334 research outputs found
Quantum feedback control of a solid-state qubit
We have studied theoretically the basic operation of a quantum feedback loop
designed to maintain a desired phase of quantum coherent oscillations in a
single solid-state qubit. The degree of oscillations synchronization with
external harmonic signal is calculated as a function of feedback strength,
taking into account available bandwidth and coupling to environment.
The feedback can efficiently suppress the dephasing of oscillations if the
qubit coupling to the detector is stronger than coupling to environment.Comment: Extended version of cond-mat/0107280 (5 pages, 5 figures); to be
published in PRB (RC
Sensitivity optimization in quantum parameter estimation
We present a general framework for sensitivity optimization in quantum
parameter estimation schemes based on continuous (indirect) observation of a
dynamical system. As an illustrative example, we analyze the canonical scenario
of monitoring the position of a free mass or harmonic oscillator to detect weak
classical forces. We show that our framework allows the consideration of
sensitivity scheduling as well as estimation strategies for non-stationary
signals, leading us to propose corresponding generalizations of the Standard
Quantum Limit for force detection.Comment: 15 pages, RevTe
Mirror quiescence and high-sensitivity position measurements with feedback
We present a detailed study of how phase-sensitive feedback schemes can be
used to improve the performance of optomechanical devices. Considering the case
of a cavity mode coupled to an oscillating mirror by the radiation pressure, we
show how feedback can be used to reduce the position noise spectrum of the
mirror, cool it to its quantum ground state, or achieve position squeezing.
Then, we show that even though feedback is not able to improve the sensitivity
of stationary position spectral measurements, it is possible to design a
nonstationary strategy able to increase this sensitivity.Comment: 25 pages, 11 figure
Feedback cooling of a nanomechanical resonator
Cooled, low-loss nanomechanical resonators offer the prospect of directly
observing the quantum dynamics of mesoscopic systems. However, the present
state of the art requires cooling down to the milliKelvin regime in order to
observe quantum effects. Here we present an active feedback strategy based on
continuous observation of the resonator position for the purpose of obtaining
these low temperatures. In addition, we apply this to an experimentally
realizable configuration, where the position monitoring is carried out by a
single-electron transistor. Our estimates indicate that with current technology
this technique is likely to bring the required low temperatures within reach.Comment: 10 pages, RevTex4, 4 color eps figure
Evolution of a qubit under the influence of a succession of unsharp measurements
We investigate the evolution of a single qubit subject to a continuous
unitary dynamics and an additional interrupting influence which occurs
periodically. One may imagine a dynamically evolving closed quantum system
which becomes open at certain times. The interrupting influence is represented
by an operation, which is assumed to equivalently describe a non-selective
unsharp measurement. It may be decomposed into a positive operator, which in
case of a measurement represents the pure measurement part, followed by an
unitary back-action operator. Equations of motion for the state evolution are
derived in the form of difference equations. It is shown that the 'free'
Hamiltonian is completed by an averaged Hamiltonian, which goes back to the
back-action. The positive operator specifies a decoherence rate and results in
a decoherence term. The continuum limit to a master equation is performed. The
selective evolution is discussed and correcting higher order terms are worked
out in an Appendix.Comment: 19 pages, no figure
One-and-a-half quantum de Finetti theorems
We prove a new kind of quantum de Finetti theorem for representations of the
unitary group U(d). Consider a pure state that lies in the irreducible
representation U_{mu+nu} for Young diagrams mu and nu. U_{mu+nu} is contained
in the tensor product of U_mu and U_nu; let xi be the state obtained by tracing
out U_nu. We show that xi is close to a convex combination of states Uv, where
U is in U(d) and v is the highest weight vector in U_mu. When U_{mu+nu} is the
symmetric representation, this yields the conventional quantum de Finetti
theorem for symmetric states, and our method of proof gives near-optimal bounds
for the approximation of xi by a convex combination of product states. For the
class of symmetric Werner states, we give a second de Finetti-style theorem
(our 'half' theorem); the de Finetti-approximation in this case takes a
particularly simple form, involving only product states with a fixed spectrum.
Our proof uses purely group theoretic methods, and makes a link with the
shifted Schur functions. It also provides some useful examples, and gives some
insight into the structure of the set of convex combinations of product states.Comment: 14 pages, 3 figures, v4: minor additions (including figures),
published versio
Continuous Quantum Measurement and the Quantum to Classical Transition
While ultimately they are described by quantum mechanics, macroscopic
mechanical systems are nevertheless observed to follow the trajectories
predicted by classical mechanics. Hence, in the regime defining macroscopic
physics, the trajectories of the correct classical motion must emerge from
quantum mechanics, a process referred to as the quantum to classical
transition. Extending previous work [Bhattacharya, Habib, and Jacobs, Phys.
Rev. Lett. {\bf 85}, 4852 (2000)], here we elucidate this transition in some
detail, showing that once the measurement processes which affect all
macroscopic systems are taken into account, quantum mechanics indeed predicts
the emergence of classical motion. We derive inequalities that describe the
parameter regime in which classical motion is obtained, and provide numerical
examples. We also demonstrate two further important properties of the classical
limit. First, that multiple observers all agree on the motion of an object, and
second, that classical statistical inference may be used to correctly track the
classical motion.Comment: 12 pages, 4 figures, Revtex
Feedback-control of quantum systems using continuous state-estimation
We present a formulation of feedback in quantum systems in which the best
estimates of the dynamical variables are obtained continuously from the
measurement record, and fed back to control the system. We apply this method to
the problem of cooling and confining a single quantum degree of freedom, and
compare it to current schemes in which the measurement signal is fed back
directly in the manner usually considered in existing treatments of quantum
feedback. Direct feedback may be combined with feedback by estimation, and the
resulting combination, performed on a linear system, is closely analogous to
classical LQG control theory with residual feedback.Comment: 12 pages, multicol revtex, revised and extende
Selective quantum evolution of a qubit state due to continuous measurement
We consider a two-level quantum system (qubit) which is continuously measured
by a detector. The information provided by the detector is taken into account
to describe the evolution during a particular realization of measurement
process. We discuss the Bayesian formalism for such ``selective'' evolution of
an individual qubit and apply it to several solid-state setups. In particular,
we show how to suppress the qubit decoherence using continuous measurement and
the feedback loop.Comment: 15 pages (including 9 figures
Continuous twin screw rheo-extrusion of an AZ91D magnesium alloy
© The Minerals, Metals & Materials Society and ASM International 2012The twin screw rheo-extrusion (TSRE) is designed to take advantage of the nondendritc microstructure and thixotropic characterization of semisolid-metal slurries and produce simple metal profiles directly from melts. The extrusion equipment consists of a rotor-stator high shear slurry maker, a twin screw extruder, and a die assembly. The process is continuous and has a potential for significantly saving energy, manufacturing cost, and enhancing efficiency. The present investigation was carried out to study the process performance for processing rods of an AZ91D magnesium alloy and the microstructure evolution during processing. The semisolid slurry prepared by the process was characterized by uniformly distributed nondendritic granular primary phase particles. AZ91D rods with uniform and fine microstructures and moderate mechanical properties were produced. For the given slurry making parameters, decreasing extrusion temperature was found to improve microstructures and properties. The mechanisms of particle granulation and refinement and the effect of processing parameters on process performance and thermal management are discussed. © 2012 The Minerals, Metals & Materials Society and ASM International.EPSRC (UK) and Rautomead Lt
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