257 research outputs found
Phase oscillations in superfluid 3He-B weak links
Oscillations in quantum phase about a mean value of , observed across
micropores connecting two \helium baths, are explained in a Ginzburg-Landau
phenomenology. The dynamics arises from the Josephson phase relation,the
interbath continuity equation, and helium boundary conditions. The pores are
shown to act as Josephson tunnel junctions, and the dynamic variables are the
inter bath phase difference and fractional difference in superfluid density at
micropores. The system maps onto a non-rigid, momentum-shortened pendulum, with
inverted-orientation oscillations about a vertical tilt angle , and
other modes are predicted
Instability of the superfluid flow as black-hole lasing effect
We show that the instability leading to the decay of the one-dimensional
superfluid flow through a penetrable barrier are due to the black-hole lasing
effect. This dynamical instability is triggered by modes resonating in an
effective cavity formed by two horizons enclosing the barrier. The location of
the horizons is set by , with being the local fluid
velocity and sound speed, respectively. We compute the critical velocity
analytically and show that it is univocally determined by the horizons
configuration. In the limit of broad barriers, the continuous spectrum at the
origin of the Hawking-like radiation and of the Landau energetic instability is
recovered.Comment: 18 pages, 3 figure
Phase Estimation With Interfering Bose-Condensed Atomic Clouds
We investigate how to estimate from atom-position measurements the relative
phase of two Bose-Einstein condensates released from a double-well potential.
We demonstrate that the phase estimation sensitivity via the fit of the average
density to the interference pattern is fundamentally bounded by shot noise.
This bound can be overcome by estimating the phase from the measurement of
(or higher) correlation function. The optimal estimation strategy
requires the measurement of the -th order correlation function. We also
demonstrate that a second estimation method -- based on the detection of the
center of mass of the interference pattern -- provides sub shot-noise
sensitivity. Yet, the implementation of both protocols might be experimentally
challenging.Comment: 4 pages, 2 figure
Staying adiabatic with unknown energy gap
We introduce an algorithm to perform an optimal adiabatic evolution that
operates without an apriori knowledge of the system spectrum. By probing the
system gap locally, the algorithm maximizes the evolution speed, thus
minimizing the total evolution time. We test the algorithm on the Landau-Zener
transition and then apply it on the quantum adiabatic computation of 3-SAT: The
result is compatible with an exponential speed-up for up to twenty qubits with
respect to classical algorithms. We finally study a possible algorithm
improvement by combining it with the quantum Zeno effect.Comment: 4 pages, 4 figure
Phase Estimation from Atom Position Measurements
We study the measurement of the position of atoms as a means to estimate the
relative phase between two Bose-Einstein condensates. First, we consider
atoms released from a double-well trap, forming an interference pattern, and
show that a simple least-squares fit to the density gives a shot-noise limited
sensitivity. The shot-noise limit can instead be overcome by using correlation
functions of order or larger. The measurement of the
-order correlation function allows to estimate the relative phase
at the Heisenberg limit. Phase estimation through the measurement of the
center-of-mass of the interference pattern can also provide sub-shot-noise
sensitivity. Finally, we study the effect of the overlap between the two clouds
on the phase estimation, when Mach-Zehnder interferometry is performed in a
double-well.Comment: 20 pages, 6 figure
Mach-Zehnder interferometry with interacting trapped Bose-Einstein condensates
We theoretically analyze a Mach-Zehnder interferometer with trapped
condensates, and find that it is surprisingly stable against the nonlinearity
induced by inter-particle interactions. The phase sensitivity, which we study
for number squeezed input states, can overcome the shot noise limit and be
increased up to the Heisenberg limit provided that a Bayesian or
Maximum-Likelihood phase estimation strategy is used. We finally demonstrate
robustness of the Mach-Zehnder interferometer in presence of interactions
against condensate oscillations and a realistic atom counting error.Comment: 4 pages, 5 figures, minor revision
Entanglement, Non-linear Dynamics, and the Heisenberg Limit
We show that the quantum Fisher information provides a sufficient condition
to recognize multi-particle entanglement in a qubit state. The same
criterion gives a necessary and sufficient condition for sub shot-noise phase
sensitivity in the estimation of a collective rotation angle . The
analysis therefore singles out the class of entangled states which are {\it
useful} to overcome classical phase sensitivity in metrology and sensors. We
finally study the creation of useful entangled states by the non-linear
dynamical evolution of two decoupled Bose-Einstein condensates or trapped ions.Comment: Phys. Rev. Lett. 102, 100401 (2009
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