477 research outputs found
Homodyne Bell's inequalities for entangled mesoscopic superpositions
We present a scheme for demonstrating violation of Bell's inequalities using
a spin-1/2 system entangled with a pair of classically distinguishable wave
packets in a harmonic potential. In the optical domain, such wave packets can
be represented by coherent states of a single light mode. The proposed scheme
involves standard spin-1/2 projections and measurements of the position and the
momentum of the harmonic oscillator system, which for a light mode can be
realized by means of homodyne detection. We discuss effects of imperfections,
including non-unit efficiency of the homodyne detector, and point out a close
link between the visibility of interference and violation of Bell's
inequalities in the described scheme.Comment: 6 pages, 3 figures. Extended version, journal reference adde
Versatile engineering of multimode squeezed states by optimizing the pump spectral profile in spontaneous parametric down-conversion
We study the quantum correlations induced by spontaneous parametric
down-conversion (SPDC) of a frequency comb. We derive a theoretical method to
find the output state corresponding to a pump with an arbitrary spectral
profile. After applying it to the relevant example of a spectrally chirped
pump, we run an optimization algorithm to numerically find the pump profiles
maximizing some target functions. These include the number of independently
squeezed modes and the variances of nullifiers defining cluster states used in
many continuous-variable quantum information protocols. To assess the
advantages of pump-shaping in real experiments we take into account the
physical limitations of the pulse shaper.Comment: Updated title, improved presentation and figures, added references,
corrected typos. Closer to the version accepted for publicatio
Fault-Tolerant Measurement-Based Quantum Computing with Continuous-Variable Cluster States
A long-standing open question about Gaussian continuous-variable cluster
states is whether they enable fault-tolerant measurement-based quantum
computation. The answer is yes. Initial squeezing in the cluster above a
threshold value of 20.5 dB ensures that errors from finite squeezing acting on
encoded qubits are below the fault-tolerance threshold of known qubit-based
error-correcting codes. By concatenating with one of these codes and using
ancilla-based error correction, fault-tolerant measurement-based quantum
computation of theoretically indefinite length is possible with finitely
squeezed cluster states.Comment: (v3) consistent with published version, more accessible for general
audience; (v2) condensed presentation, added references on GKP state
generation and a comparison of currently achievable squeezing to the
threshold; (v1) 13 pages, a few figure
Pulsed quantum optomechanics
Studying mechanical resonators via radiation pressure offers a rich avenue
for the exploration of quantum mechanical behavior in a macroscopic regime.
However, quantum state preparation and especially quantum state reconstruction
of mechanical oscillators remains a significant challenge. Here we propose a
scheme to realize quantum state tomography, squeezing and state purification of
a mechanical resonator using short optical pulses. The scheme presented allows
observation of mechanical quantum features despite preparation from a thermal
state and is shown to be experimentally feasible using optical microcavities.
Our framework thus provides a promising means to explore the quantum nature of
massive mechanical oscillators and can be applied to other systems such as
trapped ions.Comment: 9 pages, 4 figure
Consistently Simulating a Wide Range of Atmospheric Scenarios for K2-18b with a Flexible Radiative Transfer Module
The atmospheres of small, potentially rocky exoplanets are expected to cover
a diverse range in composition and mass. Studying such objects therefore
requires flexible and wide-ranging modeling capabilities. We present in this
work the essential development steps that lead to our flexible radiative
transfer module, REDFOX, and validate REDFOX for the Solar system planets
Earth, Venus and Mars, as well as for steam atmospheres. REDFOX is a
k-distribution model using the correlated-k approach with random overlap method
for the calculation of opacities used in the -two-stream approximation
for radiative transfer. Opacity contributions from Rayleigh scattering, UV /
visible cross sections and continua can be added selectively. With the improved
capabilities of our new model, we calculate various atmospheric scenarios for
K2-18b, a super-Earth / sub-Neptune with 8 M orbiting in the
temperate zone around an M-star, with recently observed HO spectral
features in the infrared. We model Earth-like, Venus-like, as well as H-He
primary atmospheres of different Solar metallicity and show resulting climates
and spectral characteristics, compared to observed data. Our results suggest
that K2-18b has an H-He atmosphere with limited amounts of HO and
CH. Results do not support the possibility of K2-18b having a water
reservoir directly exposed to the atmosphere, which would reduce atmospheric
scale heights, hence too the amplitudes of spectral features inconsistent with
the observations. We also performed tests for H-He atmospheres up to 50
times Solar metallicity, all compatible with the observations.Comment: 28 pages, 13 figures, accepted for publication in Ap
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