1,557 research outputs found
Quantum Nondemolition Charge Measurement of a Josephson Qubit
In a qubit system, the measurement operator does not necessarily commute with
the qubit Hamiltonian, so that the readout process demolishes (mixes) the qubit
energy eigenstates. The readout time is therefore limited by such a mixing time
and its fidelity will be reduced. A quantum nondemolition readout scheme is
proposed in which the charge of a flux qubit is measured. The measurement
operator is shown to commute with the qubit Hamiltonian in the reduced
two-level Hilbert space, even though the Hamiltonian contains non-commuting
charge and flux terms.Comment: 4 pages, 3 figures, a paragraph added to describe how the scheme
works in charge regim
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
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
Bayesian semi-blind component separation for foreground removal in interferometric 21-cm observations
We present in this paper a new Bayesian semi-blind approach for foreground
removal in observations of the 21-cm signal with interferometers. The
technique, which we call HIEMICA (HI Expectation-Maximization Independent
Component Analysis), is an extension of the Independent Component Analysis
(ICA) technique developed for two-dimensional (2D) CMB maps to
three-dimensional (3D) 21-cm cosmological signals measured by interferometers.
This technique provides a fully Bayesian inference of power spectra and maps
and separates the foregrounds from signal based on the diversity of their power
spectra. Only relying on the statistical independence of the components, this
approach can jointly estimate the 3D power spectrum of the 21-cm signal and,
the 2D angular power spectrum and the frequency dependence of each foreground
component, without any prior assumptions about foregrounds. This approach has
been tested extensively by applying it to mock data from interferometric 21-cm
intensity mapping observations under idealized assumptions of instrumental
effects. We also discuss the impact when the noise properties are not known
completely. As a first step toward solving the 21 cm power spectrum analysis
problem we compare the semi-blind HIEMICA technique with the commonly used
Principal Component Analysis (PCA). Under the same idealized circumstances the
proposed technique provides significantly improved recovery of the power
spectrum. This technique can be applied straightforwardly to all 21-cm
interferometric observations, including epoch of reionization measurements, and
can be extended to single-dish observations as well.Comment: 18 pages, 7 figures, added some discussions about the impact of noise
misspecificatio
Measurement induced quantum-classical transition
A model of an electrical point contact coupled to a mechanical system
(oscillator) is studied to simulate the dephasing effect of measurement on a
quantum system. The problem is solved at zero temperature under conditions of
strong non-equilibrium in the measurement apparatus. For linear coupling
between the oscillator and tunneling electrons, it is found that the oscillator
dynamics becomes damped, with the effective temperature determined by the
voltage drop across the junction. It is demonstrated that both the quantum
heating and the quantum damping of the oscillator manifest themselves in the
current-voltage characteristic of the point contact.Comment: in RevTex, 1 figure, corrected notatio
Uncollapsing the wavefunction by undoing quantum measurements
We review and expand on recent advances in theory and experiments concerning
the problem of wavefunction uncollapse: Given an unknown state that has been
disturbed by a generalized measurement, restore the state to its initial
configuration. We describe how this is probabilistically possible with a
subsequent measurement that involves erasing the information extracted about
the state in the first measurement. The general theory of abstract measurements
is discussed, focusing on quantum information aspects of the problem, in
addition to investigating a variety of specific physical situations and
explicit measurement strategies. Several systems are considered in detail: the
quantum double dot charge qubit measured by a quantum point contact (with and
without Hamiltonian dynamics), the superconducting phase qubit monitored by a
SQUID detector, and an arbitrary number of entangled charge qubits.
Furthermore, uncollapse strategies for the quantum dot electron spin qubit, and
the optical polarization qubit are also reviewed. For each of these systems the
physics of the continuous measurement process, the strategy required to ideally
uncollapse the wavefunction, as well as the statistical features associated
with the measurement is discussed. We also summarize the recent experimental
realization of two of these systems, the phase qubit and the polarization
qubit.Comment: 19 pages, 4 figure
Method for direct observation of coherent quantum oscillations in a superconducting phase qubit
Time-domain observations of coherent oscillations between quantum states in
mesoscopic superconducting systems were so far restricted to restoring the
time-dependent probability distribution from the readout statistics. We propose
a new method for direct observation of Rabi oscillations in a phase qubit. The
external source, typically in GHz range, induces transitions between the qubit
levels. The resulting Rabi oscillations of supercurrent in the qubit loop are
detected by a high quality resonant tank circuit, inductively coupled to the
phase qubit. Detailed calculation for zero and non-zero temperature are made
for the case of persistent current qubit. According to the estimates for
dephasing and relaxation times, the effect can be detected using conventional
rf circuitry, with Rabi frequency in MHz range.Comment: 5 pages, 1 figure, to appear in Phys.Rev.
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
Localization and Capacitance Fluctuations in Disordered Au Nano-junctions
Nano-junctions, containing atomic-scale gold contacts between strongly
disordered leads, exhibit different transport properties at room temperature
and at low temperature. At room temperature, the nano-junctions exhibit
conductance quantization effects. At low temperatures, the contacts exhibit
Coulomb-Blockade. We show that the differences between the room-temperature and
low temperature properties arise from the localization of electronic states in
the leads. The charging energy and capacitance of the nano-junctions exhibit
strong fluctuations with applied magnetic field at low temperature, as
predicted theoretically.Comment: 20 pages 8 figure
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
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