64,469 research outputs found
A nonlinear Ramsey interferometer operating beyond the Heisenberg limit
We show that a dynamically evolving two-mode Bose-Einstein condensate (TBEC)
with an adiabatic, time-varying Raman coupling maps exactly onto a nonlinear
Ramsey interferometer that includes a nonlinear medium. Assuming a realistic
quantum state for the TBEC, namely the SU(2) coherent spin state, we find that
the measurement uncertainty of the ``path-difference'' phase shift scales as
the standard quantum limit (1/N^{1/2}) where N is the number of atoms, while
that for the interatomic scattering strength scales as 1/N^{7/5}, overcoming
the Heisenberg limit of 1/N.Comment: 4 figures. Submitted for publicatio
Ehrenfest Dynamics and Frictionless Cooling Methods
Recently introduced methods which result in shortcuts to adiabaticity,
particularly in the context of frictionless cooling, are rederived and
discussed in the framework of an approach based on Ehrenfest dynamics. This
construction provides physical insights into the emergence of the Ermakov
equation, the choice of its boundary conditions, and the use of minimum
uncertainty states as indicators of the efficiency of the procedure.
Additionally, it facilitates the extension of frictionless cooling to more
general situations of physical relevance, such as optical dipole trapping
schemes. In this context, we discuss frictionless cooling in the short-time
limit, a complementary case to the one considered in the literature, making
explicit the limitations intrinsic to the technique when the full
three-dimensional case is analyzed.Comment: 9 pages, 4 figures, v2: To appear in Physical Review A. (some minor
typos corrected and some references added
Squeezing and robustness of frictionless cooling strategies
Quantum control strategies that provide shortcuts to adiabaticity are
increasingly considered in various contexts including atomic cooling. Recent
studies have emphasized practical issues in order to reduce the gap between the
idealized models and actual ongoing implementations. We rephrase here the
cooling features in terms of a peculiar squeezing effect, and use it to
parametrize the robustness of frictionless cooling techniques with respect to
noise-induced deviations from the ideal time-dependent trajectory for the
trapping frequency. We finally discuss qualitative issues for the experimental
implementation of this scheme using bichromatic optical traps and lattices,
which seem especially suitable for cooling Fermi-Bose mixtures and for
investigating equilibration of negative temperature states, respectively.Comment: 9 pages, 7 figures; To appear in Physical Review
An Atom Laser is not monochromatic
We study both numerically and analytically the possibility of using an
adiabatic passage control method to construct a Mach-Zehnder interferometer
(MZI) for Bose-Einstein condensates (BECs) in the time domain, in exact
one-to-one correspondence with the traditional optical MZI that involves two
beam splitters and two mirrors. The interference fringes one obtains from such
a minimum-disturbance set up clearly demonstrates that, fundamentally, an atom
laser is not monochromatic due to interatomic interactions. We also consider
how the amount of entanglement in the system correlates to the interference
fringes.Comment: 4 figures. Submitted for publicatio
Construction of optimal witness for unknown two-qubit entanglement
Whether entanglement in a state can be detected, distilled, and quantified
without full state reconstruction is a fundamental open problem. We demonstrate
a new scheme encompassing these three tasks for arbitrary two-qubit
entanglement, by constructing the optimal entanglement witness for
polarization-entangled mixed-state photon pairs without full state
reconstruction. With better efficiency than quantum state tomography, the
entanglement is maximally distilled by newly developed tunable polarization
filters, and quantified by the expectation value of the witness, which equals
the concurrence. This scheme is extendible to multiqubit
Greenberger-Horne-Zeilinger entanglement.Comment: Phys. Rev. Lett. 105, 230404 (2010); supplementary information
(OWitness_sup.pdf) is included in source zip fil
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