150 research outputs found
Fault-tolerant quantum computation with cluster states
The one-way quantum computing model introduced by Raussendorf and Briegel
[Phys. Rev. Lett. 86 (22), 5188-5191 (2001)] shows that it is possible to
quantum compute using only a fixed entangled resource known as a cluster state,
and adaptive single-qubit measurements. This model is the basis for several
practical proposals for quantum computation, including a promising proposal for
optical quantum computation based on cluster states [M. A. Nielsen,
arXiv:quant-ph/0402005, accepted to appear in Phys. Rev. Lett.]. A significant
open question is whether such proposals are scalable in the presence of
physically realistic noise. In this paper we prove two threshold theorems which
show that scalable fault-tolerant quantum computation may be achieved in
implementations based on cluster states, provided the noise in the
implementations is below some constant threshold value. Our first threshold
theorem applies to a class of implementations in which entangling gates are
applied deterministically, but with a small amount of noise. We expect this
threshold to be applicable in a wide variety of physical systems. Our second
threshold theorem is specifically adapted to proposals such as the optical
cluster-state proposal, in which non-deterministic entangling gates are used. A
critical technical component of our proofs is two powerful theorems which
relate the properties of noisy unitary operations restricted to act on a
subspace of state space to extensions of those operations acting on the entire
state space.Comment: 31 pages, 54 figure
Hybrid quantum repeater based on dispersive CQED interactions between matter qubits and bright coherent light
We describe a system for long-distance distribution of quantum entanglement,
in which coherent light with large average photon number interacts dispersively
with single, far-detuned atoms or semiconductor impurities in optical cavities.
Entanglement is heralded by homodyne detection using a second bright light
pulse for phase reference. The use of bright pulses leads to a high success
probability for the generation of entanglement, at the cost of a lower initial
fidelity. This fidelity may be boosted by entanglement purification techniques,
implemented with the same physical resources. The need for more purification
steps is well compensated for by the increased probability of success when
compared to heralded entanglement schemes using single photons or weak coherent
pulses with realistic detectors. The principle cause of the lower initial
fidelity is fiber loss; however, spontaneous decay and cavity losses during the
dispersive atom/cavity interactions can also impair performance. We show that
these effects may be minimized for emitter-cavity systems in the weak-coupling
regime as long as the resonant Purcell factor is larger than one, the cavity is
over-coupled, and the optical pulses are sufficiently long. We support this
claim with numerical, semiclassical calculations using parameters for three
realistic systems: optically bright donor-bound impurities such as 19-F:ZnSe
with a moderate-Q microcavity, the optically dim 31-P:Si system with a high-Q
microcavity, and trapped ions in large but very high-Q cavities.Comment: Please consult the published version, where assorted typos are
corrected. It is freely available at http://stacks.iop.org/1367-2630/8/18
Lasing of donor-bound excitons in ZnSe microdisks
Excitons bound to flourine atoms in ZnSe have the potential for several
quantum optical applications. Examples include optically accessible quantum
memories for quantum information processing and lasing without inversion. These
applications require the bound-exciton transitions to be coupled to cavities
with high cooperativity factors, which results in the experimental observation
of low-threshold lasing. We report such lasing from fluorine-doped ZnSe quantum
wells in 3 and 6 micron microdisk cavities. Photoluminescence and selective
photoluminescence spectroscopy confirm that the lasing is due to bound-exciton
transitions.Comment: 4 pages, 3 figures; introduction rewritte
Experimental Controlled-NOT Logic Gate for Single Photons in the Coincidence Basis
We report a proof-of-principle demonstration of a probabilistic
controlled-NOT gate for single photons. Single-photon control and target qubits
were mixed with a single ancilla photon in a device constructed using only
linear optical elements. The successful operation of the controlled-NOT gate
relied on post-selected three-photon interference effects which required the
detection of the photons in the output modes.Comment: 4 pages, 4 figures; minor change
New high-efficiency source of photon pairs for engineering quantum entanglement
We have constructed an efficient source of photon pairs using a
waveguide-type nonlinear device and performed a two-photon interference
experiment with an unbalanced Michelson interferometer. Parametric
down-converted photons from the nonlinear device are detected by two detectors
located at the output ports of the interferometer. Because the interferometer
is constructed with two optical paths of different length, photons from the
shorter path arrive at the detector earlier than those from the longer path. We
find that the difference of arrival time and the time window of the coincidence
counter are important parameters which determine the boundary between the
classical and quantum regime. When the time window of the coincidence counter
is smaller than the arrival time difference, fringes of high visibility
(80 10%) were observed. This result is only explained by quantum theory
and is clear evidence for quantum entanglement of the interferometer's optical
paths.Comment: 4 pages, 4 figures, IQEC200
Tunable Indistinguishable Photons From Remote Quantum Dots
Single semiconductor quantum dots have been widely studied within devices
that can apply an electric field. In the most common system, the low energy
offset between the InGaAs quantum dot and the surrounding GaAs material limits
the magnitude of field that can be applied to tens of kVcm^-1, before carriers
tunnel out of the dot. The Stark shift experienced by the emission line is
typically 1 meV. We report that by embedding the quantum dots in a quantum well
heterostructure the vertical field that can be applied is increased by over an
order of magnitude whilst preserving the narrow linewidths, high internal
quantum efficiencies and familiar emission spectra. Individual dots can then be
continuously tuned to the same energy allowing for two-photon interference
between remote, independent, quantum dots
Colonoscopic polyp detection rate is stable throughout the workday including evening colonoscopy sessions
Objective: Polyp detection rate (PDR) is an accepted measure of colonoscopy quality. Several factors may influence PDR including time of procedure and order of colonoscopy within a session. Our unit provides evening colonoscopy lists (6-9 pm). We examined whether colonoscopy performance declines in the evening. Design: Data for all National Health Service (NHS) outpatient colonoscopies performed at Norfolk and Norwich University Hospital in 2011 were examined. Timing, demographics, indication and colonoscopy findings were recorded. Statistical analysis was performed using multivariate regression. Results: Data from 2576 colonoscopies were included: 1163 (45.1%) in the morning, 1123 (43.6%) in the afternoon and 290 (11.3%) in the evening. Overall PDR was 40.80%. Males, increasing age and successful caecal intubation were all significantly associated with higher polyp detection. The indications ‘faecal occult blood screening’ (p<0.001) and ‘polyp surveillance’ (p<0.001) were strongly positively associated and ‘anaemia’ (p=0.01) was negatively associated with PDR. Following adjustment for covariates, there was no significant difference in PDR between sessions. With the morning as the reference value, the odds ratio for polyp detection in the afternoon and evening were 0.93 (95% CI = 0.72-1.18) and 1.15 (95%CI = 0.82-1.61) respectively. PDR was not affected by rank of colonoscopy within a list, sedation dose or trainee-involvement. Conclusions: Time of day did not affect polyp detection rate in clinical practice. Evening colonoscopy had equivalent efficacy and is an effective tool in meeting increasing demands for endoscopy. Standardisation was shown to have a considerable effect as demographics, indication and endoscopist varied substantially between sessions. Evening sessions were popular with a younger populatio
Proximal Sessile Serrated Adenomas Are More Prevalent in Caucasians, and Gastroenterologists Are Better Than Nongastroenterologists at Their Detection
Background and Aim. Proximal sessile serrated adenomas (PSSA) leading to colorectal cancer (CRC) represent an alternate pathway for CRC development. In this study, we aim to determine the prevalence of PSSAs and the impact of patient, colonoscopy, and endoscopist-related factors on PSSA detection. Methods. Patients ≥ 50 years of age undergoing a screening colonoscopy between 2012 and 2014 were included. Detection rates based on patient gender, race, colonoscopy timing, fellow participation, bowel preparation quality, and specialty of the endoscopist were calculated. t-tests were used to compare detection rates and a multivariate-adjusted analysis was performed. Results. 140 PSSAs were detected from 4151 colonoscopies, with a prevalence of 3.4%. Detection rate was higher in Caucasians compared to African-Americans (AA) (3.7 ± 4.1 versus 0.96 ± 3.5; p<0.001). Gastroenterologists detected more PSSAs compared to nongastroenterologists (3.9 ± 3.5 versus 2.2 ± 3.0; p=0.028). These findings were still significant after adjusted multivariate analysis. The rest of the factors did not make significant difference in PSSA detection rate. Conclusions. PSSAs are more prevalent in Caucasians compared to AAs. Racial difference in prevalence of PSSAs is intriguing and warrants further investigation. Gastroenterologists have a significantly higher PSSADR compared to nongastroenterologists. Educational measures should be implemented in nongastroenterologists to improve their PSSA detection rates
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