190 research outputs found
Spin entanglement, decoherence and Bohm's EPR paradox
We obtain criteria for entanglement and the EPR paradox
for spin-entangled particles and analyse the effects of decoherence caused
by absorption and state purity errors. For a two qubit photonic state,
entanglement can occur for all transmission efficiencies. In this case,
the state preparation purity must be above a threshold value. However,
Bohm’s spin EPR paradox can be achieved only above a critical level of
loss. We calculate a required efficiency of 58%, which appears achievable
with current quantum optical technologies. For a macroscopic number of
particles prepared in a correlated state, spin entanglement and the EPR
paradox can be demonstrated using our criteria for efficiencies η > 1/3
and η > 2/3 respectively. This indicates a surprising insensitivity to loss
decoherence, in a macroscopic system of ultra-cold atoms or photons
Strong Quantum Spin Correlations Observed in Atomic Spin Mixing
We have observed sub-Poissonian spin correlations generated by collisionally
induced spin mixing in a spin-1 Bose-Einstein condensate. We measure a quantum
noise reduction of -7 dB (-10 dB corrected for detection noise) below the
standard quantum limit (SQL) for the corresponding coherent spin states. The
spin fluctuations are detected as atom number differences in the spin states
using fluorescent imaging that achieves a detection noise floor of 8 atoms per
spin component for a probe time of 100 s.Comment: 5 pages, 4 figure
Rydberg excitation of a single trapped ion
We demonstrate excitation of a single trapped cold Ca ion to
Rydberg levels by laser radiation in the vacuum-ultraviolet at 122 nm
wavelength. Observed resonances are identified as 3dD to 51 F, 52 F
and 3dD to 64F. We model the lineshape and our results imply a
large state-dependent coupling to the trapping potential. Rydberg ions are of
great interest for future applications in quantum computing and simulation, in
which large dipolar interactions are combined with the superb experimental
control offered by Paul traps.Comment: 4 pages, 3 figure
Einstein-Podolsky-Rosen correlations via dissociation of a molecular Bose-Einstein condensate
Recent experimental measurements of atomic intensity correlations through
atom shot noise suggest that atomic quadrature phase correlations may soon be
measured with a similar precision. We propose a test of local realism with
mesoscopic numbers of massive particles based on such measurements. Using
dissociation of a Bose-Einstein condensate of diatomic molecules into bosonic
atoms, we demonstrate that strongly entangled atomic beams may be produced
which possess Einstein-Podolsky-Rosen (EPR) correlations in field quadratures,
in direct analogy to the position and momentum correlations originally
considered by EPR.Comment: Final published version (corrections in Ref. [32], updated
references
Towards Einstein-Podolsky-Rosen quantum channel multiplexing
A single broadband squeezed field constitutes a quantum communication
resource that is sufficient for the realization of a large number N of quantum
channels based on distributed Einstein-Podolsky-Rosen (EPR) entangled states.
Each channel can serve as a resource for, e.g. independent quantum key
distribution or teleportation protocols. N-fold channel multiplexing can be
realized by accessing 2N squeezed modes at different Fourier frequencies. We
report on the experimental implementation of the N=1 case through the
interference of two squeezed states, extracted from a single broadband squeezed
field, and demonstrate all techniques required for multiplexing (N>1). Quantum
channel frequency multiplexing can be used to optimize the exploitation of a
broadband squeezed field in a quantum information task. For instance, it is
useful if the bandwidth of the squeezed field is larger than the bandwidth of
the homodyne detectors. This is currently a typical situation in many
experiments with squeezed and two-mode squeezed entangled light.Comment: 4 pages, 4 figures. In the new version we cite recent experimental
work bei Mehmet et al., arxiv0909.5386, in order to clarify the motivation of
our work and its possible applicatio
Outcoupling from a Bose-Einstein condensate with squeezed light to produce entangled atom laser beams
We examine the properties of an atom laser produced by outcoupling from a
Bose-Einstein condensate with squeezed light. We model the multimode dynamics
of the output field and show that a significant amount of squeezing can be
transfered from an optical mode to a propagating atom laser beam. We use this
to demonstrate that two-mode squeezing can be used to produce twin atom laser
beams with continuous variable entanglement in amplitude and phase.Comment: 11 pages, 14 figure
Quantum Communication with an Accelerated Partner
An unsolved problem in relativistic quantum information research is how to
model efficient, directional quantum communication between localised parties in
a fully quantum field theoretical framework. We propose a tractable approach to
this problem based on solving the Heisenberg evolution of localized field
observables. We illustrate our approach by analysing, and obtaining approximate
analytical solutions to, the problem of communicating coherent states between
an inertial sender, Alice and an accelerated receiver, Rob. We use these
results to determine the efficiency with which continuous variable quantum key
distribution could be carried out over such a communication channel.Comment: Additional explanatory text and typo in Eq.17 correcte
A detector for continuous measurement of ultra-cold atoms in real time
We present the first detector capable of recording high-bandwidth real time
atom number density measurements of a Bose Einstein condensate. Based on a
two-color Mach-Zehnder interferometer, our detector has a response time that is
six orders of magnitude faster than current detectors based on CCD cameras
while still operating at the shot-noise limit. With this minimally destructive
system it may be possible to implement feedback to stabilize a Bose-Einstein
condensate or an atom laser.Comment: 3 pages, 3 figures, submitted to optics letter
Generating entanglement of photon-number states with coherent light via cross-Kerr nonlinearity
We propose a scheme for generating entangled states of light fields. This
scheme only requires the cross-Kerr nonlinear interaction between coherent
light-beams, followed by a homodyne detection. Therefore, this scheme is within
the reach of current technology. We study in detail the generation of the
entangled states between two modes, and that among three modes. In addition to
the Bell states between two modes and the W states among three modes, we find
plentiful new kinds of entangled states. Finally, the scheme can be extend to
generate the entangled states among more than three modes.Comment: 2 figure
Quantum projection noise limited interferometry with coherent atoms in a Ramsey type setup
Every measurement of the population in an uncorrelated ensemble of two-level
systems is limited by what is known as the quantum projection noise limit.
Here, we present quantum projection noise limited performance of a Ramsey type
interferometer using freely propagating coherent atoms. The experimental setup
is based on an electro-optic modulator in an inherently stable Sagnac
interferometer, optically coupling the two interfering atomic states via a
two-photon Raman transition. Going beyond the quantum projection noise limit
requires the use of reduced quantum uncertainty (squeezed) states. The
experiment described demonstrates atom interferometry at the fundamental noise
level and allows the observation of possible squeezing effects in an atom
laser, potentially leading to improved sensitivity in atom interferometers.Comment: 8 pages, 8 figures, published in Phys. Rev.
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