1,491 research outputs found
Spectrum of qubit oscillations from Bloch equations
We have developed a formalism suitable for calculation of the output spectrum
of a detector continuously measuring quantum coherent oscillations in a
solid-state qubit, starting from microscopic Bloch equations. The results
coincide with that obtained using Bayesian and master equation approaches. The
previous results are generalized to the cases of arbitrary detector response
and finite detector temperature.Comment: 8 page
Optical properties of Mn4+ ions in GaN:Mn codoped with Mg acceptors
The optical properties of Mn-Mg codoped epitaxial GaN were studied. Addition
of Mg acceptors quenches the weak manganese-related photoluminescence (PL) band
at 1.3 eV in GaN:Mn and a series of sharp PL peaks are observed at 1 eV in
codoped epilayers. The change in PL spectra indicates that Mg addition
stabilizes the Mn4+ charge state by decreasing the Fermi level. The 1 eV PL
peaks are tentatively attributed to intra center transitions involving Mn4+
ions. Spin allowed 3d-shell 4T2-4T1 transitions and their phonon replicas are
involved. The relative intensities of the sharp peaks are strongly dependent on
the excitation wavelength, indicating the optically active Mn4+ centers
involved in the separate peaks are different. The temperature dependence of the
PL spectrum suggests the presence of at least three distinct Mn4+ complex
centers.Comment: 14 pages, 3 figures, 1 table, accepted by Appl. Phys. Let
Full Frequency Back-Action Spectrum of a Single Electron Transistor during Qubit read-out
We calculate the spectral density of voltage fluctuations in a Single
Electron Transistor (SET), biased to operate in a transport mode where
tunneling events are correlated due to Coulomb interaction. The whole spectrum
from low frequency shot noise to quantum noise at frequencies comparable to the
SET charging energy is considered. We discuss the back-action
during read-out of a charge qubit and conclude that single-shot read-out is
possible using the Radio-Frequency SET.Comment: 4 pages, 5 figures, submitted to PR
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
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
Continuous weak measurement of quantum coherent oscillations
We consider the problem of continuous quantum measurement of coherent
oscillations between two quantum states of an individual two-state system. It
is shown that the interplay between the information acquisition and the
backaction dephasing of the oscillations by the detector imposes a fundamental
limit, equal to 4, on the signal-to-noise ratio of the measurement. The limit
is universal, e.g., independent of the coupling strength between the detector
and system, and results from the tendency of quantum measurement to localize
the system in one of the measured eigenstates
Statistics and noise in a quantum measurement process
The quantum measurement process by a single-electron transistor or a quantum
point contact coupled to a quantum bit is studied. We find a unified
description of the statistics of the monitored quantity, the current, in the
regime of strong measurement and expect this description to apply for a wide
class of quantum measurements. We derive the probability distributions for the
current and charge in different stages of the process. In the parameter regime
of the strong measurement the current develops a telegraph-noise behavior which
can be detected in the noise spectrum.Comment: 4 pages, 2 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
Charge and current fluctuations in a superconducting single electron transistor near a Cooper pair resonance
We analyze charge tunneling statistics and current noise in a superconducting
single-electron transistor in a regime where the Josephson-quasiparticle cycle
is the dominant mechanism of transport. Due to the interplay between Coulomb
blockade and Josephson coherence, the probability distribution for tunneling
events strongly deviates from a Poissonian and displays a pronounced even--odd
asymmetry in the number of transmitted charges. The interplay between charging
and coherence is reflected also in the zero-frequency current noise which is
significantly quenched when the quasi-particle tunneling rates are comparable
to the coherent Cooper-pair oscillation frequency. Furthermore the finite
frequency spectrum shows a strong enhancement near the resonant transition
frequency for Josephson tunneling.Comment: 10 pages, 11 figure
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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