185 research outputs found
A comparative study of protocols for secure quantum communication under noisy environment: single-qubit-based protocols versus entangled-state-based protocols
The effect of noise on various protocols of secure quantum communication has
been studied. Specifically, we have investigated the effect of amplitude
damping, phase damping, squeezed generalized amplitude damping, Pauli type as
well as various collective noise models on the protocols of quantum key
distribution, quantum key agreement,quantum secure direct quantum communication
and quantum dialogue. From each type of protocol of secure quantum
communication, we have chosen two protocols for our comparative study; one
based on single qubit states and the other one on entangled states. The
comparative study reported here has revealed that single-qubit-based schemes
are generally found to perform better in the presence of amplitude damping,
phase damping, squeezed generalized amplitude damping noises, while
entanglement-based protocols turn out to be preferable in the presence of
collective noises. It is also observed that the effect of noise entirely
depends upon the number of rounds of quantum communication involved in a scheme
of quantum communication. Further, it is observed that squeezing, a completely
quantum mechanical resource present in the squeezed generalized amplitude
channel, can be used in a beneficial way as it may yield higher fidelity
compared to the corresponding zero squeezing case.Comment: 23 pages 7 figure
Quantum key distribution without alternative measurements
Entanglement swapping between Einstein-Podolsky-Rosen (EPR) pairs can be used
to generate the same sequence of random bits in two remote places. A quantum
key distribution protocol based on this idea is described. The scheme exhibits
the following features. (a) It does not require that Alice and Bob choose
between alternative measurements, therefore improving the rate of generated
bits by transmitted qubit. (b) It allows Alice and Bob to generate a key of
arbitrary length using a single quantum system (three EPR pairs), instead of a
long sequence of them. (c) Detecting Eve requires the comparison of fewer bits.
(d) Entanglement is an essential ingredient. The scheme assumes reliable
measurements of the Bell operator.Comment: REVTeX, 5 pages, 2 figures. Published version with some comment
Unconditional security of the Bennett 1992 quantum key-distribution scheme with strong reference pulse
We prove the unconditional security of the original Bennett 1992 protocol
with strong reference pulse. We show that we may place a projection onto
suitably defined qubit spaces before the receiver, which makes the analysis as
simple as qubit-based protocols. Unlike the single-photon-based qubits, the
qubits identified in this scheme are almost surely detected by the receiver
even after a lossy channel. This leads to the key generation rate that is
proportional to the channel transmission rate for proper choices of
experimental parameters.Comment: More detailed presentation and a bit modified security proo
Coherent pulse implementations of quantum cryptography protocols resistant to photon number splitting attacks
A new class of quantum cryptography (QC) protocols that are robust against
the most general photon number splitting attacks in a weak coherent pulse
implementation has been recently proposed. In this article we give a quite
exhaustive analysis of several eavesdropping attacks on these schemes. The
eavesdropper (Eve) is supposed to have unlimited technological power while the
honest parties (Alice and Bob) use present day technology, in particular an
attenuated laser as an approximation of a single-photon source. They exploit
the nonorthogonality of quantum states for decreasing the information
accessible to Eve in the multi-photon pulses accidentally produced by the
imperfect source. An implementation of some of these protocols using present
day technology allow for a secure key distribution up to distances of
150 km. We also show that strong-pulse implementations, where a strong pulse is
included as a reference, allow for key distribution robust against photon
number splitting attacks.Comment: 16 pages, 11 figure
Experimental quantum key distribution with finite-key security analysis for noisy channels
In quantum key distribution implementations, each session is typically chosen
long enough so that the secret key rate approaches its asymptotic limit.
However, this choice may be constrained by the physical scenario, as in the
perspective use with satellites, where the passage of one terminal over the
other is restricted to a few minutes. Here we demonstrate experimentally the
extraction of secure keys leveraging an optimal design of the
prepare-and-measure scheme, according to recent finite-key theoretical
tight-bounds. The experiment is performed in different channel conditions, and
assuming two distinct attack models: individual attacks, or general quantum
attacks. The request on the number of exchanged qubits is then obtained as a
function of the key size and of the ambient quantum bit error rate. The results
indicate that viable conditions for effective symmetric, and even one-time-pad,
cryptography are achievable.Comment: 20 pages, 4 figure
Robust polarization-based quantum key distribution over collective-noise channel
We present two polarization-based protocols for quantum key distribution. The
protocols encode key bits in noiseless subspaces or subsystems, and so can
function over a quantum channel subjected to an arbitrary degree of collective
noise, as occurs, for instance, due to rotation of polarizations in an optical
fiber. These protocols can be implemented using only entangled photon-pair
sources, single-photon rotations, and single-photon detectors. Thus, our
proposals offer practical and realistic alternatives to existing schemes for
quantum key distribution over optical fibers without resorting to
interferometry or two-way quantum communication, thereby circumventing,
respectively, the need for high precision timing and the threat of Trojan horse
attacks.Comment: Minor changes, added reference
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