of a neutral atom quantum system under the presence of noise, while introducing a purity-preserving truncation technique that compromises between the simplicity of the matrix product state and the positivity of the matrix product density operator. We apply this simulation to a near-optimized iteration of the quantum approximate optimization algorithm on a transverse field Ising model in order to investigate the influence of large system sizes on the performance of the algorithm. We find that while circuits with a large number of qubits fail more often under noise that depletes the qubit population, their outputs on a successful measurement are just as robust under Rydberg atom dissipation or qubit dephasing as smaller systems. However, such circuits might not perform as well under coherent multiqubit errors such as Rydberg atom crosstalk. We also find that the optimized parameters are especially robust to noise, suggesting that a noisier quantum system can be used to find the optimal parameters before switching to a cleaner system for measurements of observables.

The finite operators are derived for the relativistic recoil corrections of order $m{\alpha }^6(m/M)$ to spin-averaged energy levels in hydrogenlike atoms and ions in two- and three-body formalisms beyond the adiabatic approximation. Results are presented in a form suitable for numerical evaluation.

The rovibrational transition frequencies of molecular hydrogen ions can be accurately computed using ab initio nonrelativistic quantum electrodynamics. A subset of the fundamental constants is required input. We analyze how, once upcoming ultra-high-accuracy spectroscopy data have been obtained, that subset of constants can be determined with greater accuracy. Our analysis shows that under realistic assumptions the uncertainties of the mass ratios of the proton, deuteron, and triton, relative to the electron, and of the triton charge radius can be reduced more than 100-fold compared to today (CODATA 2022). Furthermore, the Rydberg constant, as well as the proton and deuteron charge radii, can be determined with uncertainties similar to those of today, but solely using data from electronic systems. The implications are discussed.

Interaction plays a key role in the mixing properties of a multicomponent system. The miscibility-immiscibility transition (MIT) in a weakly interacting mixture of Bose gases is predominantly determined by the strengths of the intra- and intercomponent two-body contact interactions. However, in the strongly interacting regime interaction-induced processes become relevant. Despite previous studies on bosonic mixtures in optical lattices, the effects of the interaction-induced processes on the MIT remains unexplored. In this work, we investigate the MIT in the strongly interacting phases of two-component bosonic mixture trapped in a homogeneous two-dimensional square optical lattice. Particularly we examine the MIT condition when both the components are in superfluid, one-body staggered superfluid, or supersolid phases. Our study uncovers that MIT condition is significantly shaped by the interplay of competing nonlocal intra- and intercomponent density-induced tunneling effects, as well as off-site interactions. Notably, we demonstrate that the MIT condition for the staggered superfluid phase exhibits an inequality that is inverted compared to the conventional MIT condition associated with superfluid or supersolid phases driven by local contact interactions. In addition, we present the phase diagram of the Bose-Hubbard model incorporating nonlocal processes, derived using a site-decoupling mean-field approach with the Gutzwiller ansatz. Our study contributes to the better understanding of miscibility properties of multicomponent systems in the strongly interacting regime.

We have characterized the effects of thermal phase fluctuations in narrow rings of ultracold Fermi superfluids in the unitary regime and demonstrated that matter-wave interference may be reliably used to measure angular variations in the phase difference between two concentric, coplanar superfluid rings. When one of the rings is cut with a barrier, we find that phase fluctuations around that ring can be large enough to degrade the reliability of protocols for control of quantized circulation states. The effect is greatest when the one-dimensional density is low and the coupling between rings is negligible. In this limit, our measurements are consistent with a correlation length smaller than the system size, as expected from theory. These results establish the crucial role of phase fluctuations for experiments involving ultracold superfluids in elongated circuit-like geometries.

In multimode optical systems, the spectral covariance matrix encodes all the information about quantum correlations between the quadratures of Gaussian states. Recent research has revealed that, in scenarios that are more common than previously thought, part of these correlations remains inaccessible to standard homodyne detection schemes. Formally, this effect can be attributed to a nonreal spectral covariance matrix. In this work, we provide a systematic framework and explicit criteria for identifying experimental configurations leading to such a behavior. This study will facilitate the proper exploitation and optimal engineering of continuous-variable quantum resources.

The driven dynamics of localization transitions in a non-Hermitian disorder-Aubry-André (DAA) model are examined under both open boundary conditions (OBC) and periodic boundary conditions (PBC). Through an analysis of the static properties of observables, including the localization length ($\xi$), inverse participation ratio ($\mathrm{IPR}$), and energy gap ($\mathrm{\Delta }E$), we found that the critical exponents examined under PBC are also applicable under OBC. The Kibble-Zurek scaling (KZS) for the driven dynamics in the non-Hermitian DAA systems is formulated and numerically verified for different local-to-local quench directions. The hybrid KZS (HKZS) in the overlapping critical region of non-Hermitian DAA and Anderson localization is proposed and numerically confirms the validity across a local-to-skin quench direction. This study generalizes the application of the KZS to the dynamical localization transitions within systems featuring dual localization mechanisms.

Solving optimization problems is a key task for which quantum computers could possibly provide a speedup over the best known classical algorithms. Particular classes of optimization problems including semidefinite programming (SDP) and linear programming (LP) have wide applicability in many domains of computer science, engineering, mathematics, and physics. Here we focus on semidefinite and linear programs for which the dimensions of the variables involved are exponentially large, so that standard classical SDP and LP solvers are not helpful for such large-scale problems. We propose the QSlack and CSlack methods for estimating their optimal values, respectively, which work by (1) introducing slack variables to transform inequality constraints to equality constraints, (2) transforming a constrained optimization to an unconstrained one via the penalty method, and (3) replacing the optimizations over all possible nonnegative variables by optimizations over parameterized quantum states and parameterized probability distributions. Under the assumption that the SDP and LP inputs are efficiently measurable observables, it follows that all terms in the resulting objective functions are efficiently estimable by either a quantum computer in the SDP case or a quantum or probabilistic computer in the LP case. Furthermore, by making use of SDP and LP duality theory, we prove that these methods provide a theoretical guarantee that, if one could find global optima of the objective functions, then the resulting values sandwich the true optimal values from both above and below. Finally, we showcase the QSlack and CSlack methods on a variety of example optimization problems and discuss details of our implementation, as well as the resulting performance. We find that our implementations of both the primal and dual for these problems approach the ground truth, typically achieving errors of the order ${10}^{-2}$.