29 research outputs found
Quantum feedback control of a solid-state two-level system
We have studied theoretically the basic operation of a quantum feedback loop
designed to maintain the desired phase of quantum coherent oscillations in a
two-level system. Such feedback can suppress the dephasing of oscillations due
to interaction with environment. Prospective experiments can be realized using
metallic single-electron devices or GaAs technology.Comment: 4 pages, 4 figure
Quantum Nondemolition Squeezing of a Nanomechanical Resonator
We show that the nanoresonator position can be squeezed significantly below
the ground state level by measuring the nanoresonator with a quantum point
contact or a single-electron transistor and applying a periodic voltage across
the detector. The mechanism of squeezing is basically a generalization of
quantum nondemolition measurement of an oscillator to the case of continuous
measurement by a weakly coupled detector. The quantum feedback is necessary to
prevent the ``heating'' due to measurement back-action. We also discuss a
procedure of experimental verification of the squeezed state.Comment: 9 pages, 3 figure
On-chip cavity quantum phonodynamics with an acceptor qubit in silicon
We describe a chip-based, solid-state analogue of cavity-QED utilizing
acoustic phonons instead of photons. We show how long-lived and tunable
acceptor impurity states in silicon nanomechanical cavities can play the role
of a matter non-linearity for coherent phonons just as, e.g., the Josephson
qubit plays in circuit-QED. Both strong coupling (number of Rabi oscillations ~
100) and strong dispersive coupling (0.1-2 MHz) regimes can be reached in
cavities in the 1-20 GHz range, enabling the control of single phonons,
phonon-phonon interactions, dispersive phonon readout of the acceptor qubit,
and compatibility with other optomechanical components such as phonon-photon
translators. We predict explicit experimental signatures of the acceptor-cavity
system.Comment: 6 pages, 2 figures, PDFLaTeX. New version improves clarit
Qubit purification speed-up for three complementary continuous measurements
We consider qubit purification under simultaneous continuous measurement of
the three non-commuting qubit operators \sigma_x, \sigma_y, \sigma_z. The
purification dynamics is quantified by (i) the average purification rate, and
(ii) the mean time of reaching given level of purity, (1-\epsilon). Under ideal
measurements (detector efficiency \eta=1), we show in the first case an
asymptotic mean purification speed-up of 4 as compared to a standard
(classical) single-detector measurement. However by the second measure --- the
mean time of first passage T(\epsilon) of the purity --- the corresponding
speed-up is only 2. We explain these speed-ups using the isotropy of the qubit
evolution that provides an equivalence between the original measurement
directions and three simultaneous measurements, one with an axis aligned along
the Bloch vector and the other with axes in the two complementary directions.
For inefficient detectors, \eta=1-\delta <1 the mean time of first passage
T(\delta,\epsilon) increases since qubit purification competes with an
isotropic qubit dephasing. In the asymptotic high-purity limit (\epsilon,
\delta << 1) we show that the increase possesses a scaling behavior: \Delta
T(\delta,\epsilon) is a function only of the ratio {\delta}/{\epsilon}. The
increase \Delta T({\delta}/{\epsilon}) is linear for small argument but becomes
exponential ~ exp({\delta}/2{\epsilon}) for {\delta}/{\epsilon} large.Comment: 19 pages, 4 eps figures, Accepted for publication in Philosophical
Transactions of the Royal Society A: Mathematical, Physical and Engineering
Science