79 research outputs found
Practical quantum key distribution over a 48-km optical fiber network
The secure distribution of the secret random bit sequences known as "key"
material, is an essential precursor to their use for the encryption and
decryption of confidential communications. Quantum cryptography is a new
technique for secure key distribution with single-photon transmissions:
Heisenberg's uncertainty principle ensures that an adversary can neither
successfully tap the key transmissions, nor evade detection (eavesdropping
raises the key error rate above a threshold value). We have developed
experimental quantum cryptography systems based on the transmission of
non-orthogonal photon states to generate shared key material over
multi-kilometer optical fiber paths and over line-of-sight links. In both
cases, key material is built up using the transmission of a single-photon per
bit of an initial secret random sequence. A quantum-mechanically random subset
of this sequence is identified, becoming the key material after a data
reconciliation stage with the sender. Here we report the most recent results of
our optical fiber experiment in which we have performed quantum key
distribution over a 48-km optical fiber network at Los Alamos using photon
interference states with the B92 and BB84 quantum key distribution protocols.Comment: 13 pages, 7 figures, .pdf format submitted to Journal of Modern
Optic
Distribution and Movements of Moose (Alces alces) in Relation to the Trans-Alaska Oil Pipeline
During late winter 1982 and 1983, the distribution and movements of moose adjacent to the Trans-Alaska near Big Delta, Alaska, were examined. Within a 15 km wide corridor centered on the pipeline, moose distribution was independent of the distance from the pipeline. Of 175 moose trails examined, most (94%) crossed the pipeline successfully upon entering the right-of-way regardless of pipe mode or pipe height above ground. Pipe heights above 1.5 m were adequate for moose passage, but greater heights up to 2.7 m were preferred. Sections of pipe that were buried or that were specially elevated to facilitate moose passage did not receive preferential use. Moose moved in a meandering fashion whether they were crossing the pipeline or moving within habitats in distant areas. The results of this study supported the hypothesis that the distribution and local movements of moose were not significantly affected by the pipeline.Key words: moose (Alces alces), movements, pipeline, crossing success, habitat use, effects of developmentMots clés: orignal (Alces alces), déplacements, pipeline, succès de traverse, utilisation de l'habitat, effets du développemen
Experimental demonstration of four-party quantum secret sharing
Secret sharing is a multiparty cryptographic task in which some secret
information is splitted into several pieces which are distributed among the
participants such that only an authorized set of participants can reconstruct
the original secret. Similar to quantum key distribution, in quantum secret
sharing, the secrecy of the shared information relies not on computational
assumptions, but on laws of quantum physics. Here, we present an experimental
demonstration of four-party quantum secret sharing via the resource of
four-photon entanglement
Quantum key distribution with realistic states: photon-number statistics in the photon-number splitting attack
Quantum key distribution can be performed with practical signal sources such
as weak coherent pulses. One example of such a scheme is the Bennett-Brassard
protocol that can be implemented via polarization of the signals, or equivalent
signals. It turns out that the most powerful tool at the disposition of an
eavesdropper is the photon-number splitting attack. We show that this attack
can be extended in the relevant parameter regime such as to preserve the
Poissonian photon number distribution of the combination of the signal source
and the lossy channel.Comment: 4 page
Quantum authentication with unitary coding sets
A general class of authentication schemes for arbitrary quantum messages is
proposed. The class is based on the use of sets of unitary quantum operations
in both transmission and reception, and on appending a quantum tag to the
quantum message used in transmission. The previous secret between partners
required for any authentication is a classical key. We obtain the minimal
requirements on the unitary operations that lead to a probability of failure of
the scheme less than one. This failure may be caused by someone performing a
unitary operation on the message in the channel between the communicating
partners, or by a potential forger impersonating the transmitter.Comment: RevTeX4, 10 page
Locking of accessible information and implications for the security of quantum cryptography
The unconditional security of a quantum key distribution protocol is often
defined in terms of the accessible information, that is, the maximum mutual
information between the distributed key S and the outcome of an optimal
measurement on the adversary's (quantum) system. We show that, even if this
quantity is small, certain parts of the key S might still be completely
insecure when S is used in applications, such as for one-time pad encryption.
This flaw is due to a locking property of the accessible information: one
additional (physical) bit of information might increase the accessible
information by more than one bit.Comment: 5 pages; minor change
Decoherence-full subsystems and the cryptographic power of a private shared reference frame
We show that private shared reference frames can be used to perform private
quantum and private classical communication over a public quantum channel. Such
frames constitute a novel type of private shared correlation (distinct from
private classical keys or shared entanglement) useful for cryptography. We
present optimally efficient schemes for private quantum and classical
communication given a finite number of qubits transmitted over an insecure
channel and given a private shared Cartesian frame and/or a private shared
reference ordering of the qubits. We show that in this context, it is useful to
introduce the concept of a decoherence-full subsystem, wherein every state is
mapped to the completely mixed state under the action of the decoherence.Comment: 13 pages, published versio
Upper bound on the secret key rate distillable from effective quantum correlations with imperfect detectors
We provide a simple method to obtain an upper bound on the secret key rate
that is particularly suited to analyze practical realizations of quantum key
distribution protocols with imperfect devices. We consider the so-called
trusted device scenario where Eve cannot modify the actual detection devices
employed by Alice and Bob. The upper bound obtained is based on the available
measurements results, but it includes the effect of the noise and losses
present in the detectors of the legitimate users.Comment: 9 pages, 1 figure; suppress sifting effect in the figure, final
versio
Intercept-resend attacks in the Bennett-Brassard 1984 quantum key distribution protocol with weak coherent pulses
Unconditional security proofs of the Bennett-Brassard protocol of quantum key
distribution have been obtained recently. These proofs cover also practical
implementations that utilize weak coherent pulses in the four signal
polarizations. Proven secure rates leave open the possibility that new proofs
or new public discussion protocols obtain larger rates over increased distance.
In this paper we investigate limits to error rate and signal losses that can be
tolerated by future protocols and proofs.Comment: 11 pages, 3 figures. Version accepted for publication in Phys. Rev.
Upper bounds for the secure key rate of decoy state quantum key distribution
The use of decoy states in quantum key distribution (QKD) has provided a
method for substantially increasing the secret key rate and distance that can
be covered by QKD protocols with practical signals. The security analysis of
these schemes, however, leaves open the possibility that the development of
better proof techniques, or better classical post-processing methods, might
further improve their performance in realistic scenarios. In this paper, we
derive upper bounds on the secure key rate for decoy state QKD. These bounds
are based basically only on the classical correlations established by the
legitimate users during the quantum communication phase of the protocol. The
only assumption about the possible post-processing methods is that double click
events are randomly assigned to single click events. Further we consider only
secure key rates based on the uncalibrated device scenario which assigns
imperfections such as detection inefficiency to the eavesdropper. Our analysis
relies on two preconditions for secure two-way and one-way QKD: The legitimate
users need to prove that there exists no separable state (in the case of
two-way QKD), or that there exists no quantum state having a symmetric
extension (one-way QKD), that is compatible with the available measurements
results. Both criteria have been previously applied to evaluate single-photon
implementations of QKD. Here we use them to investigate a realistic source of
weak coherent pulses. The resulting upper bounds can be formulated as a convex
optimization problem known as a semidefinite program which can be efficiently
solved. For the standard four-state QKD protocol, they are quite close to known
lower bounds, thus showing that there are clear limits to the further
improvement of classical post-processing techniques in decoy state QKD.Comment: 10 pages, 3 figure
- …