1,522 research outputs found
Persistent Rabi oscillations probed via low-frequency noise correlation
The qubit Rabi oscillations are known to be non-decaying (though with a
fluctuating phase) if the qubit is continuously monitored in the weak-coupling
regime. In this paper we propose an experiment to demonstrate these persistent
Rabi oscillations via low-frequency noise correlation. The idea is to measure a
qubit by two detectors, biased stroboscopically at the Rabi frequency. The
low-frequency noise depends on the relative phase between the two combs of
biasing pulses, with a strong increase of telegraph noise in both detectors for
the in-phase or anti-phase combs. This happens because of self-synchronization
between the persistent Rabi oscillations and measurement pulses. Almost perfect
correlation of the noise in the two detectors for the in-phase regime and
almost perfect anticorrelation for the anti-phase regime indicates a presence
of synchronized persistent Rabi oscillations. The experiment can be realized
with semiconductor or superconductor qubits.Comment: 5 page
Nonideal quantum detectors in Bayesian formalism
The Bayesian formalism for a continuous measurement of solid-state qubits is
derived for a model which takes into account several factors of the detector
nonideality. In particular, we consider additional classical output and
backaction noises (with finite correlation), together with quantum-limited
output and backaction noises, and take into account possible asymmetry of the
detector coupling. The formalism is first derived for a single qubit and then
generalized to the measurement of entangled qubits.Comment: 10 page
Continuous quantum feedback of coherent oscillations in a solid-state qubit
We have analyzed theoretically the operation of the Bayesian quantum feedback
of a solid-state qubit, designed to maintain perfect coherent oscillations in
the qubit for arbitrarily long time. In particular, we have studied the
feedback efficiency in presence of dephasing environment and detector
nonideality. Also, we have analyzed the effect of qubit parameter deviations
and studied the quantum feedback control of an energy-asymmetric qubit.Comment: 11 page
Quantum Zeno stabilization in weak continuous measurement of two qubits
We have studied quantum coherent oscillations of two qubits under continuous
measurement by a symmetrically coupled mesoscopic detector. The analysis is
based on a Bayesian formalism that is applicable to individual quantum systems.
Measurement continuously collapses the two-qubit system to one of the
sub-spaces of the Bell basis. For a detector with linear response this
corresponds to measurement of the total spin of the qubits. In the other
extreme of purely quadratic response the operator \sigma_y^1 \sigma_y^2 +
\sigma_z^1 \sigma_z^2 is measured. In both cases, collapse naturally leads to
spontaneous entanglement which can be identified by measurement of the power
spectrum and/or the average current of the detector. Asymmetry between the two
qubits results in evolution between the different measurement subspaces.
However, when the qubits are even weakly coupled to the detector, a kind of
quantum Zeno effect cancels the gradual evolution and replaces it with rare,
abrupt switching events. We obtain the asymptotic switching rates for these
events and confirm them with numerical simulations. We show how such switching
affects the observable power spectrum on different time scales.Comment: 18 pages, 8 eps figures, reference adde
Quantum efficiency of binary-outcome detectors of solid-state qubits
We discuss definitions of the quantum efficiency for binary-outcome qubit
detectors with imperfect fidelity, focusing on the subclass of quantum
non-demolition detectors. Quantum efficiency is analyzed for several models of
detectors, including indirect projective measurement, linear detector in
binary-outcome regime, detector of the superconducting phase qubit, and
detector based on tunneling into continuum.Comment: 11 page
Output spectrum of a measuring device at arbitrary voltage and temperature
We calculate the noise spectrum of the electrical current in a quantum point
contact which is used for continuous measurements of a two-level system
(qubit). We generalize the previous results obtained for the regime of high
transport voltages (when is much larger than the qubit's energy level
splitting (we put )) to the case of arbitrary voltages and
temperatures. When the background output spectrum is essentially
asymmetric in frequency, i.e., it is no longer classical. Yet, the spectrum of
the amplified signal, i.e., the two coherent peaks at is still
symmetric. In the emission (negative frequency) part of the spectrum the
coherent peak can be 8 times higher than the background pedestal.
Alternatively, this ratio can be seen in the directly measureable {\it excess}
noise. For and T=0 the coherent peaks do not appear at all. We relate
these results to the properties of linear amplifiers.Comment: 7 pages, 5 figures, the results generalized for arbitrary angle
between the magnetic field and the observed component of the spin, minor
corrections and typo
Positive cross-correlations due to Dynamical Channel-Blockade in a three-terminal quantum dot
We investigate current fluctuations in a three-terminal quantum dot in the
sequential tunneling regime. In the voltage-bias configuration chosen here, the
circuit is operated like a beam splitter, i.e. one lead is used as an input and
the other two as outputs. In the limit where a double occupancy of the dot is
not possible, a super-Poissonian Fano factor of the current in the input lead
and positive cross-correlations between the current fluctuations in the two
output leads can be obtained, due to dynamical channel-blockade. When a single
orbital of the dot transports current, this effect can be obtained by lifting
the spin-degeneracy of the circuit with ferromagnetic leads or with a magnetic
field. When several orbitals participate in the electronic conduction, lifting
spin-degeneracy is not necessary. In all cases, we show that a super-Poissonian
Fano factor for the input current is not equivalent to positive
cross-correlations between the outputs. We identify the conditions for
obtaining these two effects and discuss possible experimental realizations.Comment: 18 pages, 20 Figures, submitted to Phys. rev.
Dynamical correlations in electronic transport through a system of coupled quantum dots
Current auto- and cross-correlations are studied in a system of two
capacitively coupled quantum dots. We are interested in a role of Coulomb
interaction in dynamical correlations, which occur outside the Coulomb blockade
region (for high bias). After decomposition of the current correlation
functions into contributions between individual tunneling events, we can show
which of them are relevant and lead to sub-/supper-Poissonian shot noise and
negative/positive cross-correlations. The results are differentiated for a weak
and strong inter-dot coupling. Interesting results are for the strong coupling
case when electron transfer in one of the channel is strongly correlated with
charge drag in the second channel. We show that cross-correlations are
non-monotonic functions of bias voltage and they are in general negative
(except some cases with asymmetric tunnel resistances). This is effect of local
potential fluctuations correlated by Coulomb interaction, which mimics the
Pauli exclusion principle
A Numerical Study of Transport and Shot Noise at 2D Hopping
We have used modern supercomputer facilities to carry out extensive Monte
Carlo simulations of 2D hopping (at negligible Coulomb interaction) in
conductors with the completely random distribution of localized sites in both
space and energy, within a broad range of the applied electric field and
temperature , both within and beyond the variable-range hopping region. The
calculated properties include not only dc current and statistics of localized
site occupation and hop lengths, but also the current fluctuation spectrum.
Within the calculation accuracy, the model does not exhibit noise, so
that the low-frequency noise at low temperatures may be characterized by the
Fano factor . For sufficiently large samples, scales with conductor
length as , where , and
parameter is interpreted as the average percolation cluster length. At
relatively low , the electric field dependence of parameter is
compatible with the law which follows from directed
percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference
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