15,491 research outputs found
Adaptive Phase Measurements in Linear Optical Quantum Computation
Photon counting induces an effective nonlinear optical phase shift on certain
states derived by linear optics from single photons. Although this no
nlinearity is nondeterministic, it is sufficient in principle to allow scalable
linear optics quantum computation (LOQC). The most obvious way to encode a
qubit optically is as a superposition of the vacuum and a single photon in one
mode -- so-called "single-rail" logic. Until now this approach was thought to
be prohibitively expensive (in resources) compared to "dual-rail" logic where a
qubit is stored by a photon across two modes. Here we attack this problem with
real-time feedback control, which can realize a quantum-limited phase
measurement on a single mode, as has been recently demonstrated experimentally.
We show that with this added measurement resource, the resource requirements
for single-rail LOQC are not substantially different from those of dual-rail
LOQC. In particular, with adaptive phase measurements an arbitrary qubit state
can be prepared deterministically
Conditional Production of Superpositions of Coherent States with Inefficient Photon Detection
It is shown that a linear superposition of two macroscopically
distinguishable optical coherent states can be generated using a single photon
source and simple all-optical operations. Weak squeezing on a single photon,
beam mixing with an auxiliary coherent state, and photon detecting with
imperfect threshold detectors are enough to generate a coherent state
superposition in a free propagating optical field with a large coherent
amplitude () and high fidelity (). In contrast to all
previous schemes to generate such a state, our scheme does not need photon
number resolving measurements nor Kerr-type nonlinear interactions.
Furthermore, it is robust to detection inefficiency and exhibits some
resilience to photon production inefficiency.Comment: Some important new results added, to appear in Phys.Rev.A (Rapid
Communication
Comparison of LOQC C-sign gates with ancilla inefficiency and an improvement to functionality under these conditions
We compare three proposals for non-deterministic C-sign gates implemented
using linear optics and conditional measurements with non-ideal ancilla mode
production and detection. The simplified KLM gate [Ralph et al, Phys.Rev.A {\bf
65}, 012314 (2001)] appears to be the most resilient under these conditions. We
also find that the operation of this gate can be improved by adjusting the
beamsplitter ratios to compensate to some extent for the effects of the
imperfect ancilla.Comment: to appear in PR
Damping rates and frequency corrections of Kepler LEGACY stars
Linear damping rates and modal frequency corrections of radial oscillation
modes in selected LEGACY main-sequence stars are estimated by means of a
nonadiabatic stability analysis. The selected stellar sample covers stars
observed by Kepler with a large range of surface temperatures and surface
gravities. A nonlocal, time-dependent convection model is perturbed to assess
stability against pulsation modes. The mixing-length parameter is calibrated to
the surface-convection-zone depth of a stellar model obtained from fitting
adiabatic frequencies to the LEGACY observations, and two of the nonlocal
convection parameters are calibrated to the corresponding LEGACY linewidth
measurements. The remaining nonlocal convection parameters in the 1D
calculations are calibrated so as to reproduce profiles of turbulent pressure
and of the anisotropy of the turbulent velocity field of corresponding 3D
hydrodynamical simulations. The atmospheric structure in the 1D stability
analysis adopts a temperature-optical-depth relation derived from 3D
hydrodynamical simulations. Despite the small number of parameters to adjust,
we find good agreement with detailed shapes of both turbulent pressure profiles
and anisotropy profiles with depth, and with damping rates as a function of
frequency. Furthermore, we find the absolute modal frequency corrections,
relative to a standard adiabatic pulsation calculation, to increase with
surface temperature and surface gravity.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Society (MNRAS); 15 pages, 8 figure
A long-range and long-life telemetry data-acquisition system for heart rate and multiple body temperatures from free-ranging animals
The system includes an implantable transmitter, external receiver-retransmitter collar, and a microprocessor-controlled demodulator. The size of the implant is suitable for animals with body weights of a few kilograms or more; further size reduction of the implant is possible. The ECG is sensed by electrodes designed for internal telemetry and to reduce movement artifacts. The R-wave characteristics are then specifically selected to trigger a short radio frequency pulse. Temperatures are sensed at desired locations by thermistors and then, based on a heartbeat counter, transmitted intermittently via pulse interval modulation. This modulation scheme includes first and last calibration intervals for a reference by ratios with the temperature intervals to achieve good accuracy even over long periods. Pulse duration and pulse sequencing are used to discriminate between heart rate and temperature pulses as well as RF interference
Photon number discrimination without a photon counter and its application to reconstructing non-Gaussian states
The non-linearity of a conditional photon-counting measurement can be used to
`de-Gaussify' a Gaussian state of light. Here we present and experimentally
demonstrate a technique for photon number resolution using only homodyne
detection. We then apply this technique to inform a conditional measurement;
unambiguously reconstructing the statistics of the non-Gaussian one and two
photon subtracted squeezed vacuum states. Although our photon number
measurement relies on ensemble averages and cannot be used to prepare
non-Gaussian states of light, its high efficiency, photon number resolving
capabilities, and compatibility with the telecommunications band make it
suitable for quantum information tasks relying on the outcomes of mean values.Comment: 4 pages, 3 figures. Theory section expanded in response to referee
comment
Fault-tolerant linear optical quantum computing with small-amplitude coherent states
Quantum computing using two optical coherent states as qubit basis states has
been suggested as an interesting alternative to single photon optical quantum
computing with lower physical resource overheads. These proposals have been
questioned as a practical way of performing quantum computing in the short term
due to the requirement of generating fragile diagonal states with large
coherent amplitudes. Here we show that by using a fault-tolerant error
correction scheme, one need only use relatively small coherent state amplitudes
() to achieve universal quantum computing. We study the effects
of small coherent state amplitude and photon loss on fault tolerance within the
error correction scheme using a Monte Carlo simulation and show the quantity of
resources used for the first level of encoding is orders of magnitude lower
than the best known single photon scheme. %We study this reigem using a Monte
Carlo simulation and incorporate %the effects of photon loss in this
simulation
Topological Landau-Ginzburg Theory for Vortices in Superfluid He
We propose a new Landau-Ginzburg theory for arbitrarily shaped vortex strings
in superfluid He. The theory contains a topological term and directly
describes vortex dynamics. We introduce gauge fields in order to remove
singularities from the Landau-Ginzburg order parameter of the superfluid, so
that two kinds of gauge symmetries appear, making the continuity equation and
conservation of the total vorticity manifest. The topological term gives rise
to the Berry phase term in the vortex mechanical actions.Comment: LATEX, 9 page
Production of superpositions of coherent states in traveling optical fields with inefficient photon detection
We develop an all-optical scheme to generate superpositions of
macroscopically distinguishable coherent states in traveling optical fields. It
non-deterministically distills coherent state superpositions (CSSs) with large
amplitudes out of CSSs with small amplitudes using inefficient photon
detection. The small CSSs required to produce CSSs with larger amplitudes are
extremely well approximated by squeezed single photons. We discuss some
remarkable features of this scheme: it effectively purifies mixed initial
states emitted from inefficient single photon sources and boosts negativity of
Wigner functions of quantum states.Comment: 13 pages, 9 figures, to be published in Phys. Rev.
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