390 research outputs found
Dynamics of a pulsed continuous variable quantum memory
We study the transfer dynamics of non-classical fluctuations of light to the
ground-state collective spin components of an atomic ensemble during a pulsed
quantum memory sequence, and evaluate the relevant physical quantities to be
measured in order to characterize such a quantum memory. We show in particular
that the fluctuations stored into the atoms are emitted in temporal modes which
are always different than those of the readout pulse, but which can
nevertheless be retrieved efficiently using a suitable temporal mode-matching
technique. We give a simple toy model - a cavity with variable transmission -
which accounts for the behavior of the atomic quantum memory.Comment: 6 pages, 5 figure
Unconditional security proof of long-distance continuous-variable quantum key distribution with discrete modulation
We present a continuous-variable quantum key distribution protocol combining
a discrete modulation and reverse reconciliation. This protocol is proven
unconditionally secure and allows the distribution of secret keys over long
distances, thanks to a reverse reconciliation scheme efficient at very low
signal-to-noise ratio.Comment: 4 pages, 2 figure
Towards an experimentally feasible controlled-phase gate on two blockaded Rydberg atoms
We investigate the implementation of a controlled-Z gate on a pair of Rydberg
atoms in spatially separated dipole traps where the joint excitation of both
atoms into the Rydberg level is strongly suppressed (the Rydberg blockade). We
follow the adiabatic gate scheme of Jaksch et al. [1], where the pair of atoms
are coherently excited using lasers, and apply it to the experimental setup
outlined in Ga\"etan et al. [2]. We apply optimisation to the experimental
parameters to improve gate fidelity, and consider the impact of several
experimental constraints on the gate success.Comment: 10 pages, 14 figure
Investigation of a single-photon source based on quantum interference
We report on an experimental investigation of a single-photon source based on
a quantum interference effect first demonstrated by Koashi, Matsuoka, and
Hirano [Phys. Rev. A 53, 3621 (1996)]. For certain types of measurement-based
quantum information processing applications this technique may be useful as a
high rate, but random, source of single photons.Comment: Submitted to the New J. Phys. Focus Issue on "Measurement-based
quantum information processing
Towards deterministic optical quantum computation with coherently driven atomic ensembles
Scalable and efficient quantum computation with photonic qubits requires (i)
deterministic sources of single-photons, (ii) giant nonlinearities capable of
entangling pairs of photons, and (iii) reliable single-photon detectors. In
addition, an optical quantum computer would need a robust reversible photon
storage devise. Here we discuss several related techniques, based on the
coherent manipulation of atomic ensembles in the regime of electromagnetically
induced transparency, that are capable of implementing all of the above
prerequisites for deterministic optical quantum computation with single
photons.Comment: 11 pages, 7 figure
Imaging a single atom in a time-of-flight experiment
We perform fluorescence imaging of a single 87Rb atom after its release from
an optical dipole trap. The time-of-flight expansion of the atomic spatial
density distribution is observed by accumulating many single atom images. The
position of the atom is revealed with a spatial resolution close to 1
micrometer by a single photon event, induced by a short resonant probe. The
expansion yields a measure of the temperature of a single atom, which is in
very good agreement with the value obtained by an independent measurement based
on a release-and-recapture method. The analysis presented in this paper
provides a way of calibrating an imaging system useful for experimental studies
involving a few atoms confined in a dipole trap.Comment: 14 pages, 8 figure
Energy distribution and cooling of a single atom in an optical tweezer
We investigate experimentally the energy distribution of a single rubidium
atom trapped in a strongly focused dipole trap under various cooling regimes.
Using two different methods to measure the mean energy of the atom, we show
that the energy distribution of the radiatively cooled atom is close to
thermal. We then demonstrate how to reduce the energy of the single atom, first
by adiabatic cooling, and then by truncating the Boltzmann distribution of the
single atom. This provides a non-deterministic way to prepare atoms at low
microKelvin temperatures, close to the ground state of the trapping potential.Comment: 9 pages, 6 figures, published in PR
Room temperature triggered single-photon source in the near infrared
We report the realization of a solid-state triggered single-photon source
with narrow emission in the near infrared at room temperature. It is based on
the photoluminescence of a single nickel-nitrogen NE8 colour centre in a
chemical vapour deposited diamond nanocrystal. Stable single-photon emission
has been observed in the photoluminescence under both continuous-wave and
pulsed excitations. The realization of this source represents a step forward in
the application of diamond-based single-photon sources to Quantum Key
Distribution (QKD) under practical operating conditions.Comment: 10 page
Multidimensional reconciliation for continuous-variable quantum key distribution
We propose a method for extracting an errorless secret key in a
continuous-variable quantum key distribution protocol, which is based on
Gaussian modulation of coherent states and homodyne detection. The crucial
feature is an eight-dimensional reconciliation method, based on the algebraic
properties of octonions. Since the protocol does not use any postselection, it
can be proven secure against arbitrary collective attacks, by using
well-established theorems on the optimality of Gaussian attacks. By using this
new coding scheme with an appropriate signal to noise ratio, the distance for
secure continuous-variable quantum key distribution can be significantly
extended.Comment: 8 pages, 3 figure
Entanglement of two individual atoms using the Rydberg blockade
We report on our recent progress on the manipulation of single rubidium atoms
trapped in optical tweezers and the generation of entanglement between two
atoms, each individually trapped in neighboring tweezers. To create an
entangled state of two atoms in their ground states, we make use of the Rydberg
blockade mechanism. The degree of entanglement is measured using global
rotations of the internal states of both atoms. Such internal state rotations
on a single atom are demonstrated with a high fidelity.Comment: Proceeding of the 19th International Conference on Laser Spectroscopy
ICOLS 2009, 7-13 June 2009, Hokkaido, Japa
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