Spherical Code Key Distribution Protocols for Qubits

Recently spherical codes were introduced as potentially more capable ensembles for quantum key distribution. Here we develop specific key creation protocols for the two qubit-based spherical codes, the trine and tetrahedron, and analyze them in the context of a suitably-tailored intercept/resend attack, both in standard form, and a ``gentler'' version whose back-action on the quantum state is weaker. When compared to the standard unbiased basis protocols, BB84 and six-state, two distinct advantages are found. First, they offer improved tolerance of eavesdropping, the trine besting its counterpart BB84 and the tetrahedron the six-state protocol. Second, the key error rate may be computed from the sift rate of the protocol itself, removing the need to sacrifice key bits for this purpose. This simplifies the protocol and improves the overall key rate.Comment: 4 pages revtex, 2 figures; clarified security analysis. Final version for publicatio

Security against eavesdropping in quantum cryptography

In this article we deal with the security of the BB84 quantum cryptography protocol over noisy channels using generalized privacy amplification. For this we estimate the fraction of bits needed to be discarded during the privacy amplification step. This estimate is given for two scenarios, both of which assume the eavesdropper to access each of the signals independently and take error correction into account. One scenario does not allow a delay of the eavesdropper's measurement of a measurement probe until he receives additional classical information. In this scenario we achieve a sharp bound. The other scenario allows a measurement delay, so that the general attack of an eavesdropper on individual signals is covered. This bound is not sharp but allows a practical implementation of the protocol.Comment: 11 pages including 3 figures, contains new results not contained in my Phys. Rev. A pape

Higher Security Thresholds for Quantum Key Distribution by Improved Analysis of Dark Counts

We discuss the potential of quantum key distribution (QKD) for long distance communication by proposing a new analysis of the errors caused by dark counts. We give sufficient conditions for a considerable improvement of the key generation rates and the security thresholds of well-known QKD protocols such as Bennett-Brassard 1984, Phoenix-Barnett-Chefles 2000, and the six-state protocol. This analysis is applicable to other QKD protocols like Bennett 1992. We examine two scenarios: a sender using a perfect single-photon source and a sender using a Poissonian source.Comment: 6 pages, 2 figures, v2: We obtained better results by using reverse reconciliation as suggested by Nicolas Gisi

Quantum Cryptography Based on the Time--Energy Uncertainty Relation

A new cryptosystem based on the fundamental time--energy uncertainty relation is proposed. Such a cryptosystem can be implemented with both correlated photon pairs and single photon states.Comment: 5 pages, LaTex, no figure

Accumulation of entanglement in a continuous variable memory

We study the accumulation of entanglement in a memory device built out of two continuous variable (CV) systems. We address the case of a qubit mediating an indirect joint interaction between the CV systems. We show that, in striking contrast with respect to registers built out of bidimensional Hilbert spaces, entanglement superior to a single ebit can be efficiently accumulated in the memory, even though no entangled resource is used. We study the protocol in an immediately implementable setup, assessing the effects of the main imperfections.Comment: 4 pages, 3 figures, RevTeX

Testing Bell's inequality with two-level atoms via population spectroscopy

We propose a feasible experimental scheme, employing methods of population spectroscopy with two-level atoms, for a test of Bell's inequality for massive particles. The correlation function measured in this scheme is the joint atomic $Q$ function. An inequality imposed by local realism is violated by any entangled state of a pair of atoms.Comment: 4 pages, REVTeX, no figures. More info on http://www.ligo.caltech.edu/~cbrif/science.htm

Scaling in the time-dependent failure of a fiber bundle with local load sharing

We study the scaling behaviors of a time-dependent fiber-bundle model with local load sharing. Upon approaching the complete failure of the bundle, the breaking rate of fibers diverges according to $r(t)\propto (T_f-t)^{-\xi}$, where $T_f$ is the lifetime of the bundle, and $\xi \approx 1.0$ is a quite universal scaling exponent. The average lifetime of the bundle $$ scales with the system size as $N^{-\delta}$, where $\delta$ depends on the distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.

Atom-photon entanglement generation and distribution

We extend an earlier model by Law {\it et al.} \cite{law} for a cavity QED based single-photon-gun to atom-photon entanglement generation and distribution. We illuminate the importance of a small critical atom number on the fidelity of the proposed operation in the strong coupling limit. Our result points to a promisingly high purity and efficiency using currently available cavity QED parameters, and sheds new light on constructing quantum computing and communication devices with trapped atoms and high Q optical cavities.Comment: 7 fig

Unambiguous Discrimination Between Linearly Dependent States with Multiple Copies

A set of quantum states can be unambiguously discriminated if and only if they are linearly independent. However, for a linearly dependent set, if C copies of the state are available, then the resulting C particle states may form a linearly independent set, and be amenable to unambiguous discrimination. We obtain necessary and sufficient conditions for the possibility of unambiguous discrimination between N states given that C copies are available and that the single copies span a D dimensional space. These conditions are found to be identical for qubits. We then examine in detail the linearly dependent trine ensemble. The set of C>1 copies of each state is a set of linearly independent lifted trine states. The maximum unambiguous discrimination probability is evaluated for all C>1 with equal a priori probabilities.Comment: 12 Pages RevTeX 4, 1 EPS figur