192 research outputs found
A Bell-type test of energy-time entangled qutrits
We have performed a Bell-type test for energy-time entangled qutrits. A
method of inferring the Bell violation in terms of an associated interference
visibility is derived. Using this scheme we obtained a Bell value of , representing a violation of above the limit for local
variables. The scheme has been developed for use at telecom wavelengths and
using proven long distance quantum communication architecture to optimize the
utility of this high dimensional entanglement resource.Comment: replaced lost acknowledement
Energy-time entangled qutrits: Bell tests and quantum communication
We have developed a scheme to generate, control, transmit and measure
entangled photonic qutrits (two photons each of dimension d = 3). A Bell test
of this source has previously been reported elsewhere [1], therefore, here we
focus on how the control of the system is realized. Motivated by these results,
we outline how the scheme can be used for two specific quantum protocols,
namely key distribution and coin tossing and discuss some of their advantages
and disadvantages.Comment: For the conference proceedings of QCMC 200
32 Bin Near-Infrared Time-Multiplexing Detector with Attojoule Single-Shot Energy Resolution
We present two implementations of photon counting time-multiplexing detectors
for near-infrared wavelengths, based on Peltier cooled InGaAs/InP avalanche
photo diodes (APDs). A first implementation is motivated by practical
considerations using only commercially available components. It features 16
bins, pulse repetition rates of up to 22 kHz and a large range of applicable
pulse widths of up to 100 ns. A second implementation is based on rapid gating
detectors, permitting deadtimes below 10 ns. This allows one to realize a high
dynamic-range 32 bin detector, able to process pulse repetition rates of up to
6 MHz for pulse width of up to 200 ps. Analysis of the detector response at
16.5% detection efficiency, reveals a single-shot energy resolution on the
attojoule level.Comment: 7 pages, 7 figure
High coherence photon pair source for quantum communication
This paper reports a novel single mode source of narrow-band entangled photon
pairs at telecom wavelengths under continuous wave excitation, based on
parametric down conversion. For only 7 mW of pump power it has a created
spectral radiance of 0.08 pairs per coherence length and a bandwidth of 10 pm
(1.2 GHz). The effectively emitted spectral brightness reaches 3.9*10^5 pairs
/(s pm). Furthermore, when combined with low jitter single photon detectors,
such sources allow for the implementation of quantum communication protocols
without any active synchronization or path length stabilization. A HOM-Dip with
photons from two autonomous CW sources has been realized demonstrating the
setup's stability and performance.Comment: 12 pages, 4 figure
Quasi-Superactivation of Classical Capacity of Zero-Capacity Quantum Channels
One of the most surprising recent results in quantum Shannon theory is the
superactivation of the quantum capacity of a quantum channel. This phenomenon
has its roots in the extreme violation of additivity of the channel capacity
and enables to reliably transmit quantum information over zero-capacity quantum
channels. In this work we demonstrate a similar effect for the classical
capacity of a quantum channel which previously was thought to be impossible. We
show that a nonzero classical capacity can be achieved for all zero-capacity
quantum channels and it only requires the assistance of an elementary
photon-atom interaction process - the stimulated emission.Comment: 52 pages, 6 figures, Journal-ref: Journal of Modern Optics, published
version (minor typo fixed
Decoherence-based exploration of d-dimensional one-way quantum computation
We study the effects of amplitude and phase damping decoherence in
d-dimensional one-way quantum computation (QC). Our investigation shows how
information transfer and entangling gate simulations are affected for d>=2. To
understand motivations for extending the one-way model to higher dimensions, we
describe how d-dimensional qudit cluster states deteriorate under environmental
noise. In order to protect quantum information from the environment we consider
the encoding of logical qubits into physical qudits and compare entangled pairs
of linear qubit-cluster states with single qudit clusters of equal length and
total dimension. Our study shows a significant reduction in the performance of
one-way QC for d>2 in the presence of Markovian type decoherence models.Comment: 8 pages, 11 figures, RevTeX
Mixed State Entanglement: Manipulating Polarisation-Entangled Photons
There has been much discussion recently regarding entanglement
transformations in terms of local filtering operations and whether the optimal
entanglement for an arbitrary two-qubit state could be realised. We introduce
an experimentally realisable scheme for manipulating the entanglement of an
arbitrary state of two polarisation entangled qubits. This scheme is then used
to provide some perspective to the mathematical concepts inherent in this field
with respect to a laboratory environment. Specifically, we look at how to
extract enhanced entanglement from systems with a fixed rank and in the case
where the rank of the density operator for the state can be reduced, show how
the state can be made arbitrarily close to a maximally entangled pure state. In
this context we also discuss bounds on entanglement in mixed states.Comment: 12 pages, 10 figure
Polarization ququarts
We discuss the concept of polarization states of four-dimensional quantum
systems based on frequency non-degenerate biphoton field. Several quantum
tomography protocols were developed and implemented for measurement of an
arbitrary state of ququart. A simple method that does not rely on
interferometric technique is used to generate and measure the sequence of
states that can be used for quantum communication purposes.Comment: 13 pages, 10 figure
Nonlinear interaction between two heralded single photons
Harnessing nonlinearities strong enough to allow two single photons to
interact with one another is not only a fascinating challenge but is central to
numerous advanced applications in quantum information science. Currently, all
known approaches are extremely challenging although a few have led to
experimental realisations with attenuated classical laser light. This has
included cross-phase modulation with weak classical light in atomic ensembles
and optical fibres, converting incident laser light into a non-classical stream
of photon or Rydberg blockades as well as all-optical switches with attenuated
classical light in various atomic systems. Here we report the observation of a
nonlinear parametric interaction between two true single photons. Single
photons are initially generated by heralding one photon from each of two
independent spontaneous parametric downconversion sources. The two heralded
single photons are subsequently combined in a nonlinear waveguide where they
are converted into a single photon with a higher energy. Our approach
highlights the potential for quantum nonlinear optics with integrated devices,
and as the photons are at telecom wavelengths, it is well adapted to
applications in quantum communication.Comment: 4 pages, 4 figure
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