745 research outputs found
A simple proof of the unconditional security of quantum key distribution
Quantum key distribution is the most well-known application of quantum
cryptography. Previous proposed proofs of security of quantum key distribution
contain various technical subtleties. Here, a conceptually simpler proof of
security of quantum key distribution is presented. The new insight is the
invariance of the error rate of a teleportation channel: We show that the error
rate of a teleportation channel is independent of the signals being
transmitted. This is because the non-trivial error patterns are permuted under
teleportation. This new insight is combined with the recently proposed quantum
to classical reduction theorem. Our result shows that assuming that Alice and
Bob have fault-tolerant quantum computers, quantum key distribution can be made
unconditionally secure over arbitrarily long distances even against the most
general type of eavesdropping attacks and in the presence of all types of
noises.Comment: 13 pages, extended abstract. Comments will be appreciate
Quantum key distribution with 2-bit quantum codes
We propose a prepare-and-measure scheme for quantum key distribution with
2-bit quantum codes. The protocol is unconditionally secure under whatever type
of intercept-and-resend attack. Given the symmetric and independent errors to
the transmitted qubits, our scheme can tolerate a bit error rate up to 26% in
4-state protocol and 30% in 6-state protocol, respectively. These values are
higher than all currently known threshold values for prepare-and-measure
protocols. A specific realization with linear optics is given.Comment: Approved for publication in Physical Review Letter
Security of EPR-based Quantum Cryptography against Incoherent Symmetric Attacks
We investigate a new strategy for incoherent eavesdropping in Ekert's
entanglement based quantum key distribution protocol. We show that under
certain assumptions of symmetry the effectiveness of this strategy reduces to
that of the original single qubit protocol of Bennett and Brassard
Unconditionally secure quantum bit commitment is impossible
The claim of quantum cryptography has always been that it can provide
protocols that are unconditionally secure, that is, for which the security does
not depend on any restriction on the time, space or technology available to the
cheaters. We show that this claim does not hold for any quantum bit commitment
protocol. Since many cryptographic tasks use bit commitment as a basic
primitive, this result implies a severe setback for quantum cryptography. The
model used encompasses all reasonable implementations of quantum bit commitment
protocols in which the participants have not met before, including those that
make use of the theory of special relativity.Comment: 4 pages, revtex. Journal version replacing the version published in
the proceedings of PhysComp96. This is a significantly improved version which
emphasis the generality of the resul
Modified Bennett-Brassard 1984 Quantum Key Distribution With Two-way Classical Communications
The quantum key distribution protocol without public announcement of bases is
equipped with a two-way classical communication symmetric entanglement
purification protocol. This modified key distribution protocol is
unconditionally secure and has a higher tolerable error rate of 20%, which is
higher than previous scheme without public announcement of bases.Comment: 5 pages. To appear in Physical Review
Secure quantum key distribution with an uncharacterized source
We prove the security of the Bennett-Brassard (BB84) quantum key distribution
protocol for an arbitrary source whose averaged states are basis-independent, a
condition that is automatically satisfied if the source is suitably designed.
The proof is based on the observation that, to an adversary, the key extraction
process is equivalent to a measurement in the sigma_x-basis performed on a pure
sigma_z-basis eigenstate. The dependence of the achievable key length on the
bit error rate is the same as that established by Shor and Preskill for a
perfect source, indicating that the defects in the source are efficiently
detected by the protocol.Comment: 4 pages, 1 figure, REVTeX, minor revision
A deterministic cavity-QED source of polarization entangled photon pairs
We present two cavity quantum electrodynamics proposals that, sharing the
same basic elements, allow for the deterministic generation of entangled
photons pairs by means of a three-level atom successively coupled to two single
longitudinal mode high-Q optical resonators presenting polarization degeneracy.
In the faster proposal, the three-level atom yields a polarization entangled
photon pair via two truncated Rabi oscillations, whereas in the adiabatic
proposal a counterintuitive Stimulated Raman Adiabatic Passage process is
considered. Although slower than the former process, this second method is very
efficient and robust under fluctuations of the experimental parameters and,
particularly interesting, almost completely insensitive to atomic decay.Comment: 5 pages, 5 figure
Quantum Gambling Using Three Nonorthogonal States
We provide a quantum gambling protocol using three (symmetric) nonorthogonal
states. The bias of the proposed protocol is less than that of previous ones,
making it more practical. We show that the proposed scheme is secure against
non-entanglement attacks. The security of the proposed scheme against
entanglement attacks is shown heuristically.Comment: no essential correction, 4 pages, RevTe
On bit-commitment based quantum coin flipping
In this paper, we focus on a special framework for quantum coin flipping
protocols,_bit-commitment based protocols_, within which almost all known
protocols fit. We show a lower bound of 1/16 for the bias in any such protocol.
We also analyse a sequence of multi-round protocol that tries to overcome the
drawbacks of the previously proposed protocols, in order to lower the bias. We
show an intricate cheating strategy for this sequence, which leads to a bias of
1/4. This indicates that a bias of 1/4 might be optimal in such protocols, and
also demonstrates that a cleverer proof technique may be required to show this
optimality.Comment: The lower bound shown in this paper is superceded by a result of
Kitaev (personal communication, 2001
Experimental study on Gaussian-modulated coherent states quantum key distribution over standard telecom fiber
In this paper, we present a fully fiber-based one-way Quantum Key
Distribution (QKD) system implementing the Gaussian-Modulated Coherent States
(GMCS) protocol. The system employs a double Mach-Zehnder Interferometer (MZI)
configuration in which the weak quantum signal and the strong Local Oscillator
(LO) go through the same fiber between Alice and Bob, and are separated into
two paths inside Bob's terminal. To suppress the LO leakage into the signal
path, which is an important contribution to the excess noise, we implemented a
novel scheme combining polarization and frequency multiplexing, achieving an
extinction ratio of 70dB. To further minimize the system excess noise due to
phase drift of the double MZI, we propose that, instead of employing phase
feedback control, one simply let Alice remap her data by performing a rotation
operation. We further present noise analysis both theoretically and
experimentally. Our calculation shows that the combined polarization and
frequency multiplexing scheme can achieve better stability in practice than the
time-multiplexing scheme, because it allows one to use matched fiber lengths
for the signal and the LO paths on both sides of the double MZI, greatly
reducing the phase instability caused by unmatched fiber lengths. Our
experimental noise analysis quantifies the three main contributions to the
excess noise, which will be instructive to future studies of the GMCS QKD
systems. Finally, we demonstrate, under the "realistic model" in which Eve
cannot control the system within Bob's terminal, a secure key rate of
0.3bit/pulse over a 5km fiber link. This key rate is about two orders of
magnitude higher than that of a practical BB84 QKD system.Comment: 21 pages, 9 figure
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