We present a widely accessible and experimentally realizable technique for the controlled creation of dark-bright solitons and soliton lattices in one-dimensional atomic Bose-Einstein condensates. The method is based on preparing the condensate in a dark state of a $\mathrm{\Lambda }$-coupled three-level system. Numerical simulations of the corresponding two-component system reveal that individual dark-bright solitons created through this scheme can survive over experimentally accessible timescales, even when the coupling laser fields are switched off. Meanwhile, the fate of soliton lattices upon the quench of the fields depends on the scattering lengths. When they are all equal, the lattice is found to persist on timescales comparable to the condensate lifetime, even though the analysis of dynamical stability reveals that they possess unstable modes. In this case the resulting destabilization is not found to be detrimental, as it leads to recurrent dynamics. However, for unequal scattering lengths the lattice structure gets destroyed once the instability sets in, which happens after a few tens of milliseconds after the quench of the optical fields.

Highly coherent lasers are central to modern photonics. To date, high-coherence operation has been achieved predominantly in microcavity and fiber-based platforms. More recently, free-space Brillouin-laser experiments have revealed unusually strong noise suppression whose physical origin cannot be explained by conventional continuous-medium models developed for those platforms. In conventional continuous-medium models, the optical and acoustic fields are assumed to remain continuously coupled throughout the cavity evolution, whereas in free-space implementations the coupling is confined to the nonlinear medium and interrupted by passive propagation over the rest of the round trip. To describe this interaction-propagation separation, we develop a discrete-cavity model in which the short Brillouin interaction inside the gain medium and the subsequent free-space propagation are treated as two separate stages of the round-trip evolution. This separation introduces a temporal asymmetry between optical storage and acoustic relaxation, effectively enhancing acoustic damping at the cavity level and strongly reducing pump-noise transfer to the Stokes field. When the cavity round-trip time is much longer than the nonlinear interaction time in the gain medium, the predicted noise-suppression ratio is governed by the ratio between these two timescales. These results identify the discrete interaction-propagation structure as the physical origin of the unusually strong noise suppression in free-space Brillouin laser systems.

The Pound-Drever-Hall (PDH) scheme is a well-known technique for stabilizing a laser's frequency to a reference length such as an optical cavity. However, if a PDH setup is not carefully designed, one might easily create parasitic etalons, which are caused by back reflections in the optical path. These etalons alter the electric fields being read out and, therefore, decrease the PDH performance. In this paper, we describe these etalons as the amplitude modulation of the nominal PDH signals and identify two different kinds of etalons: internal ones formed within a plane-parallel window and free-space etalons between two interfaces of different optical components. We experimentally probe these two etalons and verify our theory. Furthermore, we employ a dual-modulation scheme that is capable of measuring the optical cavity's Free Spectral Range and discuss the influence of these additional $\mathrm{G}\mathrm{Hz}$ sidebands on the etalons. We conduct this study in the framework of future space-based interferometric laser ranging instruments in DLR/NASA's GRACE-Continuity and ESA's Next-Generation Gravity Mission.

The full Schmidt decomposition of spatiotemporally entangled states generated from spontaneous parametric down-conversion (SPDC) has not been carried out until now due to the immense computational complexity arising from the large dimensionalities of the states. In this article, we utilize the rotational symmetry of the states to reduce the complexity by at least four orders of magnitude and carry out the decomposition to reveal the precise forms of the spatiotemporal Schmidt modes and the Schmidt spectrum spanning over ${10}^4$ modes. We show that the Schmidt modes have a phase profile with a transverse spatial vortex structure that endows them with orbital angular momentum at all frequencies. In the high-gain regime, these Schmidt modes broaden and the Schmidt spectrum narrows with increasing pump strength. Our work can spur novel applications in quantum imaging and spectroscopy that utilize the spatiotemporal entanglement produced from SPDC.

We provide alternative linear-combination-of-unitaries decompositions for a class of discrete elliptic differential operators. Specifically, Poisson problems augmented with periodic, Dirichlet, Neumann, Robin, and mixed boundary conditions are considered on the unit interval and on higher-dimensional rectangular domains. The number of unitary terms required for our decomposition is independent of the number of grid points used in the discretization and scales linearly with the spatial dimension. Explicit circuit constructions for each unitary are given and their complexities analyzed. The worst case depth and elementary gate cost of any such circuit is shown to scale at most logarithmically with respect to number of grid points in the underlying discrete system. We also investigate the cost of using our method within the variational quantum linear solver algorithm and show favorable scaling. Finally, we extend the proposed decomposition technique to treat problems that include first-order derivative terms with variable coefficients.

We compare several definitions of entropy production rate introduced in the literature from a large variety of situations and motivations, and then analyze their relations with memory effects. Considering a relevant experimental example of a qubit interacting with a single bosonic mode playing the role of a finite bath, we show that all definitions of entropy production coincide at weak coupling. In a stronger-coupling regime, significant discrepancies emerge between the different entropy production rates, although some similarities in the overall behavior remain. However, surprisingly, two of these definitions—one based on local quantities of the system and the other on nonlocal quantities—coincide exactly, even in the case of strong coupling. Finally, a high degree of correspondence is observed when memory effects characterized by P-divisibility are compared with the sign of all entropy production rates in the case of weak coupling. Such correspondence degrades at stronger coupling, leading us to extend the concept of entropy production to the dynamical map. We show a perfect equivalence between the sign of this enlarged concept of entropy production and P-divisibility, both numerically and analytically, in the case of phase-covariant master equations.

We study a quadratic open SSH chain with a single-site loss and show that the many-body fermionic Lindblad problem admits an exact reduction to a finite non-Hermitian one-body matrix with a rank-one imaginary impurity. Its rapidities generate the complete Liouvillian spectrum and reveal three mechanisms governing the slowest relaxation. At weak loss, decay is selected by the clean local spectral weight at the lossy site, yielding the generic law ${\mathrm{\Delta }}_{\mathcal{L}}\sim \gamma N^{-3}$ and, in the topological regime, exponentially smaller edge-controlled gaps. At intermediate loss, a centered bulk-loss geometry reaches an exact exceptional point on the real-$\gamma$ axis. Symmetry-related rapidity pairs coalesce simultaneously, including the pair at the lower rapidity edge. Exact real-space dynamics at this lower-edge exceptional point exhibits a polynomially enhanced exponential tail, whereas a matched high-energy control with the same parity-even one-body decay edge but no lower-edge defectiveness remains nearly exponential. At strong loss, one ultrafast defect mode separates from an active slow sector governed by a cut-chain Zeno problem, giving ${\mathrm{\Delta }}_{\mathcal{L}}\sim {\gamma }^{-1}$ up to a geometry-dependent prefactor. The full finite fermionic Liouvillian spectrum, including its operator-parity sectors and subset-sum structure, is statistics specific. By contrast, the elementary one-body decay spectrum and the three associated mechanisms are governed by a finite-dimensional linear drift matrix, so their spectral and dynamical signatures can also be accessed in bosonic and classical-wave platforms engineered to realize the same effective matrix. These results establish how topology, defect geometry, and local dissipation jointly organize long-time relaxation in an open dimerized lattice.

Qudits, or multilevel quantum information carriers, present a promising path for scaling quantum computers. However, their use introduces increased complexity in quantum logic, necessitating careful control of relative phases between different qudit levels. In trapped-ion systems, entangling operations accumulate phases on specific levels that are no longer global, unlike in qubit architectures. Furthermore, the structure of multilevel gates becomes increasingly intricate with higher-dimensional Hilbert spaces. This work explores the theory of these additional local and nonlocal phases, accumulated in Mølmer-Sørensen and light-shift gates. We propose methods to actively compensate for these phases, enhance gate robustness against parameter fluctuations, and simplify native gates for more efficient circuit decomposition. Our results pave the way toward the practical and scalable implementation of qudit-based quantum processors.

We investigate the quantum transport of delocalized states in continuous-time quantum walks (CTQWs) on a one-dimensional lattice containing a single defect. The defect is modeled by assigning complex-valued hopping amplitudes to the edges that connect the site corresponding to the mean position of the delocalized initial state to its nearest neighbors. We find that this single defective site is sufficient to enhance the ballistic spreading of an initially Gaussian wave packet. Extending these results, we implement a time-dependent alternation protocol between two distinct defect configurations, each individually yielding poor propagation of the state. The combination of these two unfavorable configurations improves the transport efficiency of the quantum walker, revealing a manifestation of Parrondo's paradox in CTQWs with delocalized initial states. This study provides insights into the role of complex-phase defects and time-dependent protocols in CTQWs, demonstrating that the interplay between quantum interference and graph engineering can effectively enhance quantum transport in discrete lattices.

Cavity optomagnomechanics provides a versatile platform to explore macroscopic quantum correlations, particularly nonreciprocal entanglement. In this work we propose a theoretical scheme to generate switchable bipartite and tripartite entanglement in an optomagnomechanical ring cavity by exploiting phase-controlled magnon squeezing. Indeed, two spatially separated ferrimagnetic yttrium iron garnet microbridges become entangled through their magnetostriction-mediated coupling to mechanical motion and a common cavity field via radiation-pressure interaction. The squeezing process introduces two phase-dependent contributions to the magnon response, namely, an effective detuning shift ${\mathrm{\Delta }}_{{\theta }_j}$ and a quadrature-damping contribution ${\kappa }_{{\theta }_j}$, both of which reverse sign upon a $\pi$ phase shift, providing an in situ control to switch the entanglement response. The nonreciprocal entanglement is defined operationally through the asymmetric entanglement response under the phase reversal ${\theta }_j\to {\theta }_j+\pi$, quantified by normalized contrast ratios $C_E$ and $C_{\mathcal{R}}$, which measure the relative difference between the entanglement obtained at ${\theta }_j$ and at the phase-reversed configuration ${\theta }_j+\pi$. The resulting phase-tuning method provides a flexible and robust route to achieve high-contrast bipartite and tripartite entanglement within stable parameter regions, establishing magnon squeezing as a practical quantum resource for switchable quantum correlations in hybrid platforms.

Quantum technologies provide solutions for information processing tasks that are not achievable with classical methods. Basic resources like the creation of private and unexpected random numbers or a secure key that is shared between two parties are examples of these capabilities. However, estimating the amount of generated randomness or generated key is not easy. This work presents a thorough analysis of quantum randomness generation (QRNG) and quantum key distribution (QKD) protocols using various Bell inequalities. We specifically focus on the so-called modified Clauser-Horne-Shimony-Holt and three-setting Braunstein-Caves expressions, demonstrating their efficiency in both use cases due to their robustness against noise and high asymptotic generation rates. Our findings highlight that these two inequalities not only perform best in terms of certified randomness and secure key rates, but also enable the use of the same experimental setup for both QRNG and QKD. This dual functionality offers significant design advantages for large-scale implementation. We also include an analysis of finite-data effects using the entropy accumulation theorem, providing a realistic assessment of the protocols. The insights of the investigation contribute to the development of more efficient and versatile quantum cryptography systems based on entangled photons.

We theoretically investigate the interaction between azimuthal vector vortex beams and a segmented ring medium, revealing orbital-to-spin conversion in strong-field high-order harmonic generation. We show that engineered gaps in the ring target break the light-matter symmetry and mediate the transfer of the driving field's orbital angular momentum (OAM) into the spin angular momentum (SAM) of on-axis harmonic photons. The far-field harmonics exhibit alternating spin states governed by a double selection rule dependent on both gap number and relative width, reminiscent of multislit diffraction. The Fraunhofer diffraction pattern further constitutes a fingerprint of the native $n$-fold rotational symmetry in OAM-to-SAM conversion, thereby furnishing a spectroscopic diagnostic of nonlinear coupling between structured light and patterned media.

Collective dissipation can generate useful quantum correlations, while ubiquitous individual decay destroys them. We study the interplay between these two competing processes considering a driven system of many spins (“atoms”) undergoing both collective and individual dissipation (“radiation”). In the steady state and depending on drive, we find that the system exhibits a first-order phase transition and quantum bistability: its quantum state is a mixture of two many-body states associated with the two competing decay processes. Accordingly, one of these states closely resembles a correlated “coherently radiating spin state” (CRSS)—the solution of purely collective dissipation—exhibiting spin-squeezing entanglement. We predict dynamical switching between the two stable states, manifest as many-body quantum jumps in the various observables of spin and radiation. Macroscopically, the switching rate tends to vanish and the system can reside in a correlated CRSS for long times. This reveals how correlated dissipative physics emerges at the presence of decorrelating individual decay, opening a path for unlocking collective dissipation phenomena in realistic quantum platforms and applications. We discuss consequences for experiments in collective radiation.

The scalable preparation of large photon-number (Fock) states is a long-standing frontier in quantum science, with direct implications for quantum metrology and bosonic quantum information processing. Despite substantial progress at small photon numbers, extending state generation to large photon numbers while maintaining high fidelity and operating deterministically remains a significant challenge. Here, we demonstrate a scalable and experimentally accessible control protocol for generating large photon-number states using only native spin-oscillator operations. The protocol alternates Jaynes-Cummings interactions with phase-space displacements to imprint photon-number-dependent phases and convert them into selective interference in photon-number space. It already achieves high-preparation fidelity unconditionally, while an optional final qubit projection removes residual qubit-field correlations and further enhances the fidelity. Conditioned on this final projection, photon-number state preparation with fidelities exceeding 0.95 is achieved for photon numbers in the few-hundred regime, with a success probability exceeding 0.90, placing the protocol in a near-deterministic operating regime. The resulting control sequences remain shallow and are robust against detuning, control noise, and experimentally relevant dissipation. Our results establish a scalable and experimentally realistic paradigm for synthesizing large nonclassical bosonic resource states, with immediate implications for quantum metrology and bosonic quantum information processing.

Many existing genuine multipartite entanglement (GME) witnesses for continuous-variable (CV) quantum systems typically rely on quadrature measurements, which are challenging to implement in platforms where the CV degrees of freedom can be indirectly accessed only through qubit readouts. In this work, we propose methods to implement GME witnesses through phase-space measurements in state-of-the-art experimental platforms, leveraging controlled Gaussian unitaries readily available in qubit-CV architectures. Based on two theoretical results showing that sufficient Wigner negativity can certify GME, we present five concrete implementation schemes using controlled parity, displacement, and beam-splitter operations. Our witnesses can detect paradigmatic GME states like the Dicke and multipartite $N00N$ states, which include the W states as a special case and GHZ-type entangled cat states. We analyze the performance of these witnesses under realistic noise conditions and finite measurement resolution, showing their robustness to experimental imperfections. Crucially, our implementations require exponentially fewer measurement settings than full tomography, with one scheme requiring only a single measurement on auxiliary modes. The methods are readily applicable to circuit or cavity quantum electrodynamics, circuit quantum acoustodynamics, as well as trapped ions and atomic systems, where such dichotomic phase-space measurements are already routinely performed as native readouts.

We introduce a measure for evaluating the efficiency of finite universal quantum gate sets $\mathcal{S}$, called the quantum circuit overhead (QCO), and the related notion of $T$-quantum circuit overhead $\left(T\text{−}\mathrm{QCO}\right)$. QCO compares the circuit length required by $\mathcal{S}$ with the best possible length among gate sets of the same size. The $T$-QCO adapts this idea to cost models in which only selected costly gates are counted, while cheap operations are absorbed into an effective gate set. We demonstrate the usefulness of the $\left(T\text{−}\right)\mathrm{QCO}$ by extensive numerical calculations of its upper bounds, providing insight into the efficiency of various choices of single-qubit $\mathcal{S}$, including Haar-random gate sets and the gate sets derived from finite subgroups, such as Clifford and Hurwitz groups. In particular, our results suggest that, in terms of the upper bounds on the $T\text{−}\mathrm{QCO}$, the famous T gate is a highly nonoptimal choice for the completion of the Clifford gate set, even among the gates of order 8. We identify the optimal choices of such completions for both finite subgroups.