22 research outputs found
Virtual Entanglement and Reconciliation Protocols for Quantum Cryptography with Continuous Variables
We discuss quantum key distribution protocols using quantum continuous
variables. We show that such protocols can be made secure against individual
gaussian attacks regardless the transmission of the optical line between Alice
and Bob. This is achieved by reversing the reconciliation procedure subsequent
to the quantum transmission, that is, using Bob's instead of Alice's data to
build the key. Although squeezing or entanglement may be helpful to improve the
resistance to noise, they are not required for the protocols to remain secure
with high losses. Therefore, these protocols can be implemented very simply by
transmitting coherent states and performing homodyne detection. Here, we show
that entanglement nevertheless plays a crucial role in the security analysis of
coherent state protocols. Every cryptographic protocol based on displaced
gaussian states turns out to be equivalent to an entanglement-based protocol,
even though no entanglement is actually present. This equivalence even holds in
the absence of squeezing, for coherent state protocols. This ``virtual''
entanglement is important to assess the security of these protocols as it
provides an upper bound on the mutual information between Alice and Bob if they
had used entanglement. The resulting security criteria are compared to the
separability criterion for bipartite gaussian variables. It appears that the
security thresholds are well within the entanglement region. This supports the
idea that coherent state quantum cryptography may be unconditionally secure.Comment: 18 pages, 6 figures. Submitted to QI
Controlling the quantum state of a single photon emitted from a single polariton
We investigate in detail the optimal conditions for a high fidelity transfer
from a single-polariton state to a single-photon state and subsequent homodyne
detection of the single photon. We assume that, using various possible
techniques, the single polariton has initially been stored as a spin-wave
grating in a cloud of cold atoms inside a low-finesse cavity. This state is
then transferred to a single-photon optical pulse using an auxiliary beam. We
optimize the retrieval efficiency and determine the mode of the local
oscillator that maximizes the homodyne efficiency of such a photon. We find
that both efficiencies can have values close to one in a large region of
experimental parameters.Comment: 10 pages, 8 figure
Proposal for a loophole-free Bell test using homodyne detection
We propose a feasible optical setup allowing for a loophole-free Bell test
with efficient homodyne detection. A non-gaussian entangled state is generated
from a two-mode squeezed vacuum by subtracting a single photon from each mode,
using beamsplitters and standard low-efficiency single-photon detectors. A Bell
violation exceeding 1% is achievable with 6-dB squeezed light and an homodyne
efficiency around 95%. A detailed feasibility analysis, based upon the recent
generation of single-mode non-gaussian states, confirms that this method opens
a promising avenue towards a complete experimental Bell test.Comment: 4 pages RevTex, 2 figure
On the distillation and purification of phase-diffused squeezed states
Recently it was discovered that non-Gaussian decoherence processes, such as
phase-diffusion, can be counteracted by purification and distillation protocols
that are solely built on Gaussian operations. Here, we make use of this
experimentally highly accessible regime, and provide a detailed experimental
and theoretical analysis of several strategies for purification/distillation
protocols on phase-diffused squeezed states. Our results provide valuable
information for the optimization of such protocols with respect to the choice
of the trigger quadrature, the trigger threshold value and the probability of
generating a distilled state
Experimental open air quantum key distribution with a single photon source
We present a full implementation of a quantum key distribution (QKD) system
with a single photon source, operating at night in open air. The single photon
source at the heart of the functional and reliable setup relies on the pulsed
excitation of a single nitrogen-vacancy color center in diamond nanocrystal. We
tested the effect of attenuation on the polarized encoded photons for inferring
longer distance performance of our system. For strong attenuation, the use of
pure single photon states gives measurable advantage over systems relying on
weak attenuated laser pulses. The results are in good agreement with
theoretical models developed to assess QKD security
Time-resolved homodyne characterization of individual quadrature-entangled pulses
We describe a simple and efficient setup to generate and characterize
femtosecond quadrature-entangled pulses. Quantum correlations equivalent to
about 2.5 dB squeezing are efficiently and easily reached using the
non-degenerate parametric amplification of femtosecond pulses through a
single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses
are then individually sampled to characterize the non-separability and the
entropy of formation of the states. The complete experiment is analysed in the
time-domain, from the pulsed source of quadrature entanglement to the
time-resolved homodyne detection. This particularity allows for applications in
quantum communication protocols using continuous-variable entanglement.Comment: 7 pages, 5 figure
Field test of a continuous-variable quantum key distribution prototype
We have designed and realized a prototype that implements a
continuous-variable quantum key distribution protocol based on coherent states
and reverse reconciliation. The system uses time and polarization multiplexing
for optimal transmission and detection of the signal and phase reference, and
employs sophisticated error-correction codes for reconciliation. The security
of the system is guaranteed against general coherent eavesdropping attacks. The
performance of the prototype was tested over preinstalled optical fibres as
part of a quantum cryptography network combining different quantum key
distribution technologies. The stable and automatic operation of the prototype
over 57 hours yielded an average secret key distribution rate of 8 kbit/s over
a 3 dB loss optical fibre, including the key extraction process and all quantum
and classical communication. This system is therefore ideal for securing
communications in metropolitan size networks with high speed requirements.Comment: 15 pages, 6 figures, submitted to New Journal of Physics (Special
issue on Quantum Cryptography
Improvement of continuous-variable quantum key distribution systems by using optical preamplifiers
Continuous-variable quantum key distribution protocols, based on Gaussian
modulation of the quadratures of coherent states, have been implemented in
recent experiments. A present limitation of such systems is the finite
efficiency of the detectors, which can in principle be compensated for by the
use of classical optical preamplifiers. Here we study this possibility in
detail, by deriving the modified secret key generation rates when an optical
parametric amplifier is placed at the output of the quantum channel. After
presenting a general set of security proofs, we show that the use of
preamplifiers does compensate for all the imperfections of the detectors when
the amplifier is optimal in terms of gain and noise. Imperfect amplifiers can
also enhance the system performance, under conditions which are generally
satisfied in practice.Comment: 11 pages, 7 figures, submitted to J. Phys. B (special issue on Few
Atoms Optics
Controlling excess noise in fiber optics continuous variables quantum key distribution
We describe a continuous variables coherent states quantum key distribution system working at 1550 nm, and entirely made of standard fiber optics and telecom components, such as integrated-optics modulators, couplers and fast InGaAs photodiodes. The setup is composed of an emitter randomly modulating a coherent state in the complex plane with a doubly Gaussian distribution, and a receiver based on a shot noise limited time-resolved homodyne detector. By using a reverse reconciliation protocol, the device can transfer a raw key rate up to 1 Mb/s, with a proven security against Gaussian or non-Gaussian attacks. The dependence of the secret information rate of the present fiber set-up is studied as a function of the line transmission and excess noise