18 research outputs found
Quantum Entanglement Distribution in Next-Generation Wireless Communication Systems
In this work we analyze the distribution of quantum entanglement over
communication channels in the millimeter-wave regime. The motivation for such a
study is the possibility for next-generation wireless networks (beyond 5G) to
accommodate such a distribution directly - without the need to integrate
additional optical communication hardware into the transceivers. Future
wireless communication systems are bound to require some level of quantum
communications capability. We find that direct quantum-entanglement
distribution in the millimeter-wave regime is indeed possible, but that its
implementation will be very demanding from both a system-design perspective and
a channel-requirement perspective.Comment: 6 pages, 4 figure
CV-QKD with Gaussian and non-Gaussian Entangled States over Satellite-based Channels
In this work we investigate the effectiveness of continuous-variable (CV)
entangled states, transferred through high-loss atmospheric channels, as a
means of viable quantum key distribution (QKD) between terrestrial stations and
low-Earth orbit (LEO) satellites. In particular, we investigate the role played
by the Gaussian CV states as compared to non-Gaussian states. We find that
beam-wandering induced atmospheric losses lead to QKD performance levels that
are in general quite different from those found in fixed-attenuation channels.
For example, circumstances can be found where no QKD is viable at some fixed
loss in fiber but is viable at the same mean loss in fading channels. We also
find that, in some circumstances, the QKD relative performance of Gaussian and
non-Gaussian states can in atmospheric channels be the reverse of that found in
fixed-attenuation channels. These findings show that the nature of the
atmospheric channel can have a large impact on the QKD performance. Our results
should prove useful for emerging global quantum communications that use LEO
satellites as communication relays.Comment: 7 pages, 5 figure
Multimode Entangled States in the Lossy Channel
In this work we analyse the structure of highly-entangled multimode squeezed
states, such as those generated by broadband pulses undergoing type-II
parametric down-conversion (PDC). Such down-conversion has previously been
touted as a natural and efficient means of cluster-state generation, and
therefore a viable future pathway to quantum computation. We first detail how
broadband PDC processes lead directly to a series of orthogonal supermodes that
are linear combinations of the original frequency modes. We then calculate the
total squeezing of the multimode entangled states when they are assumed to be
measured by an ideal homodyne detection in which all supermodes of the states
are detected by an optimally shaped local oscillator (LO) pulse. For
comparison, squeezing of the same entangled states are calculated when measured
by a lower-complexity homodyne detection scheme that exploits an unshaped LO
pulse. Such calculations illustrate the cost, in the context of squeezing, of
moving from higher complexity (harder to implement) homodyne detection to
lower-complexity (easier-to-implement) homodyne detection. Finally, by studying
the degradation in squeezing of the supermodes under photonic loss, multimode
entangled state evolution through an attenuation channel is determined. The
results reported here push us towards a fuller understanding of the real-world
transfer of cluster-states when they take the form of highly-entangled
multimode states in frequency space.Comment: Accepted for publication: IEEE VTC International Workshop on Quantum
Communications for Future Networks (QCFN), Sydney, Australia, June 201
Gaussian Entanglement Distribution via Satellite
In this work we analyse three quantum communication schemes for the
generation of Gaussian entanglement between two ground stations. Communication
occurs via a satellite over two independent atmospheric fading channels
dominated by turbulence-induced beam wander. In our first scheme the
engineering complexity remains largely on the ground transceivers, with the
satellite acting simply as a reflector. Although the channel state information
of the two atmospheric channels remains unknown in this scheme, the Gaussian
entanglement generation between the ground stations can still be determined. On
the ground, distillation and Gaussification procedures can be applied, leading
to a refined Gaussian entanglement generation rate between the ground stations.
We compare the rates produced by this first scheme with two competing schemes
in which quantum complexity is added to the satellite, thereby illustrating the
trade-off between space-based engineering complexity and the rate of
ground-station entanglement generation.Comment: Closer to published version (to appear in Phys. Rev. A) 13 pages, 6
figure
Optimal realistic attacks in continuous-variable quantum key distribution
Quantum cryptographic protocols are typically analysed by assuming that
potential opponents can carry out all physical operations, an assumption which
grants capabilities far in excess of present technology. Adjusting this
assumption to reflect more realistic capabilities is an attractive prospect,
but one that can only be justified with a rigorous, quantitative framework that
relates adversarial restrictions to the protocols security and performance. We
investigate the effect of limitations on the eavesdropper's (Eve's) ability to
make a coherent attack on the security of continuous-variable quantum key
distribution (CV-QKD). We consider a realistic attack, in which the total
decoherence induced during the attack is modelled by a Gaussian channel. Based
on our decoherence model we propose an optimal hybrid attack, which allows Eve
to perform a combination of both coherent and individual attacks
simultaneously. We evaluate the asymptotic and composable finite-size security
of a heterodyne CV-QKD protocol against such hybrid attacks in terms of Eve's
decoherence. We show that when the decoherence is greater than a threshold
value, Eve's most effective strategy is reduced to the individual attack. Thus,
if we are willing to assume that the decoherence caused by the memory and the
collective measurement is large enough, it is sufficient to analyse the
security of the protocol only against individual attacks, which significantly
improves the CV-QKD performance in terms of both the key rate and the maximum
secure transmission distance.Comment: 11 pages, 4 figure
Teleportation-based collective attacks in Gaussian quantum key distribution
In Gaussian quantum key distribution eavesdropping attacks are conventionally modeled through the universal entangling cloner scheme, which is based on the premise that the whole environment is under control of the adversary, i.e., the eavesdropper purifies the system. This assumption implies that the eavesdropper has either access to an identity (noiseless) channel or an infinite amount of entanglement in order to simulate such an identity channel. In this work we challenge the necessity of this assumption and we propose a teleportation-based eavesdropping attack, where the eavesdropper is not assumed to have access to the shared channel, that represents the unavoidable noise due to the environment. Under collective measurements, this attack reaches optimality in the limit of an infinite amount of entanglement, while for finite entanglement resources it outperforms the corresponding optimal individual attack. We also calculate the minimum amount of distributed entanglement that is necessary for this eavesdropping scheme, since we consider it as the operationally critical quantity capturing the limitations of a realistic attack. We conclude that the fact that an infinite amount of entanglement is required for an optimal collective eavesdropping attack signifies the robustness of Gaussian quantum key distribution