108,135 research outputs found
Advantages of gated silicon single photon detectors
We present a gated silicon single photon detector based on a commercially
available avalanche photodiode. Our detector achieves a photon detection
efficiency of 45\pm5% at 808 nm with 2x 10^-6 dark count per ns at -30V of
excess bias and -30{\deg}C. We compare gated and free-running detectors and
show that this mode of operation has significant advantages in two
representative experimental scenarios: detecting a single photon either hidden
in faint continuous light or after a strong pulse. We also explore, at
different temperatures and incident light intensities, the "charge persistence"
effect, whereby a detector clicks some time after having been illuminated
Divergence of the correlation length for critical planar FK percolation with via parafermionic observables
Parafermionic observables were introduced by Smirnov for planar FK
percolation in order to study the critical phase . This
article gathers several known properties of these observables. Some of these
properties are used to prove the divergence of the correlation length when
approaching the critical point for FK percolation when . A crucial
step is to consider FK percolation on the universal cover of the punctured
plane. We also mention several conjectures on FK percolation with arbitrary
cluster-weight .Comment: 26 page
Looking for symmetric Bell inequalities
Finding all Bell inequalities for a given number of parties, measurement
settings, and measurement outcomes is in general a computationally hard task.
We show that all Bell inequalities which are symmetric under the exchange of
parties can be found by examining a symmetrized polytope which is simpler than
the full Bell polytope. As an illustration of our method, we generate 238885
new Bell inequalities and 1085 new Svetlichny inequalities. We find, in
particular, facet inequalities for Bell experiments involving two parties and
two measurement settings that are not of the
Collins-Gisin-Linden-Massar-Popescu type.Comment: Joined the associated website as an ancillary file, 17 pages, 1
figure, 1 tabl
Dynamics and universality in noise driven dissipative systems
We investigate the dynamical properties of low dimensional systems, driven by
external noise sources. Specifically we consider a resistively shunted
Josephson junction and a one dimensional quantum liquid in a commensurate
lattice potential, subject to noise. In absence of nonlinear coupling, we
have shown previously that these systems establish a non-equilibrium critical
steady state [Nature Phys. 6, 806 (2010)]. Here we use this state as the basis
for a controlled renormalization group analysis using the Keldysh path integral
formulation to treat the non linearities: the Josephson coupling and the
commensurate lattice.
The analysis to first order in the coupling constant indicates transitions
between superconducting and localized regimes that are smoothly connected to
the respective equilibrium transitions. However at second order, the back
action of the mode coupling on the critical state leads to renormalization of
dissipation and emergence of an effective temperature. In the Josephson
junction the temperature is parametrically small allowing to observe a
universal crossover between the superconducting and insulating regimes. The IV
characteristics of the junction displays algebraic behavior controlled by the
underlying critical state over a wide range. In the noisy one dimensional
liquid the generated dissipation and effective temperature are not small as in
the junction. We find a crossover between a quasi-localized regime dominated by
dissipation and another dominated by temperature. However since in the thermal
regime the thermalization rate is parametrically small, signatures of the
non-equilibrium critical state can be seen in transient dynamics.Comment: 30 pages, 8 figures. Revised versio
Quantum storage of polarization qubits in birefringent and anisotropically absorbing materials
Storage of quantum information encoded into true single photons is an
essential constituent of long-distance quantum communication based on quantum
repeaters and of optical quantum information processing. The storage of
photonic polarization qubits is, however, complicated by the fact that many
materials are birefringent and have polarization-dependent absorption. Here we
present and demonstrate a simple scheme that allows compensating for these
polarization effects. The scheme is demonstrated using a solid-state quantum
memory implemented with an ensemble of rare-earth ions doped into a biaxial
yttrium orthosilicate () crystal. Heralded single photons generated
from a filtered spontaneous parametric downconversion source are stored, and
quantum state tomography of the retrieved polarization state reveals an average
fidelity of , which is significantly higher than what is
achievable with a measure-and-prepare strategy.Comment: 7 pages, 3 figures, 1 table, corrected typos and added ref. 3
Entanglement and non-locality are different resources
Bell's theorem states that, to simulate the correlations created by
measurement on pure entangled quantum states, shared randomness is not enough:
some "non-local" resources are required. It has been demonstrated recently that
all projective measurements on the maximally entangled state of two qubits can
be simulated with a single use of a "non-local machine". We prove that a
strictly larger amount of this non-local resource is required for the
simulation of pure non-maximally entangled states of two qubits
with
.Comment: 8 pages, 3 figure
Investigations of Optical Coherence Properties in an Erbium-doped Silicate Fiber for Quantum State Storage
We studied optical coherence properties of the 1.53 m telecommunication
transition in an Er-doped silicate optical fiber through spectral
holeburning and photon echoes. We find decoherence times of up to 3.8 s at
a magnetic field of 2.2 Tesla and a temperature of 150 mK. A strong
magnetic-field dependent optical dephasing was observed and is believed to
arise from an interaction between the electronic Er spin and the
magnetic moment of tunneling modes in the glass. Furthermore, we observed
fine-structure in the Erbium holeburning spectrum originating from
superhyperfine interaction with Al host nuclei. Our results show that
Er-doped silicate fibers are promising material candidates for quantum
state storage
Off-critical lattice models and massive SLEs
We suggest how versions of Schramm’s SLE can be used to describe the scaling limit of
some off-critical 2D lattice models. Many open questions remain
Quantum Repeaters with Photon Pair Sources and Multi-Mode Memories
We propose a quantum repeater protocol which builds on the well-known DLCZ
protocol [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature 414, 413
(2001)], but which uses photon pair sources in combination with memories that
allow to store a large number of temporal modes. We suggest to realize such
multi-mode memories based on the principle of photon echo, using solids doped
with rare-earth ions. The use of multi-mode memories promises a speedup in
entanglement generation by several orders of magnitude and a significant
reduction in stability requirements compared to the DLCZ protocol.Comment: 4 pages, 2 figures, to appear in PRL, accepted versio
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
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