2,332 research outputs found
Decoherence suppression by uncollapsing
We show that the qubit decoherence due to zero-temperature energy relaxation
can be almost completely suppressed by using the quantum uncollapsing
procedure. To protect a qubit state, a partial quantum measurement moves it
towards the ground state, where it is kept during the storage period, while the
second partial measurement restores the initial state. This procedure
preferentially selects the cases without energy decay events. Stronger
decoherence suppression requires smaller selection probability; a desired point
in this trade-off can be chosen by varying the measurement strength. The
experiment can be realized in a straightforward way using the superconducting
phase qubit.Comment: 4 page
Quantum theory of a bandpass Purcell filter for qubit readout
The readout fidelity of superconducting transmon and Xmon qubits is partially
limited by the qubit energy relaxation through the resonator into the
transmission line, which is also known as the Purcell effect. One way to
suppress this energy relaxation is to employ a filter which impedes microwave
propagation at the qubit frequency. We present semiclassical and quantum
analyses for the bandpass Purcell filter realized by E.\ Jeffrey \textit{et
al}.\ [Phys.\ Rev.\ Lett.\ 112, 190504 (2014)]. For typical experimental
parameters, the bandpass filter suppresses the qubit relaxation rate by up to
two orders of magnitude while maintaining the same measurement rate. We also
show that in the presence of a microwave drive the qubit relaxation rate
further decreases with increasing drive strength.Comment: 15 pages, 4 figures; published versio
Purcell effect with microwave drive: Suppression of qubit relaxation rate
We analyze the Purcell relaxation rate of a superconducting qubit coupled to
a resonator, which is coupled to a transmission line and pumped by an external
microwave drive. Considering the typical regime of the qubit measurement, we
focus on the case when the qubit frequency is significantly detuned from the
resonator frequency. Surprisingly, the Purcell rate decreases when the strength
of the microwave drive is increased. This suppression becomes significant in
the nonlinear regime. In the presence of the microwave drive, the loss of
photons to the transmission line also causes excitation of the qubit; however,
the excitation rate is typically much smaller than the relaxation rate. Our
analysis also applies to a more general case of a two-level quantum system
coupled to a cavity.Comment: Published versio
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
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
Simple quantum feedback of a solid-state qubit
We propose an experiment on quantum feedback control of a solid-state qubit,
which is almost within the reach of the present-day technology. Similar to the
earlier proposal, the feedback loop is used to maintain the coherent (Rabi)
oscillations in a qubit for an arbitrary long time; however, this is done in a
significantly simpler way, which requires much smaller bandwidth of the control
circuitry. The main idea is to use the quadrature components of the noisy
detector current to monitor approximately the phase of qubit oscillations.
The price for simplicity is a less-than-ideal operation: the fidelity is
limited by about 95%. The feedback loop operation can be experimentally
verified by appearance of a positive in-phase component of the detector current
relative to an external oscillating signal used for synchronization.Comment: 5 page
Resonator/zero-Qubit architecture for superconducting qubits
We analyze the performance of the Resonator/zero-Qubit (RezQu) architecture
in which the qubits are complemented with memory resonators and coupled via a
resonator bus. Separating the stored information from the rest of the
processing circuit by at least two coupling steps and the zero qubit state
results in a significant increase of the ON/OFF ratio and the reduction of the
idling error. Assuming no decoherence, we calculate such idling error, as well
as the errors for the MOVE operation and tunneling measurement, and show that
the RezQu architecture can provide high fidelity performance required for
medium-scale quantum information processing.Comment: 11 pages, 5 figure
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