249 research outputs found
Quasiparticle decay rate of Josephson charge qubit oscillations
We analyze the decay of Rabi oscillations in a charge qubit consisting of a
Cooper pair box connected to a finite-size superconductor by a Josephson
junction. We concentrate on the contribution of quasiparticles in the
superconductors to the decay rate. Passing of a quasiparticle through the
Josephson junction tunes the qubit away from the charge degeneracy, thus
spoiling the Rabi oscillations. We find the temperature dependence of the
quasiparticle contribution to the decay rate for open and isolated systems. The
former case is realized if a normal-state trap is included in the circuit, or
if just one vortex resides in the qubit; the decay rate has an activational
temperature dependence with the activation energy equal to the superconducting
gap . In a superconducting qubit isolated from the environment, the
activation energy equals if the number of electrons is even, while
for an odd number of electrons the decay rate of an excited qubit state remains
finite in the limit of zero temperature. We estimate the decay rate for
realistic parameters of a qubit.Comment: 8 pages, 3 figures, final version as published in PRB, minor change
Kinetics of the superconducting charge qubit in the presence of a quasiparticle
We investigate the energy and phase relaxation of a superconducting qubit
caused by a single quasiparticle. In our model, the qubit is an isolated system
consisting of a small island (Cooper-pair box) and a larger superconductor
(reservoir) connected with each other by a tunable Josephson junction. If such
system contains an odd number of electrons, then even at lowest temperatures a
single quasiparticle is present in the qubit. Tunneling of a quasiparticle
between the reservoir and the Cooper-pair box results in the relaxation of the
qubit. We derive master equations governing the evolution of the qubit
coherences and populations. We find that the kinetics of the qubit can be
characterized by two time scales - quasiparticle escape time from reservoir to
the box, , and quasiparticle relaxation time . The
former is determined by the dimensionless normal-state conductance of the
Josephson junction and one-electron level spacing in the reservoir
(), and the latter is due to electron-phonon
interaction. We find that phase coherence is damped on the time scale of
. The qubit energy relaxation depends on the ratio of the two
characteristic times, and , and also on the ratio of
temperature to the Josephson energy .Comment: 12 pages, 4 figures, final version as published in PRB, some changes,
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Characterization of a two-transmon processor with individual single-shot qubit readout
We report the characterization of a two-qubit processor implemented with two
capacitively coupled tunable superconducting qubits of the transmon type, each
qubit having its own non-destructive single-shot readout. The fixed capacitive
coupling yields the \sqrt{iSWAP} two-qubit gate for a suitable interaction
time. We reconstruct by state tomography the coherent dynamics of the two-bit
register as a function of the interaction time, observe a violation of the Bell
inequality by 22 standard deviations after correcting readout errors, and
measure by quantum process tomography a gate fidelity of 90%
Quantum Heating of a nonlinear resonator probed by a superconducting qubit
We measure the quantum fluctuations of a pumped nonlinear resonator, using a
superconducting artificial atom as an in-situ probe. The qubit excitation
spectrum gives access to the frequency and temperature of the intracavity field
fluctuations. These are found to be in agreement with theoretical predictions;
in particular we experimentally observe the phenomenon of quantum heating
Fluctuation-Dissipation Relations of a Tunnel Junction Driven by a Quantum Circuit
We derive fluctuation-dissipation relations for a tunnel junction driven by a
high impedance microwave resonator, displaying strong quantum fluctuations. We
find that the fluctuation-dissipation relations derived for classical forces
hold, provided the effect of the circuit's quantum fluctuations is incorporated
into a modified non-linear curve. We also demonstrate that all
quantities measured under a coherent time dependent bias can be reconstructed
from their dc counterpart with a photo-assisted tunneling relation. We confirm
these predictions by implementing the circuit and measuring the dc current
through the junction, its high frequency admittance and its current noise at
the frequency of the resonator.Comment: Publisehd as Physical Review Letters, 114, 12680
Vanishing of electron pair recession at central impact
Identity of electrons leads to description of their states by symmetrical or
anti-symmetrical combination of free coherent states.
Due to the coordinate uncertainty potential energy of the Coulomb repulsing
is limited from above and so when energy of electrons is large enough,
electrons go through each other, without noticing one another.
We show existence of set of coherent states for which wave packages recession
vanish - electrons remain close regardless of Coulomb repulsion.Comment: ICQO2006 Mins
Circuit QED with a Nonlinear Resonator : ac-Stark Shift and Dephasing
We have performed spectroscopic measurements of a superconducting qubit
dispersively coupled to a nonlinear resonator driven by a pump microwave field.
Measurements of the qubit frequency shift provide a sensitive probe of the
intracavity field, yielding a precise characterization of the resonator
nonlinearity. The qubit linewidth has a complex dependence on the pump
frequency and amplitude, which is correlated with the gain of the nonlinear
resonator operated as a small-signal amplifier. The corresponding dephasing
rate is found to be close to the quantum limit in the low-gain limit of the
amplifier.Comment: Paper : 4 pages, 3 figures; Supplementary material : 1 page, 1 figur
Measuring the Decoherence of a Quantronium Qubit with the Cavity Bifurcation Amplifier
Dispersive readouts for superconducting qubits have the advantage of speed
and minimal invasiveness. We have developed such an amplifier, the Cavity
Bifurcation Amplifier (CBA) [10], and applied it to the readout of the
quantronium qubit [2]. It consists of a Josephson junction embedded in a
microwave on-chip resonator. In contrast with the Josephson bifurcation
amplifier [17], which has an on-chip capacitor shunting a junction, the
resonator is based on a simple coplanar waveguide imposing a pre-determined
frequency and whose other RF characteristics like the quality factor are easily
controlled and optimized. Under proper microwave irradiation conditions, the
CBA has two metastable states. Which state is adopted by the CBA depends on the
state of a quantronium qubit coupled to the CBA's junction. Due to the MHz
repetition rate and large signal to noise ratio we can show directly that the
coherence is limited by 1/f gate charge noise when biased at the sweet spot - a
point insensitive to first order gate charge fluctuations. This architecture
lends itself to scalable quantum computing using a multi-resonator chip with
multiplexed readouts.Comment: 6 pages, 5 figures To be published in Physical Review
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