183 research outputs found
Building Gaussian Cluster States by Linear Optics
The linear optical creation of Gaussian cluster states, a potential resource
for universal quantum computation, is investigated. We show that for any
Gaussian cluster state, the canonical generation scheme in terms of QND-type
interactions, can be entirely replaced by off-line squeezers and beam
splitters. Moreover, we find that, in terms of squeezing resources, the
canonical states are rather wasteful and we propose a systematic way to create
cheaper states. As an application, we consider Gaussian cluster computation in
multiple-rail encoding. This encoding may reduce errors due to finite
squeezing, even when the extra rails are achieved through off-line squeezing
and linear optics.Comment: 5 Pages, 3 figure
Quantum Cloning of Continuous Variable Entangled States
We consider the quantum cloning of continuous variable entangled states. This
is achieved by introducing two symmetric entanglement cloning machines (or
e-cloners): a local e-cloner and a global e-cloner; where we look at the
preservation of entanglement in the clones under the condition that the
fidelity of the clones is maximized. These cloning machines are implemented
using simple linear optical elements such as beam splitters and homodyne
detection along with squeeze gates. We show that the global e-cloner
out-performs the local e-cloner both in terms of the fidelity of the cloned
states as well as the strength of the entanglement of the clones. There is a
minimum strength of entanglement (3dB for the inseparability criterion and
5.7dB for the EPR paradox criterion) of the input state of the global e-cloner
that is required to preserve the entanglement in the clones.Comment: 11 pages, 6 figure
Continuous-Variable Quantum Key Distribution using Thermal States
We consider the security of continuous-variable quantum key distribution
using thermal (or noisy) Gaussian resource states. Specifically, we analyze
this against collective Gaussian attacks using direct and reverse
reconciliation where both protocols use either homodyne or heterodyne
detection. We show that in the case of direct reconciliation with heterodyne
detection, an improved robustness to channel noise is achieved when large
amounts of preparation noise is added, as compared to the case when no
preparation noise is added. We also consider the theoretical limit of infinite
preparation noise and show a secure key can still be achieved in this limit
provided the channel noise is less than the preparation noise. Finally, we
consider the security of quantum key distribution at various electromagnetic
wavelengths and derive an upper bound related to an entanglement-breaking
eavesdropping attack and discuss the feasibility of microwave quantum key
distribution.Comment: 12 pages, 11 figures. Updated from published version with some minor
correction
Quantum Cryptography Approaching the Classical Limit
We consider the security of continuous-variable quantum cryptography as we
approach the classical-limit, i.e., when the unknown preparation noise at the
sender's station becomes significantly noisy or thermal (even by as much as
10,000 times the variance of the vacuum mode). We show that, provided the
channel transmission losses do not exceed 50%, the security of quantum
cryptography is not dependent on the channel transmission, and is therefore,
incredibly robust against significant amounts of excess preparation noise. We
extend these results to consider for the first time quantum cryptography at
wavelengths considerably longer than optical and find that regions of security
still exist all the way down to the microwave.Comment: Letter (4 pages) followed by appendix (4 pages). Updated from
published version with some minor correction
Universal Quantum Computation with Continuous-Variable Cluster States
We describe a generalization of the cluster-state model of quantum
computation to continuous-variable systems, along with a proposal for an
optical implementation using squeezed-light sources, linear optics, and
homodyne detection. For universal quantum computation, a nonlinear element is
required. This can be satisfied by adding to the toolbox any single-mode
non-Gaussian measurement, while the initial cluster state itself remains
Gaussian. Homodyne detection alone suffices to perform an arbitrary multi-mode
Gaussian transformation via the cluster state. We also propose an experiment to
demonstrate cluster-based error reduction when implementing Gaussian
operations.Comment: 4 pages, no figure
Continuous variable quantum key distribution with two-mode squeezed states
Quantum key distribution (QKD) enables two remote parties to grow a shared
key which they can use for unconditionally secure communication [1]. The
applicable distance of a QKD protocol depends on the loss and the excess noise
of the connecting quantum channel [2-10]. Several QKD schemes based on coherent
states and continuous variable (CV) measurements are resilient to high loss in
the channel, but strongly affected by small amounts of channel excess noise
[2-6]. Here we propose and experimentally address a CV QKD protocol which uses
fragile squeezed states combined with a large coherent modulation to greatly
enhance the robustness to channel noise. As a proof of principle we
experimentally demonstrate that the resulting QKD protocol can tolerate more
noise than the benchmark set by the ideal CV coherent state protocol. Our
scheme represents a very promising avenue for extending the distance for which
secure communication is possible.Comment: 8 pages, 5 figure
Quantum Computing with Continuous-Variable Clusters
Continuous-variable cluster states offer a potentially promising method of
implementing a quantum computer. This paper extends and further refines
theoretical foundations and protocols for experimental implementation. We give
a cluster-state implementation of the cubic phase gate through photon
detection, which, together with homodyne detection, facilitates universal
quantum computation. In addition, we characterize the offline squeezed
resources required to generate an arbitrary graph state through passive linear
optics. Most significantly, we prove that there are universal states for which
the offline squeezing per mode does not increase with the size of the cluster.
Simple representations of continuous-variable graph states are introduced to
analyze graph state transformations under measurement and the existence of
universal continuous-variable resource states.Comment: 17 pages, 5 figure
Device-Independent Quantum Key Distribution with Generalized Two-Mode Schrödinger Cat States
We show how weak nonlinearities can be used in a device-independent quantum key distribution (QKD) protocol using generalized two-mode Schrödinger cat states. The QKD protocol is therefore shown to be secure against collective attacks and for some coherent attacks. We derive analytical formulas for the optimal values of the Bell parameter, the quantum bit error rate, and the device-independent secret key rate in the noiseless lossy bosonic channel. Additionally, we give the filters and measurements which achieve these optimal values. We find that, over any distance in this channel, the quantum bit error rate is identically zero, in principle, and the states in the protocol are always able to violate a Bell inequality. The protocol is found to be superior in some regimes to a device-independent QKD protocol based on polarization entangled states in a depolarizing channel. Finally, we propose an implementation for the optimal filters and measurements
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