3,532 research outputs found
Experimentally reducing the quantum measurement back-action in work distributions by a collective measurement
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
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
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
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
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
, i.e., we look for channels that induce the same
classical transitions , but are "more coherent". To quantify the coherence
of a channel we measure the coherence of the corresponding
Jamio{\l}kowski state . We show that the classical transition matrix
can be coherified to reversible unitary dynamics if and only if is
unistochastic. Otherwise the Jamio{\l}kowski state of
the optimally coherified channel is mixed, and the dynamics must necessarily be
irreversible. To assess the extent to which an optimal process
is indeterministic we find explicit bounds on the entropy
and purity of , and relate the latter to the unitarity of
. 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
and reduces its rank (the minimal number of required Kraus operators) from
to .Comment: 20 pages, 8 figures. Published versio
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