11 research outputs found
Bogoliubov Theory of acoustic Hawking radiation in Bose-Einstein Condensates
We apply the microscopic Bogoliubov theory of dilute Bose-Einstein
condensates to analyze quantum and thermal fluctuations in a flowing atomic
condensate in the presence of a sonic horizon. For the simplest case of a
step-like horizon, closed-form analytical expressions are found for the
spectral distribution of the analog Hawking radiation and for the density
correlation function. The peculiar long-distance density correlations that
appear as a consequence of the Hawking emission features turns out to be
reinforced by a finite initial temperature of the condensate. The analytical
results are in good quantitative agreement with first principle numerical
calculations.Comment: 11 pages, 7 figure
Resonant Hawking radiation in Bose-Einstein condensates
We study double-barrier interfaces separating regions of asymptotically
subsonic and supersonic flow of Bose condensed atoms. These setups contain at
least one black hole sonic horizon from which the analog of Hawking radiation
should be generated and emitted against the flow in the subsonic region.
Multiple coherent scattering by the double-barrier structure strongly modulates
the transmission probability of phonons, rendering it very sensitive to their
frequency. As a result, resonant tunneling occurs with high probability within
a few narrow frequency intervals. This gives rise to highly non-thermal spectra
with sharp peaks. We find that these peaks are mostly associated to decaying
resonances and only occasionally to dynamical instabilities. Even at achievable
nonzero temperatures, the radiation peaks can be dominated by the spontaneous
emission, i.e. enhanced zero-point fluctuations, and not, as often in analog
models, by stimulated emission.Comment: 20 pages, 9 figures, revised versio
Quantum fluctuations around black hole horizons in Bose-Einstein condensates
We study several realistic configurations allowing to realize an acoustic
horizon in the flow of a one dimensional Bose-Einstein condensate. In each case
we give an analytical description of the flow pattern, of the spectrum of
Hawking radiation and of the associated quantum fluctuations. Our calculations
confirm that the non local correlations of the density fluctuations previously
studied in a simplified model provide a clear signature of Hawking radiation
also in realistic configurations. In addition we explain by direct computation
how this non local signal relates to short range modifications of the density
correlations
Comment on "Linear wave dynamics explains observations attributed to dark-solitons in a polariton quantum fluid"
In a recent preprint (arXiv:1401.1128v1) Cilibrizzi and co-workers report
experiments and simulations showing the scattering of polaritons against a
localised obstacle in a semiconductor microcavity. The authors observe in the
linear excitation regime the formation of density and phase patterns
reminiscent of those expected in the non-linear regime from the nucleation of
dark solitons. Based on this observation, they conclude that previous
theoretical and experimental reports on dark solitons in a polariton system
should be revised. Here we comment why the results from Cilibrizzi et al. take
place in a very different regime than previous investigations on dark soliton
nucleation and do not reproduce all the signatures of its rich nonlinear
phenomenology. First of all, Cilibrizzi et al. consider a particular type of
radial excitation that strongly determines the observed patterns, while in
previous reports the excitation has a plane-wave profile. Most importantly, the
nonlinear relation between phase jump, soliton width and fluid velocity, and
the existence of a critical velocity with the time-dependent formation of
vortex-antivortex pairs are absent in the linear regime. In previous reports
about dark soliton and half-dark soliton nucleation in a polariton fluid, the
distinctive dark soliton physics is supported both by theory (analytical and
numerical) and experiments (both continuous wave and pulsed excitation).Comment: 4 pages, 2 figure
Time-resolved density correlations as probe of squeezing in toroidal Bose-Einstein condensates
I study the evolution of mean field and linear quantum fluctuations in a
toroidal Bose-Einstein condensate, whose interaction strength is quenched from
a finite (repulsive) value to zero. The azimuthal equal-time density-density
correlation function is calculated and shows temporal oscillations with twice
the (final) excitation frequencies after the transition. These oscillations are
a direct consequence of positive and negative frequency mixing during
non-adiabatic evolution. I will argue that a time-resolved measurement of the
equal-time density correlator might be used to calculate the moduli of the
Bogoliubov coefficients and thus the amount of squeezing imposed on a mode,
i.e., the number of atoms excited out of the condensate.Comment: 18 pages, IOP styl
Numerical observation of Hawking radiation from acoustic black holes in atomic Bose-Einstein condensates
We report numerical evidence of Hawking emission of Bogoliubov phonons from a
sonic horizon in a flowing one-dimensional atomic Bose-Einstein condensate. The
presence of Hawking radiation is revealed from peculiar long-range patterns in
the density-density correlation function of the gas. Quantitative agreement
between our fully microscopic calculations and the prediction of analog models
is obtained in the hydrodynamic limit. New features are predicted and the
robustness of the Hawking signal against a finite temperature discussed.Comment: Version 2 with enlarged text and several new figure
The Cerenkov effect revisited: from swimming ducks to zero modes in gravitational analogs
We present an interdisciplinary review of the generalized Cerenkov emission
of radiation from uniformly moving sources in the different contexts of
classical electromagnetism, superfluid hydrodynamics, and classical
hydrodynamics. The details of each specific physical systems enter our theory
via the dispersion law of the excitations. A geometrical recipe to obtain the
emission patterns in both real and wavevector space from the geometrical shape
of the dispersion law is discussed and applied to a number of cases of current
experimental interest. Some consequences of these emission processes onto the
stability of condensed-matter analogs of gravitational systems are finally
illustrated.Comment: Lecture Notes at the IX SIGRAV School on "Analogue Gravity" in Como,
Italy from May 16th-21th, 201
Comment on “Linear Wave Dynamics Explains Observations Attributed to Dark Solitons in a Polariton Quantum Fluid”
Relativistic Bose-Einstein Condensates: a New System for Analogue Models of Gravity
In this paper we propose to apply the analogy between gravity and condensed matter physics to relativistic Bose-Einstein condensates (RBECs), i.e. condensates composed by relativistic constituents. While such systems are not yet a subject of experimental realization, they do provide us with a very rich analogue model of gravity, characterized by several novel features with respect to their non-relativistic counterpart. Relativistic condensates exhibit two (rather than one) quasi-particle excitations, a massless and a massive one, the latter disappearing in the non-relativistic limit. We show that the metric associated with the massless mode is a generalization of the usual acoustic geometry allowing also for non-conformally flat spatial sections. This is relevant, as it implies that these systems can allow the simulation of a wider variety of geometries. Finally, while in non-RBECs the transition is from Lorentzian to Galilean relativity, these systems represent an emergent gravity toy model where Lorentz symmetry is present (albeit with different limit speeds) at both low and high energies. Hence they could be used as a test field for better understanding the phenomenological implications of such a milder form of Lorentz violation at intermediate energies