268 research outputs found
Noise-free high-efficiency photon-number-resolving detectors
High-efficiency optical detectors that can determine the number of photons in
a pulse of monochromatic light have applications in a variety of physics
studies, including post-selection-based entanglement protocols for linear
optics quantum computing and experiments that simultaneously close the
detection and communication loopholes of Bell's inequalities. Here we report on
our demonstration of fiber-coupled, noise-free, photon-number-resolving
transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these
sensors could be made even higher at any wavelength in the visible and
near-infrared spectrum without resulting in a higher dark-count rate or
degraded photon-number resolution.Comment: 4 pages, 4 figures Published in Physical Review A, Rapid
Communications, 17 June 200
A stochastic model of Min oscillations in Escherichia coli and Min protein segregation during cell division
The Min system in Escherichia coli directs division to the centre of the cell
through pole-to-pole oscillations of the MinCDE proteins. We present a one
dimensional stochastic model of these oscillations which incorporates membrane
polymerisation of MinD into linear chains. This model reproduces much of the
observed phenomenology of the Min system, including pole-to-pole oscillations
of the Min proteins. We then apply this model to investigate the Min system
during cell division. Oscillations continue initially unaffected by the closing
septum, before cutting off rapidly. The fractions of Min proteins in the
daughter cells vary widely, from 50%-50% up to 85%-15% of the total from the
parent cell, suggesting that there may be another mechanism for regulating
these levels in vivo.Comment: 19 pages, 12 figures (25 figure files); published at
http://www.iop.org/EJ/journal/physbi
An Algebraic Approach to Linear-Optical Schemes for Deterministic Quantum Computing
Linear-Optical Passive (LOP) devices and photon counters are sufficient to
implement universal quantum computation with single photons, and particular
schemes have already been proposed. In this paper we discuss the link between
the algebraic structure of LOP transformations and quantum computing. We first
show how to decompose the Fock space of N optical modes in finite-dimensional
subspaces that are suitable for encoding strings of qubits and invariant under
LOP transformations (these subspaces are related to the spaces of irreducible
unitary representations of U(N)). Next we show how to design in algorithmic
fashion
LOP circuits which implement any quantum circuit deterministically. We also
present some simple examples, such as the circuits implementing a CNOT gate and
a Bell-State Generator/Analyzer.Comment: new version with minor modification
Low Cost and Compact Quantum Cryptography
We present the design of a novel free-space quantum cryptography system,
complete with purpose-built software, that can operate in daylight conditions.
The transmitter and receiver modules are built using inexpensive off-the-shelf
components. Both modules are compact allowing the generation of renewed shared
secrets on demand over a short range of a few metres. An analysis of the
software is shown as well as results of error rates and therefore shared secret
yields at varying background light levels. As the system is designed to
eventually work in short-range consumer applications, we also present a use
scenario where the consumer can regularly 'top up' a store of secrets for use
in a variety of one-time-pad and authentication protocols.Comment: 18 pages, 9 figures, to be published in New Journal of Physic
Quantum Forbidden-Interval Theorems for Stochastic Resonance
We extend the classical forbidden-interval theorems for a
stochastic-resonance noise benefit in a nonlinear system to a quantum-optical
communication model and a continuous-variable quantum key distribution model.
Each quantum forbidden-interval theorem gives a necessary and sufficient
condition that determines whether stochastic resonance occurs in quantum
communication of classical messages. The quantum theorems apply to any quantum
noise source that has finite variance or that comes from the family of
infinite-variance alpha-stable probability densities. Simulations show the
noise benefits for the basic quantum communication model and the
continuous-variable quantum key distribution model.Comment: 13 pages, 2 figure
Entangled state quantum cryptography: Eavesdropping on the Ekert protocol
Using polarization-entangled photons from spontaneous parametric
downconversion, we have implemented Ekert's quantum cryptography protocol. The
near-perfect correlations of the photons allow the sharing of a secret key
between two parties. The presence of an eavesdropper is continually checked by
measuring Bell's inequalities. We investigated several possible eavesdropper
strategies, including pseudo-quantum non-demolition measurements. In all cases,
the eavesdropper's presence was readily apparent. We discuss a procedure to
increase her detectability.Comment: 4 pages, 2 encapsulated postscript files, PRL (tentatively) accepte
Quantum relays and noise suppression using linear optics
Probabilistic quantum non-demolition (QND) measurements can be performed
using linear optics and post-selection. Here we show how QND devices of this
kind can be used in a straightforward way to implement a quantum relay, which
is capable of extending the range of a quantum cryptography system by
suppressing the effects of detector noise. Unlike a quantum repeater, a quantum
relay system does not require entanglement purification or the ability to store
photons.Comment: minor changes; references adde
Quantum Cryptography using larger alphabets
Like all of quantum information theory, quantum cryptography is traditionally
based on two level quantum systems. In this letter, a new protocol for quantum
key distribution based on higher dimensional systems is presented. An
experimental realization using an interferometric setup is also proposed.
Analyzing this protocol from the practical side, one finds an increased key
creation rate while keeping the initial laser pulse rate constant. Analyzing it
for the case of intercept/resend eavesdropping strategy, an increased error
rate is found compared to two dimensional systems, hence an advantage for the
legitimate users to detect an eavesdropper.Comment: 12 pages, 2 (eps) figure
Proposal to produce long-lived mesoscopic superpositions through an atom-driven field interaction
We present a proposal for the production of longer-lived mesoscopic
superpositions which relies on two requirements: parametric amplification and
squeezed vacuum reservoir for cavity-field states. Our proposal involves the
interaction of a two-level atom with a cavity field which is simultaneously
subjected to amplification processes.Comment: 12 pages, title changed, text improved and refences adde
On asymptotic continuity of functions of quantum states
A useful kind of continuity of quantum states functions in asymptotic regime
is so-called asymptotic continuity. In this paper we provide general tools for
checking if a function possesses this property. First we prove equivalence of
asymptotic continuity with so-called it robustness under admixture. This allows
us to show that relative entropy distance from a convex set including maximally
mixed state is asymptotically continuous. Subsequently, we consider it arrowing
- a way of building a new function out of a given one. The procedure originates
from constructions of intrinsic information and entanglement of formation. We
show that arrowing preserves asymptotic continuity for a class of functions
(so-called subextensive ones). The result is illustrated by means of several
examples.Comment: Minor corrections, version submitted for publicatio
- …