675 research outputs found
Quantum discord between relatively accelerated observers
We calculate the quantum discord between two free modes of a scalar field
which start in a maximally entangled state and then undergo a relative,
constant acceleration. In a regime where there is no distillable entanglement
due to the Unruh effect, we show that there is a finite amount of quantum
discord, which is a measure of purely quantum correlations in a state, over and
above quantum entanglement. Even in the limit of infinite acceleration of the
observer detecting one of the modes, we provide evidence for a non-zero amount
of purely quantum correlations, which might be exploited to gain non-trivial
quantum advantages.Comment: 4 pages, 2 figure
Signatures of non-classicality in mixed-state quantum computation
We investigate signatures of non-classicality in quantum states, in
particular, those involved in the DQC1 model of mixed-state quantum computation
[Phys. Rev. Lett. 81, 5672 (1998)]. To do so, we consider two known
non-classicality criteria. The first quantifies disturbance of a quantum state
under locally noneffective unitary operations (LNU), which are local unitaries
acting invariantly on a subsystem. The second quantifies measurement induced
disturbance (MID) in the eigenbasis of the reduced density matrices. We study
the role of both figures of non-classicality in the exponential speedup of the
DQC1 model and compare them vis-a-vis the interpretation provided in terms of
quantum discord. In particular, we prove that a non-zero quantum discord
implies a non-zero shift under LNUs. We also use the MID measure to study the
locking of classical correlations [Phys. Rev. Lett. 92, 067902 (2004)] using
two mutually unbiased bases (MUB). We find the MID measure to exactly
correspond to the number of locked bits of correlation. For three or more MUBs,
it predicts the possibility of superior locking effects.Comment: Published version, containing additional discussion on the role of
non-classicality in the locking of classical correlation
Precision metrology using weak measurements
Weak values and measurements have been proposed as means to achieve dramatic
enhancements in metrology based on the greatly increased range of possible
measurement outcomes. Unfortunately, the very large values of measurement
outcomes occur with highly suppressed probabilities. This raises three vital
questions in weak-measurement-based metrology, namely, (Q1) Does post-selection
enhance the measurement precision? (Q2) Does weak measurement offer better
precision than strong measurement? (Q3) Is it possible to beat the standard
quantum limit or to achieve the Heisenberg limit with weak measurement using
only classical resources? We analyse these questions for two prototypical, and
generic, measurement protocols and show that while the answers to the first two
questions are negative for both protocols, the answer to the last is
affirmative for measurements with phase-space interactions, and negative for
configuration space interactions. Our results, particularly the ability of weak
measurements to perform at par with strong measurements in some cases, are
instructive for the design of weak-measurement-based protocols for quantum
metrology.Comment: 5+5 pages, 2 figure
Quantum Enhanced Multiple Phase Estimation
We study the simultaneous estimation of multiple phases as a discretised
model for the imaging of a phase object. We identify quantum probe states that
provide an enhancement compared to the best quantum scheme for the estimation
of each individual phase separately, as well as improvements over classical
strategies. Our strategy provides an advantage in the variance of the
estimation over individual quantum estimation schemes that scales as O(d) where
d is the number of phases. Finally, we study the attainability of this limit
using realistic probes and photon-number-resolving detectors. This is a problem
in which an intrinsic advantage is derived from the estimation of multiple
parameters simultaneously.Comment: Accepted by Physical Review Letter
Fundamental limits of quantum-secure covert optical sensing
We present a square root law for active sensing of phase of a single
pixel using optical probes that pass through a single-mode lossy thermal-noise
bosonic channel. Specifically, we show that, when the sensor uses an -mode
covert optical probe, the mean squared error (MSE) of the resulting estimator
scales as ; improving the
scaling necessarily leads to detection by the adversary with high probability.
We fully characterize this limit and show that it is achievable using laser
light illumination and a heterodyne receiver, even when the adversary captures
every photon that does not return to the sensor and performs arbitrarily
complex measurement as permitted by the laws of quantum mechanics.Comment: 13 pages, 1 figure, submitted to ISIT 201
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