363 research outputs found
Dephasing of qubits by transverse low-frequency noise
We analyze the dissipative dynamics of a two-level quantum system subject to
low-frequency, e.g. 1/f noise, motivated by recent experiments with
superconducting quantum circuits. We show that the effect of transverse linear
coupling of the system to low-frequency noise is equivalent to that of
quadratic longitudinal coupling. We further find the decay law of quantum
coherent oscillations under the influence of both low- and high-frequency
fluctuations, in particular, for the case of comparable rates of relaxation and
pure dephasing
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
Multiplexed Readout of Transmon Qubits with Josephson Bifurcation Amplifiers
Achieving individual qubit readout is a major challenge in the development of
scalable superconducting quantum processors. We have implemented the
multiplexed readout of a four transmon qubit circuit using non-linear
resonators operated as Josephson bifurcation amplifiers. We demonstrate the
simultaneous measurement of Rabi oscillations of the four transmons. We find
that multiplexed Josephson bifurcation is a high-fidelity readout method, the
scalability of which is not limited by the need of a large bandwidth nearly
quantum-limited amplifier as is the case with linear readout resonators.Comment: 7 pages, 6 figures, and 31 reference
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
Storage and Retrieval of a Microwave Field in a Spin Ensemble
We report the storage and retrieval of a small microwave field from a
superconducting resonator into collective excitations of a spin ensemble. The
spins are nitrogen-vacancy centers in a diamond crystal. The storage time of
the order of 30 ns is limited by inhomogeneous broadening of the spin ensemble.Comment: 4 pages + supplementary material. Submitted to PR
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
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%
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,
reference adde
Tunable resonators for quantum circuits
We have designed, fabricated and measured high-Q coplanar
waveguide microwave resonators whose resonance frequency is made tunable with
magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside.
Their tunability range is 30% of the zero field frequency. Their quality factor
reaches up to 3. We present a model based on thermal fluctuations
that accounts for the dependance of the quality factor with magnetic field.Comment: subm. to JLTP (Proc. of LTD12 conference
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