243 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
Entanglement of solid-state qubits by measurement
We show that two identical solid-state qubits can be made fully entangled
(starting from completely mixed state) with probability 1/4 just measuring them
by a detector, equally coupled to the qubits. This happens in the case of
repeated strong (projective) measurements as well as in a more realistic case
of weak continuous measurement. In the latter case the entangled state can be
identified by a flat spectrum of the detector shot noise, while the
non-entangled state (probability 3/4) leads to a spectral peak at the Rabi
frequency with the maximum peak-to-pedestal ratio of 32/3.Comment: 5 pages, 2 figure
Quantum Efficiency of Charge Qubit Measurements Using a Single Electron Transistor
The quantum efficiency, which characterizes the quality of information gain
against information loss, is an important figure of merit for any realistic
quantum detectors in the gradual process of collapsing the state being
measured. In this work we consider the problem of solid-state charge qubit
measurements with a single-electron-transistor (SET). We analyze two models:
one corresponds to a strong response SET, and the other is a tunable one in
response strength. We find that the response strength would essentially bound
the quantum efficiency, making the detector non-quantum-limited. Quantum
limited measurements, however, can be achieved in the limits of strong response
and asymmetric tunneling. The present study is also associated with appropriate
justifications for the measurement and backaction-dephasing rates, which were
usually evaluated in controversial methods.Comment: 10 pages, 2 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
Numerical analysis of the radio-frequency single-electron transistor operation
We have analyzed numerically the response and noise-limited charge
sensitivity of a radio-frequency single-electron transistor (RF-SET) in a
non-superconducting state using the orthodox theory. In particular, we have
studied the performance dependence on the quality factor Q of the tank circuit
for Q both below and above the value corresponding to the impedance matching
between the coaxial cable and SET.Comment: 14 page
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
Storage Qubits and Their Potential Implementation Through a Semiconductor Double Quantum Dot
In the context of a semiconductor based implementation of a quantum computer
the idea of a quantum storage bit is presented and a possible implementation
using a double quantum dot structure is considered. A measurement scheme using
a stimulated Raman adiabatic passage is discussed.Comment: Revised version accepted for publication in Phys.Rev. B. 19 pages, 4
eps figure
Output spectrum of a detector measuring quantum oscillations
We consider a two-level quantum system (qubit) which is continuously measured
by a detector and calculate the spectral density of the detector output. In the
weakly coupled case the spectrum exhibits a moderate peak at the frequency of
quantum oscillations and a Lorentzian-shape increase of the detector noise at
low frequency. With increasing coupling the spectrum transforms into a single
Lorentzian corresponding to random jumps between two states. We prove that the
Bayesian formalism for the selective evolution of the density matrix gives the
same spectrum as the conventional master equation approach, despite the
significant difference in interpretation. The effects of the detector
nonideality and the finite-temperature environment are also discussed.Comment: 8 pages, 6 figure
Quantum Dynamics in Non-equilibrium Strongly Correlated Environments
We consider a quantum point contact between two Luttinger liquids coupled to
a mechanical system (oscillator). For non-vanishing bias, we find an effective
oscillator temperature that depends on the Luttinger parameter. A generalized
fluctuation-dissipation relation connects the decoherence and dissipation of
the oscillator to the current-voltage characteristics of the device. Via a
spectral representation, this result is generalized to arbitrary leads in a
weak tunneling regime.Comment: 4 pages, 1 figur
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
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