3,793 research outputs found
Transmission properties in waveguides: An optical streamline analysis
A novel approach to study transmission through waveguides in terms of optical
streamlines is presented. This theoretical framework combines the computational
performance of beam propagation methods with the possibility to monitor the
passage of light through the guiding medium by means of these sampler paths. In
this way, not only the optical flow along the waveguide can be followed in
detail, but also a fair estimate of the transmitted light (intensity) can be
accounted for by counting streamline arrivals with starting points
statistically distributed according to the input pulse. Furthermore, this
approach allows to elucidate the mechanism leading to energy losses, namely a
vortical dynamics, which can be advantageously exploited in optimal waveguide
design.Comment: 8 pages, 4 figure
Cavity-assisted squeezing of a mechanical oscillator
We investigate the creation of squeezed states of a vibrating membrane or a
movable mirror in an opto-mechanical system. An optical cavity is driven by
squeezed light and couples via radiation pressure to the membrane/mirror,
effectively providing a squeezed heat-bath for the mechanical oscillator. Under
the conditions of laser cooling to the ground state, we find an efficient
transfer of squeezing with roughly 60% of light squeezing conveyed to the
membrane/mirror (on a dB scale). We determine the requirements on the carrier
frequency and the bandwidth of squeezed light. Beyond the conditions of ground
state cooling, we predict mechanical squashing to be observable in current
systems.Comment: 7.1 pages, 3 figures, submitted to PR
Robustness of optimal working points for non-adiabatic holonomic quantum computation
Geometric phases are an interesting resource for quantum computation, also in
view of their robustness against decoherence effects. We study here the effects
of the environment on a class of one-qubit holonomic gates that have been
recently shown to be characterized by "optimal" working times. We numerically
analyze the behavior of these optimal points and focus on their robustness
against noise.Comment: 14 pages, 8 figure
Projective Hilbert space structures at exceptional points
A non-Hermitian complex symmetric 2x2 matrix toy model is used to study
projective Hilbert space structures in the vicinity of exceptional points
(EPs). The bi-orthogonal eigenvectors of a diagonalizable matrix are
Puiseux-expanded in terms of the root vectors at the EP. It is shown that the
apparent contradiction between the two incompatible normalization conditions
with finite and singular behavior in the EP-limit can be resolved by
projectively extending the original Hilbert space. The complementary
normalization conditions correspond then to two different affine charts of this
enlarged projective Hilbert space. Geometric phase and phase jump behavior are
analyzed and the usefulness of the phase rigidity as measure for the distance
to EP configurations is demonstrated. Finally, EP-related aspects of
PT-symmetrically extended Quantum Mechanics are discussed and a conjecture
concerning the quantum brachistochrone problem is formulated.Comment: 20 pages; discussion extended, refs added; bug correcte
Optimal Quantum Measurements of Expectation Values of Observables
Experimental characterizations of a quantum system involve the measurement of
expectation values of observables for a preparable state |psi> of the quantum
system. Such expectation values can be measured by repeatedly preparing |psi>
and coupling the system to an apparatus. For this method, the precision of the
measured value scales as 1/sqrt(N) for N repetitions of the experiment. For the
problem of estimating the parameter phi in an evolution exp(-i phi H), it is
possible to achieve precision 1/N (the quantum metrology limit) provided that
sufficient information about H and its spectrum is available. We consider the
more general problem of estimating expectations of operators A with minimal
prior knowledge of A. We give explicit algorithms that approach precision 1/N
given a bound on the eigenvalues of A or on their tail distribution. These
algorithms are particularly useful for simulating quantum systems on quantum
computers because they enable efficient measurement of observables and
correlation functions. Our algorithms are based on a method for efficiently
measuring the complex overlap of |psi> and U|psi>, where U is an implementable
unitary operator. We explicitly consider the issue of confidence levels in
measuring observables and overlaps and show that, as expected, confidence
levels can be improved exponentially with linear overhead. We further show that
the algorithms given here can typically be parallelized with minimal increase
in resource usage.Comment: 22 page
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