We study recoverable entanglement in a four-qubit cluster-state register whose two detector qubits couple to a scalar field. A local preparation phase leaves the initial reference--register entanglement and detector marginal unchanged, but controls protection under collective noise. At the optimal phase, a fixed decoder recovers a Bell pair after any channel preserving the singlet and triplet sectors. For uniformly accelerated detectors at finite separation, an exact trace-norm formula shows that the same phase is optimal at every time within the secular weak-coupling dynamics. Small separation produces a slow decay, whose rate and phase-dependent amplitude we derive. Matching the local response to an inertial thermal bath gives an analytic optimum time for the entanglement contrast. A detector measurement and conditional phase correction recover a two-qubit teleportation resource. Spatial smearing and an ordered weak-coupling then small-separation limit specify the microscopic scope of the lifetime prediction.
We study recoverable entanglement in a four-qubit cluster-state register whose two detector qubits couple to a scalar field. A local preparation phase leaves the initial reference--register entanglement and detector marginal unchanged, but controls protection under collective noise. At the optimal phase, a fixed decoder recovers a Bell pair after any channel preserving the singlet and triplet sectors. For uniformly accelerated detectors at finite separation, an exact trace-norm formula shows that the same phase is optimal at every time within the secular weak-coupling dynamics. Small separation produces a slow decay, whose rate and phase-dependent amplitude we derive. Matching the local response to an inertial thermal bath gives an analytic optimum time for the entanglement contrast. A detector measurement and conditional phase correction recover a two-qubit teleportation resource. Spatial smearing and an ordered weak-coupling then small-separation limit specify the microscopic scope of the lifetime prediction.
We relate Berry-phase control and retained entropy geometry to recoverable coherence, entanglement, and Clauser-Horne-Shimony-Holt (CHSH) correlations. For a depolarized Bell pair subject to recorded phase noise, we determine exact CHSH optima when Bob retains his record and when he compresses it with his qubit into one qubit by any deterministic quantum channel. Some records preserve a violation that every such compression loses. The retained transverse metric gives sharp resource intervals. Nevertheless, for every finite derivative order, two finite readouts of the same noise can match all pulled-back monotone-metric and relative-entropy derivatives at faithful diagonal inputs while giving opposite recovered CHSH outcomes. Complete transverse entropy response determines the reliability law. A finite transitionless Berry loop realizes the channel; zero-expectation conditional control errors give a quadratic reduced-channel accuracy certificate. Transport, thermal, and accelerated-detector applications specialize established channel criteria under explicit preparation and accuracy assumptions. These model-dependent certificates supply neither a universal entanglement lifetime nor a detector-independent acceleration bound.
We study the full entanglement dynamics of two uniformly accelerated Unruh-DeWitt detectors with no direct interaction in between but each coupled to a common quantum field and moving back-to-back in the field vacuum. For two detectors initially prepared in a separable state our exact results show that quantum entanglement between the detectors can be created by the quantum field under some specific circumstances, though each detector never enters the other's light cone in this setup. In the weak coupling limit, this entanglement creation can occur only if the initial moment is placed early enough and the proper acceleration of the detectors is not too large or too small compared to the natural frequency of the detectors. Once entanglement is created it lasts only a finite duration, and always disappears at late times. Prior result by Reznik [B. Reznik, Found. Phys. 33, 167 (2003).] derived using the time-dependent perturbation theory with extended integration domain is shown to be a limiting case of our exact solutions at some specific moment. In the strong coupling and high acceleration regime, vacuum fluctuations experienced by each detector locally always dominate over the cross correlations between the detectors, so entanglement between the detectors will never be generated.
We derive an explicit test of whether a reduced quantum-memory model overstates recoverability. For channels with orthogonal logical outputs in distinguishable sectors, the minimum recovery error follows directly from differences between environmental record amplitudes, without numerical recovery optimization. Applying this test to two-excitation oscillator–qubit and four-qubit encodings reveals that matching single-loss responses need not preserve finite-time recoverability. Under collective loss, the four-qubit encoding is perfectly recoverable, whereas oscillator emission records reveal logical information. The resulting unavoidable error initially grows as the cube of storage time. The test quantifies omitted-level effects and benchmarks reduced models and recovery algorithms.
Abstract A controlled-NOT gate acting on a coherent control qubit converts single-qubit coherence into two-qubit entanglement, ideally only if the gate is perfect and the entangled state is used at once. We analyze how this conversion degrades when the gate carries a fault and the output then idles in a finite-temperature environment of relaxation, thermal excitation, and dephasing. Four representative fault classes are treated: coherent under- or over-rotation, a coherent control-phase error, a stochastic phase flip, and a stochastic bit flip. The gate faults and the idle environment are matched in a limited but explicit sense, referred to a common set of device parameters; a commutation argument identifies when an intra-gate noise process reduces exactly to one of the four classes, with the residual error quantified in the borderline cases. Within this deliberately minimal model we obtain closed-form expressions for the conversion efficiency of each fault class, for the entanglement surviving the idle period, and for the boundary at which entanglement disappears abruptly, recovering the known amplitude-damping threshold for sudden death as a special case. The classes leave distinct signatures: coherent calibration errors are insensitive to leading order and therefore mild, stochastic faults degrade the conversion in proportion to their probability, and the bit-flip class alone carries an angle-dependent signature separating it from a phase flip. Control-phase errors leave the entanglement measure unchanged and are invisible to this benchmark, a limitation we state plainly rather than present as a strength. All analytic results are verified against independent numerical simulation, and a diagnostic inferring the dominant fault class from angle-resolved measurements identifies strong stochastic faults reliably but cannot separate weak coherent errors from an ideal gate. The results are intended as transparent analytic benchmarks for a specific resource-conversion task, not as general device characterization.
Collective coupling to the Minkowski vacuum allows uniformly accelerated quantum detectors to exhibit a strong quantum Mpemba effect, in which a state initially farther from equilibrium relaxes faster than a closer one. We establish the existence of this effect for every fixed finite number of detectors N≥2 at sufficiently small transverse separation within the pointlike secular master equation. The mechanism originates from two reflection sectors whose populations are conserved by the leading collective dynamics. Weak intersector transfer at finite separation generates an isolated, parametrically slow relaxation mode. A simple family of initial states permits its leading-order cancellation by matching the equilibrium sector populations, while a corrected preparation eliminates the slow-mode amplitude exactly. We construct a reference state that establishes a genuine reversal of trace-distance ordering. Numerical calculations and local-rate-preserving controls identify the collective origin of the effect, while spatially smeared detector calculations support its persistence within the second-order secular framework. The same mechanism extends to detectors coupled to a common thermal field, with analytically distinct slow relaxation rates. Thus, strong Mpemba relaxation is not exclusive to acceleration but emerges from a more general interplay between collective dissipation, reflection-sector conservation, and weak symmetry-breaking transfer.
The dynamics of entanglement and fidelity for a subsystem of two separate spin-1∕2 qubits prepared in Bell states is investigated. One of the subsystem qubit labeled A is under the influence of a Heisenberg XY spin bath, while another one labeled B is uncoupled with that. We discuss two cases: (i) the number of bath spins is infinite, $N\ensuremath{\rightarrow}\ensuremath{\infty}$, and (ii) N is finite, N=40. In both cases, the bath is initially prepared in a thermal equilibrium state. It is shown that the time dependence of the concurrence and the fidelity of the two subsystem qubits can be controlled by tuning the parameters of the spin bath, such as the anisotropic parameter, the temperature, and the coupling strength with qubit A. It is interesting to find that the dynamics of the concurrence is independent of four different initial Bell states and that of the fidelity is divided into two groups.