159 research outputs found
Tailoring Single and Multiphoton Probabilities of a Single Photon On-Demand Source
As typically implemented, single photon sources cannot be made to produce
single photons with high probability, while simultaneously suppressing the
probability of yielding two or more photons. Because of this, single photon
sources cannot really produce single photons on demand. We describe a
multiplexed system that allows the probabilities of producing one and more
photons to be adjusted independently, enabling a much better approximation of a
source of single photons on demand.Comment: 4 pages, LaTex, 2 figures, twocolumn and RevTex Style for PR
Distributed Relay Protocol for Probabilistic Information-Theoretic Security in a Randomly-Compromised Network
We introduce a simple, practical approach with probabilistic
information-theoretic security to mitigate one of quantum key distribution's
major limitations: the short maximum transmission distance (~200 km) possible
with present day technology. Our scheme uses classical secret sharing
techniques to allow secure transmission over long distances through a network
containing randomly-distributed compromised nodes. The protocol provides
arbitrarily high confidence in the security of the protocol, with modest
scaling of resource costs with improvement of the security parameter. Although
some types of failure are undetectable, users can take preemptive measures to
make the probability of such failures arbitrarily small.Comment: 12 pages, 2 figures; added proof of verification sub-protocol, minor
correction
Proposal of an experimental scheme for realising a translucent eavesdropping on a quantum cryptographic channel
Purpose of this paper is to suggest a scheme, which can be realised with
today's technology and could be used for entangling a probe to a photon qubit
based on polarisation. Using this probe a translucent or a coherent
eavesdropping can be performed.Comment: in pres
Quantum copying: Fundamental inequalities
How well one can copy an arbitrary qubit? To answer this question we consider
two arbitrary vectors in a two-dimensional state space and an abstract copying
transformation which will copy these two vectors. If the vectors are
orthogonal, then perfect copies can be made. If they are not, then errors will
be introduced. The size of the error depends on the inner product of the two
original vectors. We derive a lower bound for the amount of noise induced by
quantum copying. We examine both copying transformations which produce one copy
and transformations which produce many, and show that the quality of each copy
decreases as the number of copies increases.Comment: 5 pages + 1 figure, LaTeX with revtex, epsfig submitted to Phys. Rev.
Quantum identification system
A secure quantum identification system combining a classical identification
procedure and quantum key distribution is proposed. Each identification
sequence is always used just once and new sequences are ``refuelled'' from a
shared provably secret key transferred through the quantum channel. Two
identification protocols are devised. The first protocol can be applied when
legitimate users have an unjammable public channel at their disposal. The
deception probability is derived for the case of a noisy quantum channel. The
second protocol employs unconditionally secure authentication of information
sent over the public channel, and thus it can be applied even in the case when
an adversary is allowed to modify public communications. An experimental
realization of a quantum identification system is described.Comment: RevTeX, 4 postscript figures, 9 pages, submitted to Physical Review
Protocols for entanglement transformations of bipartite pure states
We present a general theoretical framework for both deterministic and
probabilistic entanglement transformations of bipartite pure states achieved
via local operations and classical communication. This framework unifies and
greatly simplifies previous works. A necessary condition for ``pure
contraction'' transformations is given. Finally, constructive protocols to
achieve both probabilistic and deterministic entanglement transformations are
presented.Comment: 7 pages, no figures. Version slightly modified on Physical Review A
reques
Security of quantum cryptography using balanced homodyne detection
In this paper we investigate the security of a quantum cryptographic scheme
which utilizes balanced homodyne detection and weak coherent pulse (WCP). The
performance of the system is mainly characterized by the intensity of the WCP
and postselected threshold. Two of the simplest intercept/resend eavesdropping
attacks are analyzed. The secure key gain for a given loss is also discussed in
terms of the pulse intensity and threshold.Comment: RevTeX4, 8pages, 7 figure
Quantum cryptography via parametric downconversion
The use of quantum bits (qubits) in cryptography holds the promise of secure
cryptographic quantum key distribution schemes. It is based usually on
single-photon polarization states. Unfortunately, the implemented ``qubits'' in
the usual weak pulse experiments are not true two-level systems, and quantum
key distribution based on these imperfect qubits is totally insecure in the
presence of high (realistic) loss rate. In this work, we investigate another
potential implementation: qubits generated using a process of parametric
downconversion. We find that, to first (two-photon) and second (four-photon)
order in the parametric downconversion small parameter, this implementation of
quantum key distribution is equivalent to the theoretical version.
Once realistic measurements are taken into account, quantum key distribution
based on parametric downconversion suffers also from sensitivity to extremely
high (nonrealistic) losses. By choosing the small parameter of the process
according to the loss rates, both implementations of quantum key distribution
can in principle become secure against the attack studied in this paper.
However, adjusting the small parameter to the required levels seems to be
impractical in the weak pulse process. On the other hand, this can easily be
done in the parametric downconversion process, making it a much more promising
implementation.Comment: 6 pages, Latex (a special style file is attached). Presented in
QCM'98 conference. Similar results regarding the insecurity of weak-pulse
schemes were also presented by Norbert Lutkenhaus in the same conferenc
A Lorentz-invariant look at quantum clock synchronization protocols based on distributed entanglement
Recent work has raised the possibility that quantum information theory
techniques can be used to synchronize atomic clocks nonlocally. One of the
proposed algorithms for quantum clock synchronization (QCS) requires
distribution of entangled pure singlets to the synchronizing parties. Such
remote entanglement distribution normally creates a relative phase error in the
distributed singlet state which then needs to be purified asynchronously. We
present a fully relativistic analysis of the QCS protocol which shows that
asynchronous entanglement purification is not possible, and, therefore, that
the proposed QCS scheme remains incomplete. We discuss possible directions of
research in quantum information theory which may lead to a complete, working
QCS protocol.Comment: 5 pages; typeset in RevTe
Solving the liar detection problem using the four-qubit singlet state
A method for solving the Byzantine agreement problem [M. Fitzi, N. Gisin, and
U. Maurer, Phys. Rev. Lett. 87, 217901 (2001)] and the liar detection problem
[A. Cabello, Phys. Rev. Lett. 89, 100402 (2002)] is introduced. The main
advantages of this protocol are that it is simpler and is based on a four-qubit
singlet state already prepared in the laboratory.Comment: REVTeX4, 4 page
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