8,066 research outputs found
Applying matrix product operators to model systems with long-range interactions
An algorithm is presented which computes a translationally invariant matrix
product state approximation of the ground state of an infinite 1D system; it
does this by embedding sites into an approximation of the infinite
``environment'' of the chain, allowing the sites to relax, and then merging
them with the environment in order to refine the approximation. By making use
of matrix product operators, our approach is able to directly model any
long-range interaction that can be systematically approximated by a series of
decaying exponentials. We apply our techniques to compute the ground state of
the Haldane-Shastry model and present results.Comment: 7 pages, 3 figures; manuscript has been expanded and restructured in
order to improve presentation of the algorith
Magnetic Trapping of Cold Bromine Atoms
Magnetic trapping of bromine atoms at temperatures in the milliKelvin regime
is demonstrated for the first time. The atoms are produced by photodissociation
of Br molecules in a molecular beam. The lab-frame velocity of Br atoms is
controlled by the wavelength and polarization of the photodissociation laser.
Careful selection of the wavelength results in one of the pair of atoms having
sufficient velocity to exactly cancel that of the parent molecule, and it
remains stationary in the lab frame. A trap is formed at the null point between
two opposing neodymium permanent magnets. Dissociation of molecules at the
field minimum results in the slowest fraction of photofragments remaining
trapped. After the ballistic escape of the fastest atoms, the trapped slow
atoms are only lost by elastic collisions with the chamber background gas. The
measured loss rate is consistent with estimates of the total cross section for
only those collisions transferring sufficient kinetic energy to overcome the
trapping potential
Mechanical Entanglement via Detuned Parametric Amplification
We propose two schemes to generate entanglement between a pair of mechanical
oscillators using parametric amplification. In contrast to existing parametric
drive-based protocols, both schemes operate in the steady-state. Using a
detuned parametric drive to maintain equilibrium and to couple orthogonal
quadratures, our approach can be viewed as a two-mode extension of previous
proposals for parametric squeezing. We find that robust steady-state
entanglement is possible for matched oscillators with well-controlled coupling.
In addition, one of the proposed schemes is robust to differences in the
damping rates of the two oscillators.Comment: 13 pages, 2 figure
Detuned Mechanical Parametric Amplification as a Quantum Non-Demolition Measurement
Recently it has been demonstrated that the combination of weak-continuous
position detection with detuned parametric driving can lead to significant
steady-state mechanical squeezing, far beyond the 3 dB limit normally
associated with parametric driving. In this work, we show the close connection
between this detuned scheme and quantum non-demolition (QND) measurement of a
single mechanical quadrature. In particular, we show that applying an
experimentally realistic detuned parametric drive to a cavity optomechanical
system allows one to effectively realize a QND measurement despite being in the
bad-cavity limit. In the limit of strong squeezing, we show that this scheme
offers significant advantages over standard backaction evasion, not only by
allowing operation in the weak measurement and low efficiency regimes, but also
in terms of the purity of the mechanical state.Comment: 17 pages, 2 figure
Semiclassical theory of cavity-assisted atom cooling
We present a systematic semiclassical model for the simulation of the
dynamics of a single two-level atom strongly coupled to a driven high-finesse
optical cavity. From the Fokker-Planck equation of the combined atom-field
Wigner function we derive stochastic differential equations for the atomic
motion and the cavity field. The corresponding noise sources exhibit strong
correlations between the atomic momentum fluctuations and the noise in the
phase quadrature of the cavity field. The model provides an effective tool to
investigate localisation effects as well as cooling and trapping times. In
addition, we can continuously study the transition from a few photon quantum
field to the classical limit of a large coherent field amplitude.Comment: 10 pages, 8 figure
Explosion of white dwarfs harboring hybrid CONe cores
Recently, it has been found that off-centre carbon burning in a subset of
intermediate-mass stars does not propagate all the way to the center, resulting
in a class of hybrid CONe cores. Here, we consider the possibility that stars
hosting these hybrid CONe cores might belong to a close binary system and,
eventually, become white dwarfs accreting from a non-degenerate companion at
rates leading to a supernova explosion. We have computed the hydrodynamical
phase of the explosion of Chandrasekhar-mass white dwarfs harboring hybrid
cores, assuming that the explosion starts at the center, either as a detonation
(as may be expected in some degenerate merging scenarios) or as a deflagration
(that afterwards transitions into a delayed detonation). We assume these hybrid
cores are made of a central CO volume, of mass M(CO), surrounded by an ONe
shell. We show that, in case of a pure detonation, a medium-sized CO-rich
region, M(CO)<0.4 Msun, results in the ejection of a small fraction of the
mantle while leaving a massive bound remnant. Part of this remnant is made of
the products of the detonation, Fe-group nuclei, but they are buried in its
inner regions, unless convection is activated during the ensuing cooling and
shrinking phase of the remnant. In contrast, and somehow paradoxically, delayed
detonations do not leave remnants but for the minimum M(CO) we have explored,
M(CO)=0.2 Msun, and even in this case the remnant is as small as 0.13 Msun. The
ejecta produced by these delayed detonations are characterized by slightly
smaller masses of 56Ni and substantially smaller kinetic energies than obtained
for a delayed detonation of a 'normal' CO white dwarf. The optical emission
expected from these explosions would hardly match the observational properties
of typical Type Ia supernovae, although they make interesting candidates for
the subluminous class of SN2002cx-like or SNIax.Comment: Accepted for Astronomy and Astrophysics, 11 pages, 4 figure
On the suppression of the diffusion and the quantum nature of a cavity mode. Optical bistability; forces and friction in driven cavities
A new analytical method is presented here, offering a physical view of driven
cavities where the external field cannot be neglected. We introduce a new
dimensionless complex parameter, intrinsically linked to the cooperativity
parameter of optical bistability, and analogous to the scaled Rabbi frequency
for driven systems where the field is classical. Classes of steady states are
iteratively constructed and expressions for the diffusion and friction
coefficients at lowest order also derived. They have in most cases the same
mathematical form as their free-space analog. The method offers a semiclassical
explanation for two recent experiments of one atom trapping in a high Q cavity
where the excited state is significantly saturated. Our results refute both
claims of atom trapping by a quantized cavity mode, single or not. Finally, it
is argued that the parameter newly constructed, as well as the groundwork of
this method, are at least companions of the cooperativity parameter and its
mother theory. In particular, we lay the stress on the apparently more
fundamental role of our structure parameter.Comment: 24 pages, 7 figures. Submitted to J. Phys. B: At. Mol. Opt. Phy
Adaptive homodyne measurement of optical phase
We present an experimental demonstration of the power of real-time feedback
in quantum metrology, confirming a theoretical prediction by Wiseman regarding
the superior performance of an adaptive homodyne technique for single-shot
measurement of optical phase. For phase measurements performed on weak coherent
states with no prior knowledge of the signal phase, we show that the variance
of adaptive homodyne estimation approaches closer to the fundamental quantum
uncertainty limit than any previously demonstrated technique. Our results
underscore the importance of real-time feedback for reaching quantum
performance limits in coherent telecommunication, precision measurement and
information processing.Comment: RevTex4, color PDF figures (separate files), submitted to PR
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