23,048 research outputs found
Undoing measurement-induced dephasing in circuit QED
We analyze the backaction of homodyne detection and photodetection on
superconducting qubits in circuit quantum electrodynamics. Although both
measurement schemes give rise to backaction in the form of stochastic phase
rotations, which leads to dephasing, we show that this can be perfectly undone
provided that the measurement signal is fully accounted for. This result
improves upon that of Phys. Rev. A, 82, 012329 (2010), showing that the method
suggested can be made to realize a perfect two-qubit parity measurement. We
propose a benchmarking experiment on a single qubit to demonstrate the method
using homodyne detection. By analyzing the limited measurement efficiency of
the detector and bandwidth of the amplifier, we show that the parameter values
necessary to see the effect are within the limits of existing technology
Cavity QED in superconducting circuits: susceptibility at elevated temperatures
We study the properties of superconducting electrical circuits, realizing
cavity QED. In particular we explore the limit of strong coupling, low
dissipation, and elevated temperatures relevant for current and future
experiments. We concentrate on the cavity susceptibility as it can be directly
experimentally addressed, i.e., as the impedance or the reflection coefficient
of the cavity. To this end we investigate the dissipative Jaynes-Cummings model
in the strong coupling regime at high temperatures. The dynamics is
investigated within the Bloch-Redfield formalism. At low temperatures, when
only the few lowest levels are occupied the susceptibility can be presented as
a sum of contributions from independent level-to-level transitions. This
corresponds to the secular (random phase) approximation in the Bloch-Redfield
formalism. At temperatures comparable to and higher than the oscillator
frequency, many transitions become important and a multiple-peak structure
appears. We show that in this regime the secular approximation breaks down, as
soon as the peaks start to overlap. In other words, the susceptibility is no
longer a sum of contributions from independent transitions. We treat the
dynamics of the system numerically by exact diagonalization of the Hamiltonian
of the qubit plus up to 200 states of the oscillator. We compare the results
obtained with and without the secular approximation and find a qualitative
discrepancy already at moderate temperatures.Comment: 7 pages, 6 figure
The information about the state of a charge qubit gained by a weakly coupled quantum point contact
We analyze the information that one can learn about the state of a quantum
two-level system, i.e. a qubit, when probed weakly by a nearby detector. We
consider the general case where the qubit Hamiltonian and the qubit's operator
probed by the detector do not commute. Because the qubit's state keeps evolving
while being probed and the measurement data is mixed with a detector-related
background noise, one might expect the detector to fail in this case. We show,
however, that under suitable conditions and by proper analysis of the
measurement data useful information about the initial state of the qubit can be
extracted. Our approach complements the usual master-equation and
quantum-trajectory approaches, which describe the evolution of the qubit's
quantum state during the measurement process but do not keep track of the
acquired measurement information.Comment: 5 pages, 3 figures; Published in the proceedings of the Nobel
Symposium 141: Qubits for Future Quantum Informatio
Nanowire Acting as a Superconducting Quantum Interference Device
We present the results from an experimental study of the magneto-transport of
superconducting wires of amorphous Indium-Oxide, having widths in the range 40
- 120 nm. We find that, below the superconducting transition temperature, the
wires exhibit clear, reproducible, oscillations in their resistance as a
function of magnetic field. The oscillations are reminiscent of those which
underlie the operation of a superconducting quantum interference device.Comment: 4 pages, 4 figures, 1 tabl
Is the Mott transition relevant to f-electron metals ?
We study how a finite hybridization between a narrow correlated band and a
wide conduction band affects the Mott transition. At zero temperature, the
hybridization is found to be a relevant perturbation, so that the Mott
transition is suppressed by Kondo screening. In contrast, a first-order
transition remains at finite temperature, separating a local moment phase and a
Kondo- screened phase. The first-order transition line terminates in two
critical endpoints. Implications for experiments on f-electron materials such
as the Cerium alloy CeLaTh are discussed.Comment: 5 pages, 3 figure
Quantum information processing using frequency control of impurity spins in diamond
Spin degrees of freedom of charged nitrogen-vacancy (NV) centers in
diamond have large decoherence times even at room temperature, can be
initialized and read out using optical fields, and are therefore a promising
candidate for solid state qubits. Recently, quantum manipulations of NV-
centers using RF fields were experimentally realized. In this paper we show;
first, that such operations can be controlled by varying the frequency of the
signal, instead of its amplitude, and NV- centers can be selectively
addressed even with spacially uniform RF signals; second, that when several \NV
- centers are placed in an off-resonance optical cavity, a similar application
of classical optical fields provides a controlled coupling and enables a
universal two-qubit gate (CPHASE). RF and optical control together promise a
scalable quantum computing architecture
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