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Latest articles for New Journal of Physics
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Phase-matching quantum key distribution with heralded single-photon sources and local detection
Channel loss in long-distance phase-matching quantum key distribution (PM-QKD) amplifies vacuum emissions and dark counts, impairing parameter estimation and limiting secure transmission distance. In this paper, we propose a PM-QKD protocol based on heralded single-photon sources with local detection. In the key sifting stage, local heralding outcomes are filtered to suppress vacuum-dominated noise and enhance the single-photon fraction of the retained ensemble. A three-intensity decoy-state method is used to estimate the single-photon contribution and bound the phase-error rate. An asymptotic security analysis is provided. Simulation results show that the proposed protocol surpasses the Pirandola–Laurenza–Ottaviani–Banchi bound at and achieves a practical key rate of at , while the conventional PM-QKD protocol based on weak coherent sources reaches the same key rate only at . These gains are mainly attributed to the vacuum-event filtering strategy, which effectively improves both key-rate performance and transmission distance. Furthermore, our evaluation of a practical threshold detector implementation demonstrates that photon-number-resolving detectors are not mandatory to achieve this advantage.
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The resource theory of causal influence and knowledge of causal influence
Understanding and quantifying causal relationships between variables is essential for reasoning about the physical world. A challenge to this is the fact that different functional causal models can induce the same conditional probability distribution over observed variables while encoding different amounts of causal influence. Motivated by this observation, we study the underlying functional mechanisms, including uncertainty over them, as meaningful objects in their own right and develop a resource-theoretic framework for quantifying causal influence and knowledge of causal influence. We focus on the simplest nontrivial setting—two variables that are causally ordered, meaning that the first has the potential to influence the second, without hidden confounding. First, we introduce the resource theory that directly quantifies causal influence of a functional dependence in this setting and show that the problem of deciding convertibility of resources and identifying a complete set of monotones has a relatively straightforward solution. Following this, we introduce the resource theory that arises naturally when one has uncertainty about the functional dependence. We describe a linear program for deciding the question of whether one resource (i.e. state of knowledge about the functional dependence) can be converted to another. Then, we focus on the case where the variables are binary. In this case, we identify a triple of monotones that are complete in the sense that they capture the partial order over the set of all resources, and we provide an interpretation of each.
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Odd transport in a two-temperature Brownian dimer
We investigate a two-temperature Brownian dimer with odd mobility, characterized by an antisymmetric component of the transport response, as a controlled paradigm for odd nonequilibrium dynamics. Two confined Brownian particles are elastically coupled and connected to reservoirs at different temperatures, while odd mobility converts conservative forces into transverse motion. For the symmetric harmonic reference model we obtain the nonequilibrium steady state exactly. Odd mobility generates a handed transverse interparticle correlation while the one-particle distributions remain isotropic, and the correlation exists only when temperature imbalance, elastic coupling, and odd response act together. The steady probability current separates into an ordinary irreversible component and a chiral contribution which, upon marginalization, produces counter-rotating one-particle currents. We derive the corresponding heat transfer and entropy production and show that odd mobility enhances thermal conductance, whereas scalar thermodynamic observables are unchanged when the odd handedness is reversed. We further solve the harmonically confined dimer with unequal trap stiffnesses and find that moderate mechanical asymmetry can enhance the normalized transverse correlation while preserving counter-rotation. Finally, small-mass Langevin simulations with quartic confinement show that the transverse odd correlation and counter-rotating currents persist when the steady state becomes explicitly non-Gaussian.
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Quantum criticality beyond thermodynamic stability
For a many-body system in equilibrium, described by a thermodynamically stable Hamiltonian, quantum criticality is associated with structural changes of the many-body ground state. However, there exist physically relevant models, notably, certain quadratic bosonic Hamiltonians (QBHs), which fail to have a ground state. From a dynamical system-theory standpoint, QBHs can be either dynamically stable or unstable. We show that criticality is a meaningful concept for the entire class of QBHs that are dynamically stable or at the boundary of instability—regardless of thermodynamic stability—and that the key state for such QBHs is a naturally and unambiguously defined quasiparticle vacuum(QPV). This state is Gaussian, and coincides with the ground state if the QBH is thermodynamically stable. We identify a relevant spectral gap, the Krein gap, associated to the minimal spectral separation between creation and annihilation operators, and show that the QPV is unique when the Krein gap is positive. In addition, we prove that, for dynamically stable QBHs with finite-range couplings, correlations are exponentially bounded unless the Krein gap closes—which is associated with one of two kinds of spectral degeneracies: an exceptional point (physically, a free-particle-like collective mode) or a Krein collision (often a bosonic zero mode). As a consequence, long-range correlations can ensue in the QPV. We conclude that (i) the Krein gap takes the role of the spectral gap for dynamically stable QBHs, and (ii) the boundary of dynamical stability and criticality (associated to exceptional points) or multicriticality (associated to Krein collisions) are one and the same. Alongside long-range correlations, we find that bosonic critical behavior beyond thermodynamic stability is witnessed by the scaling of the entanglement entropy and other indicators of equilibrium criticality from information geometry. Hence, our framework opens the door to investigating all dynamically stable QBHs through the lens of critical phenomena, including thermodynamically unstable models from photonics, opto-mechanics, cavity-QED, and magnonics.
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The chiral random walk: a quantum-inspired framework for odd diffusion
Chirality in active and passive fluids gives rise to odd transport properties, most notably the emergence of robust edge currents that defy standard dissipative dynamics. While these phenomena are well-described by continuum hydrodynamics, the topological analogy with quantum systems, widely invoked to explain their robustness, has lacked a concrete discrete realization. Here, we present a lattice model for an isotropic chiral random walk that bridges the gap between classical stochastic diffusion and unitary quantum evolution. By equipping the walker with an internal degree of freedom and a tunable chirality parameter, , we interpolate between a standard diffusive random walk and a deterministic, topologically non-trivial quantum walk. We show that signatures of the topologically protected edge states characteristic of the unitary limit ( ) persist into the dissipative regime ( ) This correspondence allows us to theoretically ground the robustness of edge flows in classical chiral systems using the bulk-boundary correspondence of Floquet topological insulators. Our results provide a discrete, effective description for odd diffusion, offering a powerful toolkit to predict transport in confined geometries and disordered chiral media.