377 research outputs found
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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Manipulating the Quantum State of an Electrical Circuit
We have designed and operated a superconducting tunnel junction circuit that
behaves as a two-level atom: the ``quantronium''. An arbitrary evolution of its
quantum state can be programmed with a series of microwave pulses, and a
projective measurement of the state can be performed by a pulsed readout
sub-circuit. The measured quality factor of quantum coherence Qphi=25000 is
sufficiently high that a solid-state quantum processor based on this type of
circuit can be envisioned.Comment: 4 figures include
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
The Bright Side of Coulomb Blockade
We explore the photonic (bright) side of dynamical Coulomb blockade (DCB) by
measuring the radiation emitted by a dc voltage-biased Josephson junction
embedded in a microwave resonator. In this regime Cooper pair tunneling is
inelastic and associated to the transfer of an energy 2eV into the resonator
modes. We have measured simultaneously the Cooper pair current and the photon
emission rate at the resonance frequency of the resonator. Our results show two
regimes, in which each tunneling Cooper pair emits either one or two photons
into the resonator. The spectral properties of the emitted radiation are
accounted for by an extension to DCB theory.Comment: 4 pages, 4 figures + 3 pages, 1 figure supplementary materia
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%
Antibunched photons emitted by a dc-biased Josephson junction
We show experimentally that a dc biased Josephson junction in series with a high-enough-impedance microwave resonator emits antibunched photons. Our resonator is made of a simple microfabricated spiral coil that resonates at 4.4 GHz and reaches a 1.97kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6×10^7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain
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
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
Spectroscopy of superconducting charge qubits coupled by a Josephson inductance
We have designed and experimentally implemented a circuit of
inductively-coupled superconducting charge qubits, where a Josephson junction
is used as an inductance, and the coupling between the qubits is controlled by
an applied magnetic flux. Spectroscopic measurements on the circuit are in good
agreement with theoretical calculations. We observed anticrossings which
originate from the coupling between the qubit and the plasma mode of the
Josephson junction. Moreover, the size of the anticrossing depends on the
external magnetic flux, which demonstrates the controllability of the coupling.Comment: Accepted for publication in PRB. 11 pages, 7 figure
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