361 research outputs found
QPSK coherent state discrimination via a hybrid receiver
We propose and experimentally demonstrate a near-optimal discrimination
scheme for the quadrature phase shift keying protocol (QPSK). We show in theory
that the performance of our hybrid scheme is superior to the standard scheme -
heterodyne detection - for all signal amplitudes and underpin the predictions
with our experimental results. Furthermore, our scheme provides the hitherto
best performance in the domain of highly attenuated signals. The discrimination
is composed of a quadrature measurement, a conditional displacement and a
threshold detector
Multiple-Access Bosonic Communications
The maximum rates for reliably transmitting classical information over
Bosonic multiple-access channels (MACs) are derived when the transmitters are
restricted to coherent-state encodings. Inner and outer bounds for the ultimate
capacity region of the Bosonic MAC are also presented. It is shown that the
sum-rate upper bound is achievable with a coherent-state encoding and that the
entire region is asymptotically achievable in the limit of large mean input
photon numbers.Comment: 11 pages, 5 figures, corrected two figures, accepted for publication
in Phys. Rev.
Classical capacity of the lossy bosonic channel: the exact solution
The classical capacity of the lossy bosonic channel is calculated exactly. It
is shown that its Holevo information is not superadditive, and that a
coherent-state encoding achieves capacity. The capacity of far-field,
free-space optical communications is given as an example.Comment: 4 pages, 2 figures (revised version
Binary optical communication in single-mode and entangled quantum noisy channels
We address binary optical communication in single-mode and entangled quantum
noisy channels. For single-mode we present a systematic comparison between
direct photodetection and homodyne detection in realistic conditions, i.e.
taking into account the noise that occurs both during the propagation and the
detection of the signals. We then consider entangled channels based on
twin-beam state of radiation, and show that with realistic heterodyne detection
the error probability at fixed channel energy is reduced in comparison to the
single-mode cases for a large range of values of quantum efficiency and noise
parameters
Gaussian benchmark for optical communication aiming towards ultimate capacity
We establish the fundamental limit of communication capacity within Gaussian
schemes under phase-insensitive Gaussian channels, which employ multimode
Gaussian states for encoding and collective Gaussian operations and
measurements for decoding. We prove that this Gaussian capacity is additive,
i.e., its upper bound occurs with separable encoding and separable receivers so
that a single-mode communication suffices to achieve the largest capacity under
Gaussian schemes. This rigorously characterizes the gap between the ultimate
Holevo capacity and the capacity within Gaussian communication, showing that
Gaussian regime is not sufficient to achieve the Holevo bound particularly in
the low-photon regime. Furthermore the Gaussian benchmark established here can
be used to critically assess the performance of non-Gaussian protocols for
optical communication. We move on to identify non-Gaussian schemes to beat the
Gaussian capacity and show that a non-Gaussian receiver recently implemented by
Becerra et al. [Nat. Photon. 7, 147 (2013)] can achieve this aim with an
appropriately chosen encoding strategy.Comment: 9 pages, 6 figures, with supplemental materia
Second-order coding rates for pure-loss bosonic channels
A pure-loss bosonic channel is a simple model for communication over
free-space or fiber-optic links. More generally, phase-insensitive bosonic
channels model other kinds of noise, such as thermalizing or amplifying
processes. Recent work has established the classical capacity of all of these
channels, and furthermore, it is now known that a strong converse theorem holds
for the classical capacity of these channels under a particular photon number
constraint. The goal of the present paper is to initiate the study of
second-order coding rates for these channels, by beginning with the simplest
one, the pure-loss bosonic channel. In a second-order analysis of
communication, one fixes the tolerable error probability and seeks to
understand the back-off from capacity for a sufficiently large yet finite
number of channel uses. We find a lower bound on the maximum achievable code
size for the pure-loss bosonic channel, in terms of the known expression for
its capacity and a quantity called channel dispersion. We accomplish this by
proving a general "one-shot" coding theorem for channels with classical inputs
and pure-state quantum outputs which reside in a separable Hilbert space. The
theorem leads to an optimal second-order characterization when the channel
output is finite-dimensional, and it remains an open question to determine
whether the characterization is optimal for the pure-loss bosonic channel.Comment: 18 pages, 3 figures; v3: final version accepted for publication in
Quantum Information Processin
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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