183 research outputs found
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
Approaches to Single Photon Detection
We present recent results on our development of single photon detectors,
including: gated and free-running InGaAs/InP avalanche photodiodes; hybrid
detection systems based on sum-frequency generation and Si APDs; and SSPDs
(superconducting single photon detectors), for telecom wavelengths; as well as
SiPM (Silicon photomultiplier) detectors operating in the visible regime.Comment: NDIP08 Conf proceedings. 4 pages, 4 figure
Self-critical thinking and overgeneralisation in depression and eating disorders:An experimental study
Background: Self-critical thinking is common across psychological disorders. This study hypothesized that it may play an important role in ‘overgeneralization’, the process of drawing general implications from an isolated negative experience. Aims: To explore the impact of two experimental tasks designed to elicit self-critical thoughts on the endorsement of general negative self-views of clinical and non-clinical populations. Method: Three groups (depression, eating disorders and non-clinical controls), completed standardized questionnaires and the two tasks. Participants rated their self-critical thinking and general negative self-beliefs before and after each task. Results: Following a failure experience, both clinical groups showed a greater increase in general negative self-views compared with controls, indicating greater overgeneralization. Both habitual and increases in state self-critical thinking were associated with overgeneralization while negative perfectionism was not. Overgeneralization was more strongly associated with post-task reduced mood than self-criticism. Conclusions: Self-critical thinking may be an important factor in the process of overgeneralization, and the increase in general negative self-views may be particularly crucial for lowering of mood
Purification of Single-photon Entanglement
Single-photon entanglement is a simple form of entanglement that exists
between two spatial modes sharing a single photon. Despite its elementary form,
it provides a resource as useful as polarization-entangled photons and it can
be used for quantum teleportation and entanglement swapping operations. Here,
we report the first experiment where single-photon entanglement is purified
with a simple linear-optics based protocol. Besides its conceptual interest,
this result might find applications in long distance quantum communication
based on quantum repeaters.Comment: Main article: 5 pages, 4 figure
Comparing different approaches for generating random numbers device-independently using a photon pair source
What is the most efficient way to generate random numbers
device-independently using a photon pair source based on spontaneous parametric
down conversion (SPDC)? We consider this question by comparing two
implementations of a detection-loophole-free Bell test. In particular, we study
in detail a scenario where a heralded single photon source (HSPS) is used to
herald path-entangled states, i.e. entanglement between two spatial modes
sharing a single photon and where non-locality is revealed using photon
counting preceded by small displacement operations. We start by giving a
theoretical description of such a measurement. We then show how to optimize the
Bell-CHSH violation through a non-perturbative calculation, taking the main
experimental imperfections into account. We finally bound the amount of
randomness that can be extracted and compare it to the one obtained with the
conventional scenario using photon pairs entangled e.g. in polarization and
analyzed through photon counting. While the former requires higher overall
detection efficiencies, it is far more efficient in terms of both the entropy
per experimental run and the rate of random bit generation.Comment: 12 pages, 5 figure
Ancilla-assisted quantum process tomography
Complete and precise characterization of a quantum dynamical process can be
achieved via the method of quantum process tomography. Using a source of
correlated photons, we have implemented several methods investigating a wide
range of processes, e.g., unitary, decohering, and polarizing. One of these
methods, ancilla-assisted process tomography (AAPT), makes use of an additional
``ancilla system,'' and we have theoretically determined the conditions when
AAPT is possible. All prior schemes for AAPT make use of entangled states. Our
results show that, surprisingly, entanglement is not required for AAPT, and we
present process tomography data obtained using an input state that has no
entanglement. However, the use of entanglement yields superior results.Comment: To appear in Physical Review Letter
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
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
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