Increasing the utility of currently available noisy intermediate-scale quantum (NISQ) devices requires developing efficient methods to mitigate hardware errors. In this work we propose a novel cyclic layout permutations based zero noise extrapolation (CLP-ZNE) protocol for such a task. The method leverages the inherent nonuniformity of gate errors in NISQ hardware to extrapolate the expectation value, averaged over cyclic circuit layout permutations, to the level of zero noise. In contrast to the previous layout permutation based approaches, for $n$-qubit circuit CLP-ZNE requires execution of only $O\left(n\right)$, and at most, $O\left(n^2\right)$ different circuit layouts for circuits of one-dimensional and arbitrary connectivity, respectively. When benchmarked against noise channels modeling the IBM Torino quantum computer, the method reduces a typical error in expectation values of $n=12$ qubit circuits by an order of magnitude, outperforming standard unitary folding ZNE. By demonstrating the ability to mitigate noise of real hardware specifications, including both depolarizing and $T_1/T_2$ relaxation processes, these results give evidence for the applicability of CLP-ZNE to present-day NISQ processors.

We propose and systematically analyze a practical scheme for implementing a one-dimensional non-Hermitian (NH) Su-Schrieffer-Heeger model using individually addressable Rydberg atom arrays. Our setup consists of an atomic chain with three-atom unit cells, in which a synthetic gauge field is generated by applying multicolor laser fields. By engineering fast dissipative channels for one auxiliary atom in each unit cell, adiabatic elimination effectively gives rise to a NH skin effect. We examine how fluctuations in the experimental parameters influence both the skin effect and the topological invariant in real space and find that both features remain highly robust. This work establishes a versatile, controllable, and programmable open-system quantum simulator with neutral atoms, providing a clear route for exploring rich NH topological phenomena.

We introduce a method for determining the sensitivity of any given entangled two-photon absorption (ETPA) measurement. By modeling all signal and noise contributions to the measurement, we derive a single numerical value that describes the sensitivity of the ETPA measurement in Göppert-Mayer units. This allows us to directly compare vastly different experimental approaches and determine whether ETPA will be detectable under the given conditions. Therefore we can quantify the effect of any change to a given experimental apparatus and identify the ideal optimization pathway.

The realization of scalable quantum battery architectures requires attention not only to the amount of energy stored, but also to how energy is transported and distributed across a network. While previous studies focused on collective charging in multicell quantum batteries, topology-dependent transport laws in quantum battery networks remain less explored. Here we investigate quantum battery networks with reciprocal and engineered nonreciprocal couplings and compare cascaded and parallel architectures within a unified transport framework. In the nonreciprocal regime, the optimal coupling follows distinct topology-dependent scaling laws, $J_{\mathrm{op}}^c\propto N$ for cascaded transport and $J_{\mathrm{op}}^p\propto N^{-1/2}$ for parallel charging in the large-$N$ limit. In reciprocal cascaded networks, a parity-dependent spectral response produces an odd-even transport effect that is absent in the nonreciprocal and parallel configurations. We further analyze the role of thermal and squeezed reservoirs and show that thermal noise mainly increases passive energy, whereas squeezing enhances the useful fraction of the stored energy, thereby increasing the ergotropy. These results shift the emphasis from charging enhancement to transport engineering and provide architecture-level design principles for quantum battery networks.

We establish that the exact quantum dynamics of a Brownian particle in the Caldeira-Leggett model, with at most quadratic external potential, can be mapped, at any temperature, onto a classical, non-Markovian stochastic process in phase space. Starting from a correlated thermal equilibrium state between the particle and bath, we demonstrate that this correspondence is exact for quadratic potentials under arbitrary quantum state preparations of the particle itself. Our approach allows us to consider arbitrary initial quantum states—including highly nonclassical superpositions—which are incorporated via their Wigner functions, which serve as statistical weights for trajectory ensembles. Furthermore, the formalism naturally accommodates external manipulations and measurements modeled by preparation functions acting at arbitrary times, enabling the simulation of complex driven-dissipative quantum protocols. For more general, smooth potentials, we identify a natural small parameter: the density matrix becomes strongly quasidiagonal in the coordinate representation, with its off-diagonal width shrinking as the bath's spectral cutoff increases, suggesting a controlled parameter for a possible approximation.

Local unitary equivalence is central to entanglement quantification and classification. Identifying the local unitary equivalence remains a formidable challenge. We address this problem for a class of quantum states with one highly degenerate eigenvalue and the rest nondegenerate simple eigenvalues that are pivotal to structured resources in quantum resource theory. We introduce a reduction procedure that maps each state to a reduced state by nullifying the highest-multiplicity eigenvalue and prove that the local unitary equivalence of the original states is equivalent to that of their reduced counterparts. For the resulting pure or nondegenerate reduced states, we employ the existing invariants or fixed-point subgroup criteria to establish a complete discrimination framework, although the existing criteria cannot directly identify the local unitary equivalence of the original states. We also verify the local unitary equivalence of two families of single-parametrized multipartite mixed states constructed by perturbing absolutely maximally entangled states from distinct combinatorial origins, demonstrating the efficacy and generality of our approach.

Quantum entanglement in systems of identical particles is often obscured by the interplay between exchange-induced correlations and the operational framework used to define entanglement. To study the role of exchange statistics, we propose a scheme using two distinguishable particles where an exchange symmetry is artificially engineered via a relative phase $\theta$ in the initial state. This approach allows continuous tuning from bosonic ($\theta =0$) to fermionic ($\theta =\pi$) statistics. By monitoring the interplay between purity and coherence, we uncover distinct dynamical regimes dictated by the interaction strength $U$ and the phase $\theta$. For particles initially loaded in a bound state, strong $U$ suppresses coherence development by avoiding the scattering band, reducing the purity toward its minimum. For particles initially on neighboring sites, coherence grows linearly in time. While nonsymmetric inputs feature a sharp purity reduction at intermediate $U$, due to the competition between bound and unbound states, symmetric initial conditions produce transient coherence bursts that significantly enhance the purity. More generally, tuning the phase $\theta$ reveals a high-purity region over a range of $\theta$ at intermediate interactions, with the purity collapsing to $1/2$ as $\theta$ approaches the fermionic limit. Our results show that the imposed statistics, or lack thereof, reshapes the entanglement dynamics and its response to the interaction $U$.

We propose an architecture for fault-tolerant quantum computing that incorporates strong single-photon nonlinearities into a photonic Greenberger-Horne-Zeilinger-measurement-based architecture. The nonlinearities substantially reduce resource overhead compared to conventional linear-optics-based architectures, which require significant redundancy to accommodate probabilistic photon generation and probabilistic entangling operations. By removing linear-optical failure modes, our nonlinear architecture can also tolerate much higher optical losses than linear approaches, with a baseline loss tolerance of approximately $12\%$ using a 32-photon resource state and a foliated surface code. Nonlinear photonic architectures provide a route to dramatically improving practical implementations of fault-tolerant quantum computing.

We report a comprehensive characterization of the intrinsic noise of a single-beam ${}^4\mathrm{He}$-based vector optically pumped magnetometer, and identify its main origins. In its optimal regime, the sensor provided vector magnetic readings with noise below $30\mathrm{fT}/\sqrt{\mathrm{Hz}}$ on two axes. We show that photon shot noise dominates at the optimum, and that the standard quantum limit set by spin projection of the aligned ensemble lies around $4\mathrm{fT}/\sqrt{\mathrm{Hz}}$. An extra noise is observed on one axis. It fits well with our model of metastable atom transit, suggesting interesting links with spin-noise spectroscopy.

Quantum electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: The vacuum optical index should increase when it is stressed by intense electromagnetic fields. The Deflection of Light by Light project aims to measure it by using intense and ultrashort laser pulses. The experiment uses a Sagnac interferometer to amplify the tiny deflection signal of a low-intensity probe pulse crossing the vacuum refractive-index gradient produced by an external high-intensity pump pulse. The measurement of the amplified signal by a CCD camera requires a high spatial resolution, which is limited by the ultimate quantum noise of the CCD. However, interferometric phase noise induced by the mechanical vibrations of the interferometer is also amplified and degrades spatial resolution. To overcome this, we propose a method named high-frequency phase noise suppression (HFPNS), based on the addition of a delayed replica (5 ns) of the probe pulse. The delayed pulse, which is not affected by the pump but is subject to the same vibration noise, enables offline subtraction of correlated phase noise. In this work, we present an experimental proof of concept on a prototype interferometer operating with a limited amplification factor $(\mathcal{A}\simeq 25)$, about ten times smaller than the required value of the final experiment. We have succeeded in reducing phase noise by a factor of 40, resulting in a residual noise level 2.3 times higher than the expected quantum noise. The residual noise is linked to delay-line instabilities and incident beam-pointing fluctuations present during these tests. This result validates HFPNS as a robust method for future quantum-noise-limited interferometric measurements of vacuum optical nonlinearity, though additional stabilization and higher interferometric amplification are still needed.

Compatibility with noisy intermediate-scale quantum (NISQ) devices is crucial for the realistic implementation of quantum cryptographic protocols. We investigate a cryptographic scheme based on discrete-time quantum walks on cyclic graphs that exploits Parrondo dynamics, wherein periodic evolution emerges from a deterministic sequence of individually chaotic coin operators. We construct an explicit quantum circuit realization tailored to NISQ architectures and analyze its performance through numerical simulations in qiskit under both ideal and noisy conditions. Protocol performance is quantified using probability distributions, Hellinger fidelity, and total variation distance. To assess security at the circuit level, we model intercept-resend and man-in-the-middle attacks and evaluate the resulting quantum bit error rate. In the absence of adversarial intervention, the protocol enables reliable message recovery, whereas eavesdropping induces characteristic disturbances that disrupt the periodic reconstruction mechanism. We further examine hardware feasibility on contemporary NISQ processors, specifically ibm_torino, incorporating qubit connectivity and state-transfer constraints into the circuit design. Our analysis demonstrates that communication between spatially separated logical modules increases circuit depth via SWAP operations, leading to cumulative noise effects. By exploring hybrid state-transfer strategies, we show that qubit selection and connectivity play a decisive role in determining fidelity and overall protocol performance, highlighting hardware-dependent trade-offs in NISQ implementations.

Long-distance quantum communication necessitates the use of quantum repeaters, which typically include highly coherent quantum memories. We provide a theoretical analysis of the secret key rates for a quantum repeater system incorporating bosonic error correction and memory components. Specifically, we focus on the application of binomial codes for two repeater segments. Using these codes, our investigation aims to suppress memory loss errors that commonly affect bosonic systems such as atomic gases, excitations of spin ensembles, and microwave cavities, in contrast to dephasing errors in single-spin memories. We further discuss a physical implementation of such a quantum repeater comprising a microwave cavity and a superconducting transmon, capable of state engineering with high fidelities ($>97\%$) and logical Bell state measurements for successful entanglement swapping. As an alternative approach, we also discuss a realization in the all-optical domain.

Limited connectivity makes nonlocal quantum gates expensive on near-neighbor hardware, where compilation typically relies on swap transport, inheriting both depth overhead and path congestion. We present a swap-free routing framework in which higher levels of a qudit act as orthogonal spectral buses that transport control information without moving the computational state. We show that exact congestion relief in nearest-neighbor architectures requires local Hilbert-space expansion. In this model, a single nonlocal operation over a path of length $L$ requires a best-case minimum of $2L+1$ logical routing primitives. For comparison, a layout-restoring swap implementation requires $2L$ swap operations ($6L$ cnot-equivalent layers). Overlapping routes remain distinguishable through bus labels encoded in the same physical qudits. This routing algebra extends to Boolean fan-in at a common target: Multiple controls arriving on distinct buses trigger a local unitary based on an arbitrary Boolean function of bus digits, yielding multicontrol operations of depth $2L+D_g+O\left(K\right)$ for fan-in size $K$ to account for shared-target serialization, where $D_g$ is the target-synthesis cost. We prove decodability, reversibility, and correctness for cnot and Boolean fan-in, along with a state-count lower bound $d\ge 2^{K+1}$ for exact overlap routing. Cirq simulations confirm single-control correctness and zero crosstalk. Compiler-level benchmarks on quantum Fourier transform, quantum approximate optimization algorithm, and mirror-interaction circuits verify the predicted congestion law and transport reduction. Noisy QuTiP simulations show that the architectural advantage depends on higher-level coherence and speed. These results identify spectral qudit routing as a congestion-relief architecture that separates nonlocal control delivery from local target-side aggregation, providing a minimal mechanism for overcoming qubit routing limitations.

Since complex absorbing potentials (CAPs) are nonphysical in nature, they can introduce artificial perturbations into the system, which may lead to shifts in the calculated energy. This study presents a methodology to eliminate artificial reflections caused by the CAP using a backward parametric equation of motion with the CAP approach (PEMCAP approach), starting from a higher CAP strength parameter $\eta$ to limit of $\eta \to 0^{+}$. During backward PEMCAP approach, the eigenstates as a function of $\eta$ do not follow the same path as the forward trajectory, i.e., starting from $\eta =0$ to higher $\eta$. Instead, the energy converges to the true numerically exact resonance energy, which includes both its position and lifetime. This technique enables us to reach the physical limit $\eta \to 0^{+}$, effectively removing the perturbation caused by the CAP. The effectiveness of this approach is demonstrated by accurate calculations of resonance complex energies that are independent of both the CAP box-size parameters and the optimal value of $\eta$. This makes the optimization of the CAP onset parameters unnecessary. Moreover, this method also allows for the determination of resonance energies corresponding to multiple resonances in the physical limit $\eta \to 0^{+}$ through a single calculation, making this approach extremely beneficial.

The localization transition in the Hermitian Aubry-André model is known to have a classical origin, with the critical point being exactly predictable from an analysis of classical phase-space trajectories. Motivated by this correspondence, we investigate whether a similar classical origin exists for localization transitions in non-Hermitian quasiperiodic Hamiltonians. Using semiclassical Husimi dynamics together with a detailed phase-space stability analysis, we show that localization transitions persist even in the semiclassical limit of such non-Hermitian models. However, in sharp contrast to the Hermitian Aubry-André case, the transition point inferred from classical phase-space analysis does not coincide with the quantum critical point. Instead, we find that the semiclassical transition depends sensitively on the choice of the irrational parameter defining the quasiperiodic potential, indicating the absence of a universal classical-quantum correspondence for the localization transition in the non-Hermitian setting. Nonetheless, we identify a suitable parameter regime in which the classical dynamics can faithfully mimic the quantum dynamics over a finite but appreciable time window.

We conduct a comprehensive investigation into the excitation of turnkey Stokes pure quartic solitons (PQSs) and platicons (PQPs) in Kerr microresonators under the influence of Raman nonlinearities. Analytical expressions of the power threshold required for Stokes PQS formation are derived using a local approximation method and subsequently validated by numerical simulations. The minimal impact on Stokes PQSs in the consideration of free-spectral-range mismatch, backscattering, and stochastic noise exhibits fairly good robustness of the system. Utilizing a modified Lugiato-Lefever equation incorporating a laser self-injection-locking mechanism, we also demonstrate that single- and multi-PQS Stokes states can be realized and controlled by adjusting the initial detuning or feedback phase, thereby obviating the need for photonic and electronic control circuitry. Furthermore, we analyze the parameter space for Stokes PQPs characterized by normal fourth-order dispersion (FOD), revealing complex dynamical behaviors, including stable, chaotic, and breathing states. These findings are expected to deepen the understanding of soliton dynamics influenced by the Raman effect in self-injection-locked systems and provide valuable guidance for the robust generation of emerging Stokes solitons.

We investigate the excitation of a three-level ladder-type atom by a unidirectional field with a pair of indistinguishable photons. Starting from an analytical expression for the two-photon absorption probability, we determine the two-photon state that maximizes the population of the upper atomic state at a chosen time and show that in the limit of an infinitely long pulse, perfect excitation is possible. The optimal state is identified as the time-reversed counterpart of the two-photon state emitted in spontaneous cascade decay. We then compare this ideal excitation strategy with experimentally accessible families of states, including symmetrized Gaussian product states, temporally correlated Gaussian states, and coherent pulses. We analyze how the optimal excitation conditions depend on the ratio of atomic decay rates and on the separation of the atomic transition frequencies. For indistinguishable photons, quantum interference may shift the maxima of the marginal spectral distribution away from the atomic resonances and qualitatively modify the optimal excitation strategy. Our results clarify the role of indistinguishability and correlations in two-photon absorption and provide guidance for designing realistic excitation schemes in quantum-optical light-matter interfaces.

We construct holonomic quantum gates for qubits that are encoded in the near-degenerate vibrational $E$ doublet of a deformable three-body system. Using Kendall's shape theory, we derive the Wilczek-Zee connection that governs adiabatic transport within the $E$ manifold. We show that its restricted holonomy group is $\mathrm{SU}\left(2\right)$, implying universal single-qubit control by closed loops in shape space. We provide explicit loops implementing a $\pi /2$ phase gate and a Hadamard-type gate. For two-qubit operations, we outline how linked holonomic cycles in arrays generate a controlled Chern-Simons phase, enabling an entangling controlled-$X$ (cnot) gate. We present a Ramsey-echo interferometric protocol that measures the Wilson loop trace of the Wilczek-Zee connection for a control cycle, providing a gauge-invariant signature of the non-Abelian holonomy. As a physically realizable demonstrator, we propose bond-length modulations of a Cs($6s$)-Cs($6s$)-Cs($n{d}_{3/2}$) Rydberg trimer in optical tweezers and specify operating conditions that suppress leakage out of the $E$ manifold.

Fault tolerance using constant space overhead against long-range correlated errors is an important practical question. In the pioneering works of Terhal and Burkhard [Phys. Rev. A 71, 012336 (2005)], Aliferis, Gottesman, and Preskill [Quant. Inf. Comput. 6, 97 (2006)], and Aharonov, Kitaev, and Preskill [Phys. Rev. Lett. 96, 050504 (2006)], fault tolerance using polylogarithmic overhead against long-range correlation modeled by a pairwise joint Hamiltonian was proven when the total correlation of an error at a qubit location with errors at other locations was $O\left(1\right)$, i.e., the total correlation at a location did not scale with the number of qubits. This condition, under spatial symmetry, can simply be stated as the correlation between locations decaying faster than $\frac{1}{{\mathrm{dist}}^{\dim }}$. However, the pairwise Hamiltonian model remained intractable for constant overhead quantum low density parity check (qLDPC) codes. Recently, Bagewadi and Chatterjee [Phys. Rev. A 112, 022422 (2025)] introduced and analyzed the generalized hidden Markov random field model, which provably captures all stationary distributions, including long-range correlations [Kunsch, Geman, and Kehagias, Ann. Appl. Prob. 5, 577 (1995)]. It resulted in a noise threshold in the case of long-range correlation for memory corrected by the linear-distance Tanner codes [Leverrier and Zémor, in IEEE 63rd Annual Symposium on the Foundations of Computer Science (FOCS) (IEEE, 2022)] for superpolynomial time. In this paper, we prove a similar result for square-root distance qLDPC codes, and we provide an explicit expression for the noise threshold in terms of the code rate, for up to $o\left(\sqrt{\#\mathrm{qubits}}\right)$ scaling of the total correlation of error at a location with errors at other locations.