Papers

Finding the eigenstates of a many-body Hamiltonian is a fundamental challenge in physics and computational science. Since the search space grows exponentially with system size, numerous classical and quantum algorithms have been developed to address this problem. A practical strategy is to identify a physics-informed low-dimensional subspace that effectively accommodates the low-lying eigenstates, thereby reducing the computational complexity. In this paper, we propose a recursive module-coupling algorithm, which iteratively treats a system as a composition of locally-coupled smaller modules, with low-energy subspace estimated successively according to the same recursive structure. Unlike the density matrix renormalization group (DMRG) approach that optimizes a global matrix product state through repeated local sweeps and obtains excited states sequentially, our algorithm constructs a physically tailored variational basis from module eigenstates and obtains several low-energy states on an equal footing, leading to substantial speedups if targeting moderate accuracy. Our proposed method further leads naturally to a recursive quantum variational algorithm, providing a systematic and modular circuit-construction framework compatible with contemporary gate-based quantum architectures. At each recursive level, block encoders are trained to map logical basis states onto the retained physical subspace, within which a variational circuit is subsequently optimized. Such a quantum-circuit implementation provides not only a quantum multistate eigensolver, but also a systematic prescription for hierarchically constructing quantum state-preparation circuits. Classical simulations demonstrate the accuracy and efficiency of the proposed method, whereas experiments on IBM quantum processors show that eigenstate preparation with reasonable fidelities is achievable even in the current NISQ era.

Precise control of multi-qubit architectures remains a critical bottleneck in superconducting quantum processors. In this work, we investigate the synthesis of high-fidelity quantum operations and state transfer protocols within an extended superconducting linear chain, scaling from three to seven sites. Using Floquet theory, we model the periodic drive as a train of delta-like pulses, mapping the quantum control problem onto quasi-energy resonance conditions. Combining the Baker-Campbell-Hausdorff expansion with Floquet spectral decomposition, we analytically identify optimal driving parameters, refined via the Covariance Matrix Adaptation Evolution Strategy (CMA-ES). In the three-qubit architecture, this enables high-fidelity synthesis of the iSWAP gate. Extending to a seven-site chain, we implement periodic trains of finite-width Gaussian pulses to activate distinct double-excitation transport channels with ultra-short gate durations t_gate (~170 ns). This achieves a clear scale separation from energy-relaxation times (T1) typical of fixed-frequency transmon devices with tunable couplers, such as IBM Quantum hardware. Finally, we benchmark stability under realistic imperfections, revealing a heightened sensitivity to static parameter disorder at the sub-percent level (eta ~ 10^-3) driven by spectral crowding, and discuss how closed-loop topologies could mitigate this constraint. This framework bridges time-periodic control theory and practical quantum gate engineering.

Tensor networks have attracted much attention as a powerful tool for modeling quantum many-body systems. Their contraction is a significant challenge, however, especially in highly connected networks, as memory requirements become prohibitive and the optimal contraction order is increasingly hard to find. The belief propagation (BP) algorithm has emerged as an alternative to exact contraction. Being formulated in a graph-agnostic way, it offers great flexibility, but its accuracy suffers in the presence of loops. In this work, we combine BP with the DMRG algorithm to solve ground-state problems, thereby extending DMRG to higher dimensions and arbitrary lattices. We demonstrate the viability of BP-DMRG on the transverse-field Ising model on a $2\times 2$ hexagonal lattice, finding that it produces states with a fidelity between $0.9$ and $0.99$ to the true ground state, and energy estimates with a relative error between $10^{-2}$ and $10^{-3}$. Additionally, BP-DMRG can find ground states on randomly generated lattices, with fidelity improving as the transverse field increases. We conclude with a discussion of the limitations we encounter when using belief propagation, highlighting that the TFI tensor network operators lead to larger errors during BP iterations in BP-DMRG.

Hexagonal boron nitride (hBN) is known as a promising solid-state platform to host room temperature quantum emitters that produce high purity single photons. The bright and spectrally adaptable hBN emitters are space-compatible, making hBN well-suited for satellite-based free-space optical (FSO) quantum key distribution (QKD) at wavelengths where the atmospheric background is naturally suppressed. This paper presents a pathway to develop hBN emitters operating near the Ca-II Fraunhofer line (at 854 nm), enabling daylight FSO operation. Moreover, due to its compatibility with the first telecommunication window at 850 nm, it is possible to interface with optical fibers to bridge the `last mile' in a scenario where multiple end-users connect through a single optical ground station to a QKD satellite. We therefore introduce a concept that combines continuous operation of a quantum network, hybrid links to minimize deployment costs, and high data rates due to the use of realistic single photon sources. The realization would be an important milestone for the development of the quantum internet.

A central challenge in diagnosing quantum chaos is to distinguish genuine many-body scrambling from kinematic effects of the spectrum. Krylov state complexity, or spread complexity, has emerged as a powerful diagnostic, with its characteristic growth, peak, and relaxation often taken as signatures of chaos. However, previous work has shown that this criterion can give false positives: saddle-dominated integrable systems may display prominent peaks even without random-matrix level correlations. We argue, based on complementary numerical and analytical evidence, that this ambiguity can be resolved by unfolding the spectrum prior to constructing the ensuing Krylov dynamics. By removing the non-universal smooth density of states while retaining microscopic spectral correlations, unfolding suppresses spurious peaks in integrable systems while preserving the universal spectral signatures of chaotic systems. Analytically, the formulation of the Lanczos iteration in terms of orthogonal polynomials yields an exact complexity kernel with a robust near-diagonal structure whose fine-grained features reflect the underlying spectral correlations. Moreover, for the logarithmic model, unfolding can be performed exactly, mapping the spectrum to a uniform lattice and yielding an analytic spread complexity that removes the false-positive peak. These findings establish unfolded Krylov complexity as a more reliable probe of genuine many-body scrambling.

Microwave-to-optical quantum transducers are essential for entangling remote superconducting qubits. Among the available transduction platforms, ensembles of Er$^{3+}$ and Yb$^{3+}$ ions doped into solids have emerged as leading candidates. While external magnetic fields are needed to split the Zeeman levels of erbium ions and enable a microwave–qubit interface, superconducting qubits suffer decoherence in such fields. In contrast, ytterbium ions exhibit zero-first-order Zeeman transitions and large hyperfine splittings at zero magnetic field (when doped into inorganic crystals). Owing to its long optical and spin coherence times, Yb:YSO has been widely used as a quantum memory, yet its potential for quantum transduction remains largely unexplored. Investigating this material could enable the integration of quantum memory and transduction in a single platform. Here, we demonstrate microwave-to-optical transduction in the continuous-wave regime using a 5\,ppm doped Yb:YSO crystal. The internal transduction efficiency is $2\times10^{-8}$ with a bandwidth of 200\,kHz, achieved using a 3D loop-gap microwave resonator (LGR) and a single-pass optical configuration. We explore all the ground states that form a V-type three-level system with the first and second optical excited states and assert the use of the ground state, which provides the highest efficiency and isolated optical transition. We further establish strong spin-microwave coupling from avoided crossing measurements. With a strong microwave drive to saturate the spin transition, we estimate the spin population pumped into the excited state, close to the simulated value. Finally, we calculate target parameter values for maximum efficiency with our system and suggest using 50\,ppm doped Yb:YSO crystal. With the calculated target parameters, the internal transduction efficiency is predicted to reach up to $10^{-4}$ in the current LGR.

Light-matter interactions underlie many quantum technologies, yet whether quasiparticles formed from such interactions preserve the full quantum state of light remains unresolved. Surface plasmon polaritons (SPPs), a class of polaritons formed by interacting photons with free-electron oscillations at metal-dielectric interfaces, are prime candidates to explore this question. Here we demonstrate quantum interference between single photons and SPPs using an Au-SiN$_{\mathrm{x}}$ integrated photonic-plasmonic device. Our results reveal that SPPs retain the indistinguishability of their excitation photons, establishing SPP as a viable quantum information carrier and opening a potential route toward photonic-plasmonic quantum circuitry.

Quantum generative models offer a promising framework for exploring whether quantum computation can enhance generative machine learning. Flow matching is a generative method in which samples are generated by transporting a simple, known distribution to the target data distribution with a learned velocity field. Its quantum counterpart, known as quantum flow matching (QFM), was introduced recently, and, like its classical counterpart, requires integrating an ordinary differential equation over many time steps during inference. As each step requires the output from the previous step, the circuit submission is sequential and a drawback on quantum computers as they have high input/output costs. To alleviate this problem, we introduce Quantum MeanFlow (QMF), the quantum analogue of the MeanFlow formulation, which allows single-step sample generation. While the QFM learns an instantaneous velocity field at each time step, QMF learns the average velocity over a time interval. We use a parameterized quantum circuit to learn these velocity fields and benchmark the two methods on the MNIST dataset. We show that while single-step QMF has lower image quality compared to multi-step QFM, it performs better than the single-step QFM sampling at every shot count. Both of our models are executed on IBM quantum computers and best-of-N rejection sampling recovers most of the accuracy lost to device noise without modifying the circuit. This is especially advantageous for QMF which has only one circuit evaluation per image. Here, We establish QMF as a viable method for single-step quantum generative sampling, saving on quantum circuit evaluations per generated sample.

A departure of a mixed quantum state from a local Gibbs description is generally invisible to local observables but can be detected by the conditional mutual information (CMI). Here we study the relationship between the renormalization group (RG) and CMI, and in particular, how RG constrains CMI. We first show that the CMI between nonadjacent regions $A$ and $C$, conditioned on the buffer region $B$, is UV-finite whenever the state admits a locally reversible RG with a fixed on-site Hilbert space dimension. We then study two hierarchical models that have long-range CMI and yet admit a simple RG description. The first model has a divergent Markov length at every temperature $0<T<\infty$ but nevertheless flows to an infinite-temperature product state under RG. The second model satisfies the local Markov condition while violating the global one and is stable against weak noise. At the critical noise strength, the two-point CMI decays only polynomially as a function of the system size, while the two-point mutual information vanishes.

Semidefinite programming plays a key role in optimization, with broad impact across control theory, machine learning, and combinatorial optimization. Although semidefinite programs are polynomially solvable, several commonly used algorithms rest on a linear-algebraic step whose running time grows cubically with the matrix dimension and which requires the matrix itself to be held in memory, at quadratic cost. In this study, we propose replacing it with a variational quantum eigensolver, whose qubit requirement is logarithmic in the matrix dimension, and present the first end-to-end implementation of such an approach within a cutting-plane framework, together with an operator-derived ansatz whose entanglement structure is read directly from the Pauli support of the candidate matrix. Evaluated on the control family of SDPLIB against an identical scheme driven by an exact eigendecomposition, the variational oracle produces valid cuts throughout, closing 32 to 82% of the initial optimality gap against a near-constant 75 to 82% for the exact oracle. Implementing and measuring the method end to end surfaces several effects not visible from theoretical analyses alone: where memory is actually consumed, how the padding required to fit a matrix onto a quantum register can mislead the variational optimizer, and why the candidate matrices prove dense in the Pauli basis, reducing the operator-derived ansatz to full entanglement. We report these findings and discuss their implications for near-term hybrid quantum-classical approaches.

Classical and current quantum approaches to protein structure prediction (QPSP) face limitations, notably massive qubit requirements restricting near-term models to simplistic on-lattice simulations. We propose a logarithmic-scale variational quantum eigensolver (VQE) that reduces qubit requirements for N torsional degrees of freedom to O(log2N), enabling off-lattice, all-atom simulations. Our architecture extracts molecular torsions from relative phases in statevector simulations. On quantum hardware, a decoder maps the empirical cumulative distribution function (CDF) from basis-state probabilities to bounded torsional variables. These feed a classical algorithm to build heavy-atom coordinates. We use an EfficientSU2 ansatz and multi-stage relaxation to mitigate barren plateaus. Structures are evaluated via a custom hybrid quantum-classical Hamiltonian, alongside Rosetta and OpenMM benchmarks. Evaluation on chignolin and Trp-cage yielded native-like conformations. Chignolin reached a 0.623 Å C{\alpha} RMSD in retained snapshots and 1.199 Å in final models; Trp-cage achieved a 2.501 Å RMSD among snapshots (3.512 Å in final models). Execution on IBM processors (ibm_cleveland, ibm_miami) successfully recovered native-like structures with a best RMSD of 1.758 Å. The custom energy function performed best overall, though energy-ranking imbalances persisted across sampled landscapes for all functions. This introduces the first all-atom, continuous-space quantum algorithm for QPSP. By converting physical qubit constraints into circuit depth constraints, it proves high-resolution prediction is feasible with exponentially fewer qubits. Despite current limits like computational overhead and energy function sensitivity, it establishes a scalable foundation for hybrid quantum biophysics.

We revisit the calculation of the phase shift experienced by swift electrons on passing through the electromagnetic field of a laser. Such phase shifts are now utilized in the form of `laser phase plates' in transmission electron microscopes (TEMs), for example. We calculate the phase shift using three different methods, namely, perturbation theory applied to the Dirac equation, the Volkov solution to the Klein-Gordon equation, and the relativistic Hamilton-Jacobi equation. We find that all three methods are in agreement, and that the calculated phase shift is independent of the relative orientation of the electron and laser beams. The agreement between the quantum and classical theories is explained. The duality between the phase shifts experienced by electrons, photons and neutrons is described. Our Lorentz invariant result for the phase shift differs from certain results published in the literature.

Distributed quantum sensing requires spatially separated probes to acquire local parameters while maintaining compatibility with network-level quantum information processing. We develop an entanglement-assisted stabilizer framework based on the extended structure of entanglement-assisted quantum error-correcting (EAQEC) codes, in which the remote halves of pre-shared ebits are used directly as local phase probes while the joint state simultaneously carries an encoded logical subsystem. Each remote probe acquires a local Z-axis phase and subsequently returns through an X-type noise channel. Within the extended EAQEC stabilizer structure, the stabilizer containing $X_{B_j}$ provides phase-dependent measurement statistics, whereas its partner containing $Z_{B_j}$ records the corresponding return-error syndrome. A graph-code formulation is introduced to make this structure explicit, together with an illustrative [[5,1,3;2]] construction. We further show that, conditioned on the joint sensing-and-syndrome measurement record, the post-sensing state differs from the original encoded state only by a known Pauli transformation, so that the logical information remains available for subsequent encoded operations. For the local-phase model considered here, the stabilizer readout attains the available quantum Fisher information, while the finite-shot estimator approaches the corresponding $1/\sqrt{M}$ scaling as the number of repetitions increases. The framework therefore provides a common entanglement-assisted stabilizer structure for distributed local-phase sensing, restricted return-error identification, and post-sensing logical-state retention, without relying on an intrinsic metrological enhancement from EAQEC itself.

Every finite-dimensional real or complex Lie algebra has a largest solvable ideal, its radical. In the compact dynamical Lie algebra (DLA) of a closed quantum system, this radical is the center; projecting onto it isolates directions that commute with the supplied algebra. With sparse Lie-bracket data and a known spectral gap, we construct a quantum circuit that approximately encodes this coefficient-space projector. The general construction combines the derived-algebra map with the Killing form, whereas compactness makes the Killing form alone sufficient. This distinction matters for numerical sensitivity, and our main theorem gives its exact law under an invariant-orthonormal basis adapted to the center and semisimple part. In a compact real algebra with a nonzero semisimple part, the condition number of the general operator is the three-halves power of the Killing-form condition number on that part. The compact projector in turn yields a bounded-error test for whether the center is trivial, provided that any nonzero center has at least a stated minimum dimension. This gives a controlled test for internal conserved directions in compact quantum dynamics.

We analytically characterize the maximal quantum violation of the extended Mermin-Ardehali-Belinskii-Klyshko (EMABK) family of inequalities by $n$-qubit generalized Greenberger-Horne-Zeilinger (GHZ) states. We develop a correlation-tensor approach in which the Bell value is expressed as the Frobenius inner product of the quantum correlation tensor and an effective coefficient tensor. A rank constraint on the latter, together with the von Neumann trace inequality, yields an upper bound governed by the two largest singular values of the reshaped correlation-tensor. For generalized GHZ states, we determine the complete singular spectrum and obtain a piecewise analytic upper bound. We then construct two complementary measurement strategies that saturate the bound for EMABK throughout the entire parameter range. The first is a purely MABK strategy in which all measurement directions lie in the equatorial plane of the Bloch sphere. The second is a hybrid strategy that recursively combines lower-order MABK anti-diagonal operators with the fully-$\sigma_z$ tensor product. The resulting piecewise analytic expression recovers the standard MABK Tsirelson bound $2^{(n-1)/2}$ in the maximally entangled limit and approaches the classical local-hidden-variable bound in the product-state limits. It further shows that every entangled generalized GHZ state exhibits a strict quantum-classical separation under the EMABK inequality, thereby eliminating the nonviolation region of the standard MABK inequality in the partially entangled regime.

The exact bound-state wavefunctions and the corresponding energy equation are calculated for a new generalized harmonic oscillator problem describing a spin-1/2 fermion in 3+1-dimensions, involving scalar, vector, and tensor couplings acting simultaneously within a particular plane of motion. For the scalar and vector coupling, singular harmonic oscillator shapes are considered, such that the singular term is needed to allow analytical solutions for the wavefunctions while preserving binding under adequate conditions. For the tensor sector,the Dirac oscillator potential is employed, which adds another independent binding mechanism to the problem. The exact bound-state solutions are computed by specifically tuning coefficients for an appropriate pair of Ansätze for the radial functions, which leads to wavefunctions in terms of generalized Laguerre polynomials. Although the energy equation cannot provide a general expression for the energy spectrum, specific constraints on the quantum numbers as simple functions of the external potential parameters can be derived for it, which determines the conditions for bound solutions to exist, and of what type: particle, antiparticle or both. It is shown that the results can be simply mapped to the spherically symmetric analogue problem, and this is used to show that the general result encompass several previous particular cases of spherically symmetric harmonic oscillator problems in the Dirac equation available in the literature.

Bell's theorem revealed the fundamental incompatibility between the predictions of quantum mechanics and local realism. Bell inequality violations have since demonstrated quantum nonlocality using photons and internal states of massive particles, but never using their motional states. Here we report the first Bell inequality violation in the motional states of massive particles. Using momentum-entangled pairs of metastable helium atoms manipulated by matter-wave interferometry, we measure a Clauser-Horne-Shimony-Holt (CHSH) Bell parameter of $S = 2.52 \pm 0.17$, violating the CHSH-Bell inequality ($S \le 2$). Our work completes a long-standing objective in quantum atom optics by extending Bell tests from internal quantum variables to the external degrees of freedom of massive particles, opening a new regime for exploring quantum nonlocality in matter waves and for investigating the interplay between quantum mechanics and gravity.

Entanglement and nonstabilizerness capture distinct aspects of quantum complexity, yet their relation through the entanglement spectrum remains poorly understood. Here we develop a spectral framework for bipartite nonlocal nonstabilizerness. We introduce a generalized anti-flatness and derive upper and lower bounds on the nonlocal stabilizer Rényi entropy (SRE) in terms of Rényi entanglement entropy and spectral non-uniformity. We further reduce the local-unitary optimization to a single-unitary problem and prove that the ordered Schmidt reference state is a local minimum of the SRE for integer $\alpha\geq2$. Applying these results to exponentially and algebraically decaying spectra reveals parametrically distinct relations between entanglement and nonlocal nonstabilizerness. For broad spectra, we introduce a dyadic-shell sandwich construction that determines the asymptotic SRE scaling. At one-dimensional conformal critical points, it yields a universal hierarchy of double-logarithmic scaling laws for integer $\alpha\geq2$. Our spectral bounds and dyadic-shell sandwich construction provide general tools for analyzing nonlocal SRE, offering a flexible framework that can be applied to a wide range of entanglement spectra in quantum many-body systems.

Teleportation performance is commonly characterized by the average teleportation fidelity, while its variation over input states provides additional information captured by the fidelity deviation. In controlled teleportation, the controller's measurement introduces an additional source of fidelity variation through its measurement outcomes. We investigate the fidelity deviation in controlled teleportation with three-qubit pure states. We derive a lower bound on the fidelity deviation and show that it is attainable together with the maximal average teleportation fidelity. Among the measurements attaining the maximal average fidelity, however, the fidelity deviation can vary depending on the controller's measurement. We further examine the effect of controller assistance by comparing controlled teleportation with direct teleportation using the reduced state. Unlike the maximal average fidelity, which cannot decrease with controller assistance, the minimal fidelity deviation is not necessarily reduced by controller assistance. In particular, for any W-class pure state, controller assistance cannot reduce the fidelity deviation. These results reveal contrasting effects of control on teleportation fidelity and its deviation in controlled teleportation.

Effective field theories (EFTs) are a powerful framework for performing high-precision calculations at reduced complexity compared to their fundamental counterparts. A particularly important class of EFTs arises in the non-relativistic (NR) regime. Their construction relies on a different realization of the underlying symmetries, since Lorentz invariance is no longer manifest in covariant form in the NR regime. This behavior imposes a link between certain matching coefficients, and therefore additional constraints, commonly referred to as hidden Lorentz invariance. These constraints are established in quantum field theories on inertial flat spacetime, such as NR quantum electrodynamics. However, deriving these constraints becomes considerably more involved for theories involving physics beyond the Standard Model or formulated in non-inertial spacetime backgrounds, where the hidden symmetry structure is less transparent. In this work, we present an approach to obtain the NR EFT by first constructing a relativistic EFT and then performing a generalized NR reduction based on an extended Foldy-Wouthuysen transformation. We illustrate this method by a quantum chromo-electrodynamics EFT for inertial flat spacetime, describing both electromagnetic and strong interactions, and show how it reduces to the established Lagrangian of NR quantum chromodynamics and electrodynamics. The hidden Lorentz invariance emerges as a direct consequence of the construction. This approach provides a route to obtain the NR limits of more complex theories, \eg Dirac fields in non-inertial spacetime or extensions involving physics beyond the Standard Model. As an example, we apply the method to add the coupling of a pseudoscalar axion field in a simplified model and derive its NR limit.

Continuous-time quantum walks on a lattice spread ballistically and converge to a limiting distribution for the rescaled position. On the hard-wall half line the boundary reflects the walker but does not change the bulk dispersion, so the ballistic front remains set by the maximal group velocity. We ask how close one can get to this front when the initial state is constrained to occupy only the first $M$ sites. We show that optimizing the moment-generating function of the limiting-velocity distribution over this finite-support class reduces to the principal eigenvalue of an explicit $M\times M$ Hermitian matrix, which yields the optimal state by direct diagonalization. For large $M$, a near-front scaling limit produces a continuum description that controls the entire peak region. In particular, the maximal mean drift approaches the front with an inverse-square deficit in $M$ and a constant prefactor $\pi^2/4$. Numerical results verify both the finite-$M$ spectral formulation and the predicted scaling.

The idea that information can be localized is pervasive in physics. In thermodynamics, entropy flows from one reservoir to another, and in quantum gravity, the entropy content of some spatial regions is bounded. In quantum information theory, information is transmitted from one place to another, and entanglement can provide an advantage when there are constraints on such transmission. Shannon entropy quantifies localizable information in the sense that marginalization defines an outer measure on the set of parts of a classical multipartite system. In contrast, von Neumann entropy does not quantify localizable information in this sense. However, a variant of von Neumann entropy, which originates in noncommutative geometry, does. We prove that this adjusted von Neumann entropy is the minimum quantum entropy that quantifies localizable information. We also prove that this is the unique quantum entropy that characterizes Bell states in terms of redundant information, directly generalizing the classical case. We work with finite-dimensional $C^*$-algebras throughout, modeling finite quantum systems that may have superselection sectors.

We introduce a cipher-structure-aware variational framework for key recovery from public data in reduced block ciphers. Public S-box differential constraints and candidate-key-dependent inverse-round residuals are converted into phase separators through reversible compute-phase-uncompute circuits. The principal separator is a hybrid DDT construction that combines pairwise and synchronized multi-pair differential constraints before residual refinement. The construction is specified by public functions and does not require a key-indexed phase table in the physical circuit. We formalize the public-instance model, prove one-sided completeness of the structural diagnostics, and derive a conditional lower-tail bound for false-key diagnostic gaps under an explicitly stated sub-Gaussian moment condition. This statistical statement concerns the diagnostic landscape, not convergence of a finite-depth QAOA optimizer. On ten independent random three-pair instances of 16-bit simplified AES, an 18-layer Hybrid-DDT schedule reaches a mean public-consistent-key probability of 0.529785, with values from 0.388892 to 0.770937, ranks the correct key first in every run, and has largest observed false-key probability 0.008723. The result is a reduced-cipher mechanism study, not an attack on full AES.

The effective decay rate of traditional single mode model for an open quantum system can't accurately described the exchange between intermodes and dissipation caused by the environment,therefore we can not clearly see the occupation pathway of target mode or the transient participation of auxiliary modes.At the same time,we also can't address the selective dissipation pathways resulting from the circuit design.In order to solving above problem,we adopt fluxonium-transmon-transmon (FTT)-based three mode model to describe dynamical characteristics of dissipation spectrum in superconducting quantum circuits.In terms of periodic flux modulation,we find out the pathways of intermode occupation transfer and environmental release for the target fluxonium changes from a high occupation nonequilibrium condensate-like state to low occupation equilibrium Bose-Einstein condensation(BEC)state.Without increasing the intrinsic loss of the target mode in the process of adjusting the auxiliary mode,we furthermore obtain additional selectively engaged environmental dissipative pathway.Our model can provides a new design perspective for non-equilibrium state recovery in a controllable dissipation superconducting quantum circuit.

One-dimensional quantum–classical hybrid walks are introduced by combining quantum and classical steps under periodic and random protocols. Exact diffusion coefficients are derived for both protocols throughout the diffusive regime. The exact results show how diffusion depends on the average frequency and temporal arrangement of classical steps. As the average frequency approaches zero, both diffusion coefficients diverge inversely with the frequency. At matched average frequencies, the ratio of the diffusion coefficients for the random and periodic protocols approaches two. A finite-mode approximation is also developed and tested against the exact diffusion coefficient for the random protocol. The exact results for the one-dimensional model provide a foundation for future studies of more general quantum–classical hybrid walks.

We present a cohesive framework for simulating seismic wave propagation utilizing quantum computing paradigms and their classical tensor network equivalents. We detail a quantum circuit-based formulation for the explicit finite-difference time-domain (FDTD) solution of the two-dimensional acoustic wave equation and map this quantum architecture onto a tensor train representation, namely for Matrix Product State (MPS). The MPS solver enables deterministic simulation of large-scale wavefield dynamics on classical high-performance computing systems. We demonstrate the MPS representation by computing 2D seismic wavefields on the Marmousi model. Our results indicate that the MPS representation is a viable direction for computing and scaling wavefield propagation.

Fault-tolerant quantum computation fundamentally relies on encoding a logical qubit into a structured block of physical qubits, typically in the tens to hundreds. As a trade-off for improved fault-tolerance, logical gate operations will incur a linear overhead in terms of both the amount of time and quantum resources than before. For example, in monolithic quantum computing, performing a gate operation on two distant logical qubits will first require using a linear number of SWAP operations in order to move the logical qubits next to each other; while in distributed quantum computing, doing so will first require a linear number of ancilla qubits in order to form entanglement connections (or a logical Bell pair). In this paper, we focus on significantly reducing the overhead involved in logical CNOT operations, a fundamental primitive. We propose NOBOL, a novel approach that requires only one Bell pair to perform a logical CNOT operation on two distant qubits encoded in arbitrary CSS codes. More importantly, NOBOL only requires performing gate operations on the logical X or Z operator subsets of the logical qubits. For many CSS codes, such as the surface code, these subsets are significantly smaller than the size of the code itself. In this paper, we describe various circuit realizations of NOBOL, including a depth-optimal circuit with logarithmic depth in terms of the size of the logical operators. Finally, we propose effective methods to contain error propagation without incurring much additional overhead. Since NOBOL can be effectively applied to a wide range of quantum error-correcting (QEC) codes and, in addition, is agnostic to qubit modalities and effective for various architectures, including those based on either a monolithic QPU or distributed QPUs.

We study relativistic beam-like wave packets governed by a quasi-Hermitian massive Dirac Hamiltonian and uncover anomalous Gouy-phase behavior in non-Hermitian dynamics. We show that the Gouy phase provides a sensitive probe of the global $\mathcal{PT}$-symmetry-breaking threshold: it remains purely real in the globally unbroken, quasi-Hermitian regime, while, after crossing the exceptional point, the Gouy phase changes sign and acquires an imaginary component. At the exceptional point, the Gouy-phase variation vanishes in the small-mass limit but becomes maximal for large masses, revealing a counterintuitive crossover from effectively classical to increasingly wave-like quantum behavior. We propose an experimental scheme to measure the components of the non-Hermitian Gouy phase in the broken regime by monitoring the attenuation of a light beam propagating through a lossy waveguide. These results highlight the potential of the non-Hermitian Gouy phase for photonic applications, including the determination of threshold conditions and the design and control of systems with gain and loss.

The asymptotic McEliece–Rodemich–Rumsey–Welch bound (1977) limits the largest attainable rate of binary codes as a function of the relative distance. After a nearly half-century hiatus, this result was recently improved in two concurrent works, by OpenAI and by O. Alrabiah and V. Guruswami. The two arguments look entirely different, a Delsarte certificate on the one hand, a classical-quantum channel and the pretty good measurement on the other, and they yield the same bound. The purpose of this note is to explain why: in both proofs, a subspace is attached to every codeword and moved with it, and the bound counts how many such subspaces fit in the ambient space, exactly in the first case and in the probabilistic sense of typicality in the second. We also present the OpenAI proof in the language and context of coding theory, as an extension of the spectral method in which the single vector attached to a codeword is replaced by a subspace.

Present-day quantum processors are open systems in which dissipation and decoherence degrade the information carried by a quantum state as it propagates through a circuit. We study this degradation in dissipative random quantum circuits, modeling each two-qubit gate as a diluted unitary that interpolates between the intended unitary operation and a random dissipative quantum channel. Using the fidelity between the ideal and noisy output states as a diagnostic, we show that sufficiently random unitary gates or dissipative Kraus operators lead to a universal decay of the average fidelity. This decay is determined only by the dissipation strength, system size, and circuit depth, while microscopic details of the gates and noise, including the Kraus rank, appear only in subleading corrections to higher moments. We derive a closed-form expression for the average fidelity in terms of three physically meaningful error parameters, dissipation strength, coherent two-qubit gate-error strength, and connectivity-error probability. Finally, we identify the cases when dissipative effects can be reproduced by an unitary-noise model and when dissipation-induced decoherence remains distinguishable from coherent noise at the level of the average fidelity.