3,532 research outputs found

    Experimentally reducing the quantum measurement back-action in work distributions by a collective measurement

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    In quantum thermodynamics, the standard approach to estimate work fluctuations in unitary processes is based on two projective measurements, one performed at the beginning of the process and one at the end. The first measurement destroys any initial coherence in the energy basis, thus preventing later interference effects. In order to decrease this back-action, a scheme based on collective measurements has been proposed in~[PRL 118, 070601 (2017)]. Here, we report its experimental implementation in an optical system. The experiment consists of a deterministic collective measurement on identically prepared two qubits, encoded in the polarisation and path degree of a single photon. The standard two projective measurement approach is also experimentally realized for comparison. Our results show the potential of collective schemes to decrease the back-action of projective measurements, and capture subtle effects arising from quantum coherence.Comment: 9 pages, 4 figure

    Cohering and decohering power of massive scalar fields under instantaneous interactions

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    Employing a non-perturbative approach based on an instantaneous interaction between a two-level Unruh-DeWitt detector and a massive scalar field, we investigate the ability of the field to generate or destroy coherence in the detector by deriving the cohering and decohering power of the induced quantum evolution channel. For a field in a coherent state a previously unnoticed effect is reported whereby the amount of coherence that the field generates displays a revival pattern with respect to the size of the detector. It is demonstrated that by including mass in a thermal field the set of maximally coherent states of the detector decoheres less compared to a zero mass. In both of the examples mentioned, by making a suitable choice of detector radius, field energy and coupling strength it is possible to infer the mass of the field by either measuring the coherence present in the detector in the case of an interaction with a coherent field or the corresponding decoherence of a maximally coherent state in the case of a thermal field. In view of recent advances in the study of Proca metamaterials, these results suggest the possibility of utilising the theory of massive electromagnetism for the construction of novel applications for use in quantum technologies

    Coherence generating power of unitary transformations via probabilistic average

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    We study the ability of a quantum channel to generate quantum coherence when it applies to incoherent states. Based on probabilistic averages, we define a measure of such coherence generating power (CGP) for a generic quantum channel, based on the average coherence generated by the quantum channel acting on a uniform ensemble of incoherent states. Explicit analytical formula of the CGP for any unitary channels are presented in terms of subentropy. An upper bound for CGP of unital quantum channels has been also derived. Detailed examples are investigated.Comment: 16 pages, 2 figures, LaTeX, accepted versio

    Phase-resolved spectroscopy of low frequency quasi-periodic oscillations in GRS 1915+105

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    X-ray radiation from black hole binary (BHB) systems regularly displays quasi-periodic oscillations (QPOs). In principle, a number of suggested physical mechanisms can reproduce their power spectral properties, thus more powerful diagnostics which preserve phase are required to discern between different models. In this paper, we first find for two Rossi X-ray Timing Explorer (RXTE) observations of the BHB GRS 1915+105 that the QPO has a well defined average waveform. That is, the phase difference and amplitude ratios between the first two harmonics vary tightly around a well defined mean. This enables us to reconstruct QPO waveforms in each energy channel, in order to constrain QPO phase-resolved spectra. We fit these phase resolved spectra across 16 phases with a model including Comptonisation and reflection (Gaussian and smeared edge components) to find strong spectral pivoting and a modulation in the iron line equivalent width. The latter indicates the observed reflection fraction is changing throughout the QPO cycle. This points to a geometric QPO origin, although we note that the data presented here do not entirely rule out an alternative interpretation of variable disc ionisation state. We also see tentative hints of modulations in the iron line centroid and width which, although not statistically significant, could result from a non-azimuthally symmetric QPO mechanism.Comment: Accepted for publication in MNRA

    Coherifying quantum channels

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    Is it always possible to explain random stochastic transitions between states of a finite-dimensional system as arising from the deterministic quantum evolution of the system? If not, then what is the minimal amount of randomness required by quantum theory to explain a given stochastic process? Here, we address this problem by studying possible coherifications of a quantum channel Φ\Phi, i.e., we look for channels ΦC\Phi^{\mathcal{C}} that induce the same classical transitions TT, but are "more coherent". To quantify the coherence of a channel Φ\Phi we measure the coherence of the corresponding Jamio{\l}kowski state JΦJ_{\Phi}. We show that the classical transition matrix TT can be coherified to reversible unitary dynamics if and only if TT is unistochastic. Otherwise the Jamio{\l}kowski state JΦCJ_\Phi^{\mathcal{C}} of the optimally coherified channel is mixed, and the dynamics must necessarily be irreversible. To assess the extent to which an optimal process ΦC\Phi^{\mathcal{C}} is indeterministic we find explicit bounds on the entropy and purity of JΦCJ_\Phi^{\mathcal{C}}, and relate the latter to the unitarity of ΦC\Phi^{\mathcal{C}}. We also find optimal coherifications for several classes of channels, including all one-qubit channels. Finally, we provide a non-optimal coherification procedure that works for an arbitrary channel Φ\Phi and reduces its rank (the minimal number of required Kraus operators) from d2d^2 to dd.Comment: 20 pages, 8 figures. Published versio
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