309 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
Quantum error correction for continuously detected errors
We show that quantum feedback control can be used as a quantum error
correction process for errors induced by weak continuous measurement. In
particular, when the error model is restricted to one, perfectly measured,
error channel per physical qubit, quantum feedback can act to perfectly protect
a stabilizer codespace. Using the stabilizer formalism we derive an explicit
scheme, involving feedback and an additional constant Hamiltonian, to protect
an ()-qubit logical state encoded in physical qubits. This works for
both Poisson (jump) and white-noise (diffusion) measurement processes. In
addition, universal quantum computation is possible in this scheme. As an
example, we show that detected-spontaneous emission error correction with a
driving Hamiltonian can greatly reduce the amount of redundancy required to
protect a state from that which has been previously postulated [e.g., Alber
\emph{et al.}, Phys. Rev. Lett. 86, 4402 (2001)].Comment: 11 pages, 1 figure; minor correction
Information dynamics in cavity QED
A common experimental setup in cavity quantum electrodynamics (QED) consists
of a single two-level atom interacting with a single mode of the
electromagnetic field inside an optical cavity. The cavity is externally driven
and the output is continuously monitored via homodyne measurements. We derive
formulas for the optimal rates at which these measurements provide information
about (i) the quantum state of the system composed of atom and electromagnetic
field, and (ii) the coupling strength between atom and field. We find that the
two information rates are anticorrelated.Comment: 11 pages, 1 figure, final versio
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
Removal of a single photon by adaptive absorption
We present a method to remove, using only linear optics, exactly one photon
from a field-mode. This is achieved by putting the system in contact with an
absorbing environment which is under continuous monitoring. A feedback
mechanism then decouples the system from the environment as soon as the first
photon is absorbed. We propose a possible scheme to implement this process and
provide the theoretical tools to describe it
Adiabatic Elimination in Compound Quantum Systems with Feedback
Feedback in compound quantum systems is effected by using the output from one
sub-system (``the system'') to control the evolution of a second sub-system
(``the ancilla'') which is reversibly coupled to the system. In the limit where
the ancilla responds to fluctuations on a much shorter time scale than does the
system, we show that it can be adiabatically eliminated, yielding a master
equation for the system alone. This is very significant as it decreases the
necessary basis size for numerical simulation and allows the effect of the
ancilla to be understood more easily. We consider two types of ancilla: a
two-level ancilla (e.g. a two-level atom) and an infinite-level ancilla (e.g.
an optical mode). For each, we consider two forms of feedback: coherent (for
which a quantum mechanical description of the feedback loop is required) and
incoherent (for which a classical description is sufficient). We test the
master equations we obtain using numerical simulation of the full dynamics of
the compound system. For the system (a parametric oscillator) and feedback
(intensity-dependent detuning) we choose, good agreement is found in the limit
of heavy damping of the ancilla. We discuss the relation of our work to
previous work on feedback in compound quantum systems, and also to previous
work on adiabatic elimination in general.Comment: 18 pages, 12 figures including two subplots as jpeg attachment
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
Non-Markovian homodyne-mediated feedback on a two-level atom: a quantum trajectory treatment
Quantum feedback can stabilize a two-level atom against decoherence
(spontaneous emission), putting it into an arbitrary (specified) pure state.
This requires perfect homodyne detection of the atomic emission, and
instantaneous feedback. Inefficient detection was considered previously by two
of us. Here we allow for a non-zero delay time in the feedback circuit.
Because a two-level atom is a nonlinear optical system, an analytical solution
is not possible. However, quantum trajectories allow a simple numerical
simulation of the resulting non-Markovian process. We find the effect of the
time delay to be qualitatively similar to that of inefficient detection. The
solution of the non-Markovian quantum trajectory will not remain fixed, so that
the time-averaged state will be mixed, not pure. In the case where one tries to
stabilize the atom in the excited state, an approximate analytical solution to
the quantum trajectory is possible. The result, that the purity () of the average state is given by (where
is the spontaneous emission rate) is found to agree very well with the
numerical results.Comment: Changed content, Added references and Corrected typo
Universal quantum interfaces
To observe or control a quantum system, one must interact with it via an
interface. This letter exhibits simple universal quantum interfaces--quantum
input/output ports consisting of a single two-state system or quantum bit that
interacts with the system to be observed or controlled. It is shown that under
very general conditions the ability to observe and control the quantum bit on
its own implies the ability to observe and control the system itself. The
interface can also be used as a quantum communication channel, and multiple
quantum systems can be connected by interfaces to become an efficient universal
quantum computer. Experimental realizations are proposed, and implications for
controllability, observability, and quantum information processing are
explored.Comment: 4 pages, 3 figures, 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
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