9,780 research outputs found
The Curci-Ferrari model with massive quarks at two loops
Massive quarks are included in the Curci-Ferrari model and the theory is
renormalized at two loops in the MSbar scheme in an arbitrary covariant gauge.Comment: 8 latex page
Stronger Quantum Correlations with Loophole-free Post-selection
One of the most striking non-classical features of quantum mechanics is in
the correlations it predicts between spatially separated measurements. In local
hidden variable theories, correlations are constrained by Bell inequalities,
but quantum correlations violate these. However, experimental imperfections
lead to "loopholes" whereby LHV correlations are no longer constrained by Bell
inequalities, and violations can be described by LHV theories. For example,
loopholes can emerge through selective detection of events. In this letter, we
introduce a clean, operational picture of multi-party Bell tests, and show that
there exists a non-trivial form of loophole-free post-selection. Surprisingly,
the same post-selection can enhance quantum correlations, and unlock a
connection between non-classical correlations and non-classical computation.Comment: 4 pages, 2 figures, substantially revised in response to referee
suggestion
Model-independent assessment of current direct searches for spin-dependent dark matter
I evaluate the current results of spin-dependent weakly interacting massive
particle (WIMP) searches within a model-independent framework, showing the most
restrictive limits to date derive from the combination of xenon and sodium
iodide experiments. The extension of this analysis to the case of positive
signal experiments is elaborated.Comment: 4 pages, 4 figures, revised and accepted for publication on Phys.
Rev. Let
The one-way quantum computer - a non-network model of quantum computation
A one-way quantum computer works by only performing a sequence of one-qubit
measurements on a particular entangled multi-qubit state, the cluster state. No
non-local operations are required in the process of computation. Any quantum
logic network can be simulated on the one-way quantum computer. On the other
hand, the network model of quantum computation cannot explain all ways of
processing quantum information possible with the one-way quantum computer. In
this paper, two examples of the non-network character of the one-way quantum
computer are given. First, circuits in the Clifford group can be performed in a
single time step. Second, the realisation of a particular circuit --the
bit-reversal gate-- on the one-way quantum computer has no network
interpretation. (Submitted to J. Mod. Opt, Gdansk ESF QIT conference issue.)Comment: 7 pages, 3 figure
Impulse Generation by an Open Shock Tube
We perform experimental and numerical studies of a shock tube with an open end. The purpose is to investigate the impulse due to the exhaust of gases through the open end of the tube as a model for a partially filled detonation tube as used in pulse detonation engine testing. We study the effects of the pressure ratio (varied from 3 to 9.2) and the volume ratio (expressed as fill fractions) between the driver and driven section. Two different driver gases, helium and nitrogen, and fill fractions between 5 and 100% are studied; the driven section is filled with air. For both driver gases, increasing the pressure ratio leads to larger specific impulses. The specific impulse increases for a decreasing fill fraction for the helium driver, but the impulse is almost independent of the fill fraction for the nitrogen driver. Two-dimensional (axisymmetric) numerical simulations are carried out for both driver gases. The simulation results show reasonable agreement with experimental measurements at high pressure ratios or small fill fractions, but there are substantial discrepancies for the smallest pressure ratios studied. Empirical models for the impulse in the limits of large and small fill fractions are also compared with the data. Reasonable agreement is found for the trends with fill fractions using the Gurney or Sato model at large fill fractions, but only Cooper’s bubble model is able to predict the small fill fraction limit. Computations of acoustic impedance and numerical simulations of unsteady gas dynamics indicate that the interaction of waves with the driver-driven gas interface and the propagation of waves in the driven gas play an essential role in the partial-fill effect
Loss tolerant linear optical quantum memory by measurement-based quantum computing
We give a scheme for loss tolerantly building a linear optical quantum memory which itself is tolerant to qubit loss. We use the encoding recently introduced in Varnava et al 2006 Phys. Rev. Lett. 97 120501, and give a method for efficiently achieving this. The entire approach resides within the 'one-way' model for quantum computing (Raussendorf and Briegel 2001 Phys. Rev. Lett. 86 5188–91; Raussendorf et al 2003 Phys. Rev. A 68 022312). Our results suggest that it is possible to build a loss tolerant quantum memory, such that if the requirement is to keep the data stored over arbitrarily long times then this is possible with only polynomially increasing resources and logarithmically increasing individual photon life-times
Fault-tolerant error correction with the gauge color code
The constituent parts of a quantum computer are inherently vulnerable to
errors. To this end we have developed quantum error-correcting codes to protect
quantum information from noise. However, discovering codes that are capable of
a universal set of computational operations with the minimal cost in quantum
resources remains an important and ongoing challenge. One proposal of
significant recent interest is the gauge color code. Notably, this code may
offer a reduced resource cost over other well-studied fault-tolerant
architectures using a new method, known as gauge fixing, for performing the
non-Clifford logical operations that are essential for universal quantum
computation. Here we examine the gauge color code when it is subject to noise.
Specifically we make use of single-shot error correction to develop a simple
decoding algorithm for the gauge color code, and we numerically analyse its
performance. Remarkably, we find threshold error rates comparable to those of
other leading proposals. Our results thus provide encouraging preliminary data
of a comparative study between the gauge color code and other promising
computational architectures.Comment: v1 - 5+4 pages, 11 figures, comments welcome; v2 - minor revisions,
new supplemental including a discussion on correlated errors and details on
threshold calculations; v3 - Author accepted manuscript. Accepted on
21/06/16. Deposited on 29/07/16. 9+5 pages, 17 figures, new version includes
resource scaling analysis in below threshold regime, see eqn. (4) and methods
sectio
GALEX ultraviolet observations of stellar variability in the Hyades and Pleiades clusters
We present GALEX near ultraviolet (NUV:1750 - 2750A) and far ultraviolet
(FUV: 1350 - 1750A) imaging observations of two 1.2 degree diameter fields in
the Hyades and Pleiades open clusters in order to detect possible UV
variability of the member stars. We have performed a detailed software search
for short-term UV flux variability during these observations of the approx 400
sources detected in each of the Hyades and Pleiades fields to identify
flare-like (dMe) stellar objects. This search resulted in the detection of 16
UV variable sources, of which 13 can be directly associated with probable
M-type stars. The other UV sources are G-type stars and one newly discovered RR
Lyrae star, USNOB1.0 1069-0046050, of period 0.624 day and distance 4.5-7.0
kpc. Light curves of photon flux versus time are shown for 7 flare events
recorded on six probable dMe stars. UV energies for these flares span the range
2E27 to 5E29 erg, with a corresponding NUV variability change of 1.82 mag. Only
one of these flare events (on the star Cl* Melotte 25 LH129) can definitely be
associated with an origin on a member the Hyades cluster itself. Finally, many
of our M-type candidates show long periods of enhanced UV activity but without
the associated rapid increase in flux that is normally associated with a flare
event. However, the total UV energy output during such periods of increased
activity is greater than that of many short-term UV flares. These intervals of
enhanced low-level UV activity concur with the idea that, even in quiescence,
the UV emission from dMe stars may be related to a superposition of many small
flare events possessing a wide range of energies.Comment: PASP Submitte
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