32 research outputs found
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
Covert Communication over Classical-Quantum Channels
The square root law (SRL) is the fundamental limit of covert communication
over classical memoryless channels (with a classical adversary) and quantum
lossy-noisy bosonic channels (with a quantum-powerful adversary). The SRL
states that covert bits, but no more, can be reliably
transmitted in channel uses with bits of secret
pre-shared between the communicating parties. Here we investigate covert
communication over general memoryless classical-quantum (cq) channels with
fixed finite-size input alphabets, and show that the SRL governs covert
communications in typical scenarios. %This demonstrates that the SRL is
achievable over any quantum communications channel using a product-state
transmission strategy, where the transmitted symbols in every channel use are
drawn from a fixed finite-size alphabet. We characterize the optimal constants
in front of for the reliably communicated covert bits, as well as
for the number of the pre-shared secret bits consumed. We assume a
quantum-powerful adversary that can perform an arbitrary joint (entangling)
measurement on all channel uses. However, we analyze the legitimate
receiver that is able to employ a joint measurement as well as one that is
restricted to performing a sequence of measurements on each of channel uses
(product measurement). We also evaluate the scenarios where covert
communication is not governed by the SRL
Complete elimination of information leakage in continuous-variable quantum communication channels
In all lossy communication channels realized to date, information is
inevitably leaked to a potential eavesdropper. Here we present a communication
protocol that does not allow for any information leakage to a potential
eavesdropper in a purely lossy channel. By encoding information into a
restricted Gaussian alphabet of squeezed states we show, both theoretically and
experimentally, that the Holevo information between the eavesdropper and the
intended recipient can be exactly zero in a purely lossy channel while
minimized in a noisy channel. This result is of fundamental interest, but might
also have practical implications in extending the distance of secure quantum
key distribution.Comment: 9 pages, 5 figure
Fundamental Limits of Thermal-noise Lossy Bosonic Multiple Access Channel
Bosonic channels describe quantum-mechanically many practical communication
links such as optical, microwave, and radiofrequency. We investigate the
maximum rates for the bosonic multiple access channel (MAC) in the presence of
thermal noise added by the environment and when the transmitters utilize
Gaussian state inputs. We develop an outer bound for the capacity region for
the thermal-noise lossy bosonic MAC. We additionally find that the use of
coherent states at the transmitters is capacity-achieving in the limits of high
and low mean input photon numbers. Furthermore, we verify that coherent states
are capacity-achieving for the sum rate of the channel. In the non-asymptotic
regime, when a global mean photon-number constraint is imposed on the
transmitters, coherent states are the optimal Gaussian state. Surprisingly
however, the use of single-mode squeezed states can increase the capacity over
that afforded by coherent state encoding when each transmitter is photon number
constrained individually.Comment: 8 pages, 3 figure
Covert sensing using floodlight illumination
We propose a scheme for covert active sensing using floodlight illumination
from a THz-bandwidth amplified spontaneous emission (ASE) source and heterodyne
detection. We evaluate the quantum-estimation-theoretic performance limit of
covert sensing, wherein a transmitter's attempt to sense a target phase is kept
undetectable to a quantum-equipped passive adversary, by hiding the signal
photons under the thermal noise floor. Despite the quantum state of each mode
of the ASE source being mixed (thermal), and hence inferior compared to the
pure coherent state of a laser mode, the thousand-times higher optical
bandwidth of the ASE source results in achieving a substantially superior
performance compared to a narrowband laser source by allowing the probe light
to be spread over many more orthogonal temporal modes within a given
integration time. Even though our analysis is restricted to single-mode phase
sensing, this system could be applicable extendible for various practical
optical sensing applications.Comment: We present new results and discuss some results found in
arXiv:1701.06206. Comments are welcom
Optimal Throughput for Covert Communication Over a Classical-Quantum Channel
This paper considers the problem of communication over a memoryless
classical-quantum wiretap channel subject to the constraint that the
eavesdropper on the channel should not be able to learn whether the legitimate
parties are using the channel to communicate or not. Specifically, the relative
entropy between the output quantum states at the eavesdropper when a codeword
is transmitted and when no input is provided must be sufficiently small.
Extending earlier works, this paper proves the "square-root law" for a broad
class of classical-quantum channels: the maximum amount of information that can
be reliably and covertly transmitted over uses of such a channel scales
like . The scaling constant is also determined.Comment: Corrected version of a paper presented at ITW 2016. In the ITW paper,
the denominator in the main formula (10) was incorrect. The current version
corrects this mistake and adds an appendix for its derivatio
Quantum limits of covert target detection
In covert target detection, Alice attempts to send optical or microwave
probes to detect whether or not a weakly-reflecting target embedded in thermal
background radiation is present in a target region while remaining undetected
herself by an adversary Willie who is co-located with the target and collects
all the light that does not return to Alice. We formulate this problem in a
realistic setting and derive quantum-mechanical limits on Alice's error
probability performance in entanglement-assisted target detection for any fixed
level of her detectability by Willie. In particular, we show that Alice must
expend a minimum energy in her probe light to maintain a given covertness
level, but is also able to achieve a nonzero error probability exponent while
remaining perfectly covert. We compare the performance of two-mode squeezed
vacuum probes and Gaussian-distributed coherent states to our performance
limits. We also obtain quantum limits for discriminating any two thermal loss
channels and for non-adversarial quantum illumination without the
no-passive-signature assumption.Comment: 18 pages, 5 figure