818 research outputs found
Robust and Efficient Sifting-Less Quantum Key Distribution Protocols
We show that replacing the usual sifting step of the standard
quantum-key-distribution protocol BB84 by a one-way reverse reconciliation
procedure increases its robustness against photon-number-splitting (PNS)
attacks to the level of the SARG04 protocol while keeping the raw key-rate of
BB84. This protocol, which uses the same state and detection than BB84, is the
m=4 member of a protocol-family using m polarization states which we introduce
here. We show that the robustness of these protocols against PNS attacks
increases exponentially with m, and that the effective keyrate of optimized
weak coherent pulses decreases with the transmission T like T^{1+1/(m-2)}
Heisenberg-limited eavesdropping on the continuous-variable quantum cryptographic protocol with no basis switching is impossible
The Gaussian quantum key distribution protocol based on coherent states and
heterodyne detection [Phys. Rev. Lett. 93, 170504 (2004)] has the advantage
that no active random basis switching is needed on the receiver's side. Its
security is, however, not very satisfyingly understood today because the bounds
on the secret key rate that have been derived from Heisenberg relations are not
attained by any known scheme. Here, we address the problem of the optimal
Gaussian individual attack against this protocol, and derive tight upper bounds
on the information accessible to an eavesdropper. The optical scheme achieving
this bound is also exhibited, which concludes the security analysis of this
protocol.Comment: 10 pages, 6 figure
Virtual noiseless amplification and Gaussian post-selection in continuous-variable quantum key distribution
The noiseless amplification or attenuation are two heralded filtering
operations that enable respectively to increase or decrease the mean field of
any quantum state of light with no added noise, at the cost of a small success
probability. We show that inserting such noiseless operations in a transmission
line improves the performance of continuous-variable quantum key distribution
over this line. Remarkably, these noiseless operations do not need to be
physically implemented but can simply be simulated in the data post-processing
stage. Hence, virtual noiseless amplification or attenuation amounts to perform
a Gaussian post-selection, which enhances the secure range or tolerable excess
noise while keeping the benefits of Gaussian security proofs.Comment: 8 pages, 5 figure
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
Continuous Variable Quantum Cryptography using Two-Way Quantum Communication
Quantum cryptography has been recently extended to continuous variable
systems, e.g., the bosonic modes of the electromagnetic field. In particular,
several cryptographic protocols have been proposed and experimentally
implemented using bosonic modes with Gaussian statistics. Such protocols have
shown the possibility of reaching very high secret-key rates, even in the
presence of strong losses in the quantum communication channel. Despite this
robustness to loss, their security can be affected by more general attacks
where extra Gaussian noise is introduced by the eavesdropper. In this general
scenario we show a "hardware solution" for enhancing the security thresholds of
these protocols. This is possible by extending them to a two-way quantum
communication where subsequent uses of the quantum channel are suitably
combined. In the resulting two-way schemes, one of the honest parties assists
the secret encoding of the other with the chance of a non-trivial superadditive
enhancement of the security thresholds. Such results enable the extension of
quantum cryptography to more complex quantum communications.Comment: 12 pages, 7 figures, REVTe
Security of continuous-variable quantum key distribution against general attacks
We prove the security of Gaussian continuous-variable quantum key
distribution against arbitrary attacks in the finite-size regime. The novelty
of our proof is to consider symmetries of quantum key distribution in phase
space in order to show that, to good approximation, the Hilbert space of
interest can be considered to be finite-dimensional, thereby allowing for the
use of the postselection technique introduced by Christandl, Koenig and Renner
(Phys. Rev. Lett. 102, 020504 (2009)). Our result greatly improves on previous
work based on the de Finetti theorem which could not provide security for
realistic, finite-size, implementations.Comment: 5 pages, plus 11 page appendi
Experimental implementation of non-Gaussian attacks on a continuous-variable quantum key distribution system
An intercept-resend attack on a continuous-variable quantum-key-distribution
protocol is investigated experimentally. By varying the interception fraction,
one can implement a family of attacks where the eavesdropper totally controls
the channel parameters. In general, such attacks add excess noise in the
channel, and may also result in non-Gaussian output distributions. We implement
and characterize the measurements needed to detect these attacks, and evaluate
experimentally the information rates available to the legitimate users and the
eavesdropper. The results are consistent with the optimality of Gaussian
attacks resulting from the security proofs.Comment: 4 pages, 5 figure
Continuous variable quantum cryptography using coherent states
We propose several methods for quantum key distribution (QKD) based upon the
generation and transmission of random distributions of coherent or squeezed
states, and we show that they are are secure against individual eavesdropping
attacks. These protocols require that the transmission of the optical line
between Alice and Bob is larger than 50 %, but they do not rely on
"non-classical" features such as squeezing. Their security is a direct
consequence of the no-cloning theorem, that limits the signal to noise ratio of
possible quantum measurements on the transmission line. Our approach can also
be used for evaluating various QKD protocols using light with gaussian
statistics.Comment: 5 pages, 1 figure. In v2 minor rewriting for clarity, references
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