41 research outputs found

### Propagation of Light in the Field of Stationary and Radiative Gravitational Multipoles

Extremely high precision of near-future radio/optical interferometric
observatories like SKA, Gaia, SIM and the unparalleled sensitivity of LIGO/LISA
gravitational-wave detectors demands more deep theoretical treatment of
relativistic effects in the propagation of electromagnetic signals through
variable gravitational fields of the solar system, oscillating and precessing
neutron stars, coalescing binary systems, exploding supernova, and colliding
galaxies. Especially important for future gravitational-wave observatories is
the problem of propagation of light rays in the field of multipolar
gravitational waves emitted by a localized source of gravitational radiation.
Present paper suggests physically-adequate and consistent mathematical solution
of this problem in the first post-Minkowskian approximation of General
Relativity which accounts for all time-dependent multipole moments of an
isolated astronomical system.Comment: 36 pages, no figure

### Optomechanical cooling of levitated spheres with doubly-resonant fields

Optomechanical cooling of levitated dielectric particles represents a
promising new approach in the quest to cool small mechanical resonators towards
their quantum ground state. We investigate two-mode cooling of levitated
nanospheres in a self-trapping regime. We identify a rich structure of split
sidebands (by a mechanism unrelated to usual strong-coupling effects) and
strong cooling even when one mode is blue detuned. We show the best regimes
occur when both optical fields cooperatively cool and trap the nanosphere,
where cooling rates are over an order of magnitude faster compared to
corresponding single-sideband cooling rates.Comment: 8 Pages, 7 figure

### Cooling of a mirror by radiation pressure

We describe an experiment in which a mirror is cooled by the radiation
pressure of light. A high-finesse optical cavity with a mirror coated on a
mechanical resonator is used as an optomechanical sensor of the Brownian motion
of the mirror. A feedback mechanism controls this motion via the radiation
pressure of a laser beam reflected on the mirror. We have observed either a
cooling or a heating of the mirror, depending on the gain of the feedback loop.Comment: 4 pages, 6 figures, RevTe

### Universal detector efficiency of a mesoscopic capacitor

We investigate theoretically a novel type of high frequency quantum detector
based on the mesoscopic capacitor recently realized by Gabelli et al., [Science
{\bf 313}, 499 (2006)], which consists of a quantum dot connected via a single
channel quantum point contact to a single lead. We show that the state of a
double quantum dot charge qubit capacitively coupled to this detector can be
read out in the GHz frequency regime with near quantum limited efficiency. To
leading order, the quantum efficiency is found to be universal owing to the
universality of the charge relaxation resistance of the mesoscopic capacitor.Comment: 4 pages, 2 figures, submitted to PR

### Quantum limits in interferometric measurements

Quantum noise limits the sensitivity of interferometric measurements. It is
generally admitted that it leads to an ultimate sensitivity, the ``standard
quantum limit''. Using a semi-classical analysis of quantum noise, we show that
a judicious use of squeezed states allows one in principle to push the
sensitivity beyond this limit. This general method could be applied to large
scale interferometers designed for gravitational wave detection.Comment: 4 page

### Practical quantum metrology with large precision gains in the low photon number regime

Quantum metrology exploits quantum correlations to make precise measurements with limited particle numbers. By utilizing inter- and intra- mode correlations in an optical interferometer, we find a state that combines entanglement and squeezing to give a 7-fold enhancement in the quantum Fisher information (QFI) - a metric related to the precision - over the shot noise limit, for low photon numbers. Motivated by practicality we then look at the squeezed cat-state, which has recently been made experimentally, and shows further precision gains over the shot noise limit and a 3-fold improvement in the QFI over the optimal Gaussian state. We present a conceptually simple measurement scheme that saturates the QFI, and we demonstrate a robustness to loss for small photon numbers. The squeezed cat-state can therefore give a significant precision enhancement in optical quantum metrology in practical and realistic conditions

### Nondemolition Principle of Quantum Measurement Theory

We give an explicit axiomatic formulation of the quantum measurement theory
which is free of the projection postulate. It is based on the generalized
nondemolition principle applicable also to the unsharp, continuous-spectrum and
continuous-in-time observations. The "collapsed state-vector" after the
"objectification" is simply treated as a random vector of the a posteriori
state given by the quantum filtering, i.e., the conditioning of the a priori
induced state on the corresponding reduced algebra. The nonlinear
phenomenological equation of "continuous spontaneous localization" has been
derived from the Schroedinger equation as a case of the quantum filtering
equation for the diffusive nondemolition measurement. The quantum theory of
measurement and filtering suggests also another type of the stochastic equation
for the dynamical theory of continuous reduction, corresponding to the counting
nondemolition measurement, which is more relevant for the quantum experiments.Comment: 23 pages. See also related papers at
http://www.maths.nott.ac.uk/personal/vpb/research/mes_fou.html and
http://www.maths.nott.ac.uk/personal/vpb/research/cau_idy.htm

### On Quantum State Observability and Measurement

We consider the problem of determining the state of a quantum system given
one or more readings of the expectation value of an observable. The system is
assumed to be a finite dimensional quantum control system for which we can
influence the dynamics by generating all the unitary evolutions in a Lie group.
We investigate to what extent, by an appropriate sequence of evolutions and
measurements, we can obtain information on the initial state of the system. We
present a system theoretic viewpoint of this problem in that we study the {\it
observability} of the system. In this context, we characterize the equivalence
classes of indistinguishable states and propose algorithms for state
identification

### Testing the Principle of Equivalence by Solar Neutrinos

We discuss the possibility of testing the principle of equivalence with solar
neutrinos. If there exists a violation of the equivalence principle quarks and
leptons with different flavors may not universally couple with gravity. The
method we discuss employs a quantum mechanical phenomenon of neutrino
oscillation to probe into the non-universality of the gravitational couplings
of neutrinos. We develop an appropriate formalism to deal with neutrino
propagation under the weak gravitational fields of the sun in the presence of
the flavor mixing. We point out that solar neutrino observation by the next
generation water Cherenkov detectors can improve the existing bound on
violation of the equivalence principle by 3-4 orders of magnitude if the
nonadiabatic Mikheyev-Smirnov-Wolfenstein mechanism is the solution to the
solar neutrino problem.Comment: Latex, 17 pages + 6 uuencoded postscript figures, KEK-TH-396,
TMUP-HEL-9402 (unnecessary one reference was removed

### Theory of Pseudomodes in Quantum Optical Processes

This paper deals with non-Markovian behaviour in atomic systems coupled to a
structured reservoir of quantum EM field modes, with particular relevance to
atoms interacting with the field in high Q cavities or photonic band gap
materials. In cases such as the former, we show that the pseudo mode theory for
single quantum reservoir excitations can be obtained by applying the Fano
diagonalisation method to a system in which the atomic transitions are coupled
to a discrete set of (cavity) quasimodes, which in turn are coupled to a
continuum set of (external) quasimodes with slowly varying coupling constants
and continuum mode density. Each pseudomode can be identified with a discrete
quasimode, which gives structure to the actual reservoir of true modes via the
expressions for the equivalent atom-true mode coupling constants. The quasimode
theory enables cases of multiple excitation of the reservoir to now be treated
via Markovian master equations for the atom-discrete quasimode system.
Applications of the theory to one, two and many discrete quasimodes are made.
For a simple photonic band gap model, where the reservoir structure is
associated with the true mode density rather than the coupling constants, the
single quantum excitation case appears to be equivalent to a case with two
discrete quasimodes