10,972 research outputs found
Optimal control for one-qubit quantum sensing
Quantum systems can be exquisite sensors thanks to their sensitivity to
external perturbations. This same characteristic also makes them fragile to
external noise. Quantum control can tackle the challenge of protecting quantum
sensors from environmental noise, while leaving their strong coupling to the
target field to be measured. As the compromise between these two conflicting
requirements does not always have an intuitive solution, optimal control based
on numerical search could prove very effective. Here we adapt optimal control
theory to the quantum sensing scenario, by introducing a cost function that,
unlike the usual fidelity of operation, correctly takes into account both the
unknown field to be measured and the environmental noise. We experimentally
implement this novel control paradigm using a Nitrogen Vacancy center in
diamond, finding improved sensitivity to a broad set of time varying fields.
The demonstrated robustness and efficiency of the numerical optimization, as
well as the sensitivity advantaged it bestows, will prove beneficial to many
quantum sensing applications
Static quantum corrections to the Schwarzschild spacetime
We study static quantum corrections of the Schwarzschild metric in the
Boulware vacuum state. Due to the absence of a complete analytic expression for
the full semiclassical Einstein equations we approach the problem by
considering the s-wave approximation and solve numerically the associated
backreaction equations. The solution, including quantum effects due to pure
vacuum polarization, is similar to the classical Schwarzschild solution up to
the vicinity of the classical horizon. However, the radial function has a
minimum at a time-like surface close to the location of the classical event
horizon. There the g_{00} component of the metric reaches a very small but
non-zero value. The analysis unravels how a curvature singularity emerges
beyond this bouncing point. We briefly discuss the physical consequences of
these results by extrapolating them to a dynamical collapsing scenario.Comment: 10 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Probing correlations of gaseous microwires in lattice potentials via inelastic light scattering
In this work, inelastic light-scattering (Bragg spectroscopy) is used to study strongly correlated phases of ultracold 1D gases in optical lattices. We investigate
the crossover from correlated superfluids to Mott insulators. Light-scattering creates in the system elementary excitations with non-zero momentum, and the response of the correlated gases is in the linear regime. This allows for extracting information about the atomic many-body state in terms of its particle-hole excitations,
as common in solid-state physics. In particular, we characterize the Mott state both via intra-band and inter-band spectroscopy, the former giving access to the dynamical structure factor S(q, ω) and the latter to the one-particle spectral function A(q, ω)
Multi-band spectroscopy of inhomogeneous Mott-insulator states of ultracold bosons
In this work, we use inelastic scattering of light to study the response of
inhomogeneous Mott-insulator gases to external excitations. The experimental
setup and procedure to probe the atomic Mott states are presented in detail. We
discuss the link between the energy absorbed by the gases and accessible
experimental parameters as well as the linearity of the response to the
scattering of light. We investigate the excitations of the system in multiple
energy bands and a band-mapping technique allows us to identify band and
momentum of the excited atoms. In addition the momentum distribution in the
Mott states which is spread over the entire first Brillouin zone enables us to
reconstruct the dispersion relation in the high energy bands using a single
Bragg excitation with a fixed momentum transfer.Comment: 19 pages, 7 figure
Quantum effects in Acoustic Black Holes: the Backreaction
We investigate the backreaction equations for an acoustic black hole formed
in a Laval nozzle under the assumption that the motion of the fluid is
one-dimensional. The solution in the near-horizon region shows that as phonons
are (thermally) radiated the sonic horizon shrinks and the temperature
decreases. This contrasts with the behaviour of Schwarzschild black holes, and
is similar to what happens in the evaporation of (near-extremal)
Reissner-Nordstrom black holes (i.e. infinite evaporation time). Finally, by
appropriate boundary conditions the solution is extended in both the asymptotic
regions of the nozzle.Comment: 23 pages, latex, 1 figure; revised version, to appear in Phys. Rev.
Hawking radiation from extremal and non-extremal black holes
The relationship between Hawking radiation emitted by non extremal and
extremal Reissner Nordstrom black holes is critically analyzed. A careful study
of a series of regular collapsing geometries reveals that the stress energy
tensor stays regular in the extremal limit and is smoothly connected to that of
non extremal black holes. The unexpected feature is that the late time
transients which played little role in the non extremal case are necessary to
preserve the well defined character of the flux in the extremal case. The known
singular behavior of the static energy density of extremal black holes is
recovered from our series by neglecting these transients, when performing what
turns out to be an illegitimate late time limit. Although our results are
derived in two dimensional settings, we explain why they should also apply to
higher dimensional black holes.Comment: 18 pages, late
Entanglement generation in relativistic quantum fields
We present a general, analytic recipe to compute the entanglement that is
generated between arbitrary, discrete modes of bosonic quantum fields by
Bogoliubov transformations. Our setup allows the complete characterization of
the quantum correlations in all Gaussian field states. Additionally, it holds
for all Bogoliubov transformations. These are commonly applied in quantum
optics for the description of squeezing operations, relate the mode
decompositions of observers in different regions of curved spacetimes, and
describe observers moving along non-stationary trajectories. We focus on a
quantum optical example in a cavity quantum electrodynamics setting: an
uncharged scalar field within a cavity provides a model for an optical
resonator, in which entanglement is created by non-uniform acceleration. We
show that the amount of generated entanglement can be magnified by initial
single-mode squeezing, for which we provide an explicit formula. Applications
to quantum fields in curved spacetimes, such as an expanding universe, are
discussed.Comment: 8 pages, 2 figures, Ivette Fuentes previously published as Ivette
Fuentes-Guridi and Ivette Fuentes-Schuller; v2: published version (online),
to appear in the J. Mod. Opt. Special Issue on the Physics of Quantum
Electronic
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