200 research outputs found
High-gain weakly nonlinear flux-modulated Josephson parametric amplifier using a SQUID-array
We have developed and measured a high-gain quantum-limited microwave
parametric amplifier based on a superconducting lumped LC resonator with the
inductor L including an array of 8 superconducting quantum interference devices
(SQUIDs). This amplifier is parametrically pumped by modulating the flux
threading the SQUIDs at twice the resonator frequency. Around 5 GHz, a maximum
gain of 31 dB, a product amplitude-gain x bandwidth above 60 MHz, and a 1 dB
compression point of -123 dBm at 20 dB gain are obtained in the non-degenerate
mode of operation. Phase sensitive amplification-deamplification is also
measured in the degenerate mode and yields a maximum gain of 37 dB. The
compression point obtained is 18 dB above what would be obtained with a single
SQUID of the same inductance, due to the smaller nonlinearity of the SQUID
array.Comment: 7 pages, 4 figures, 23 reference
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
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%
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
Improved qubit bifurcation readout in the straddling regime of circuit QED
We study bifurcation measurement of a multi-level superconducting qubit using
a nonlinear resonator biased in the straddling regime, where the resonator
frequency sits between two qubit transition frequencies. We find that
high-fidelity bifurcation measurements are possible because of the enhanced
qubit-state-dependent pull of the resonator frequency, the behavior of
qubit-induced nonlinearities and the reduced Purcell decay rate of the qubit
that can be realized in this regime. Numerical simulations find up to a
threefold improvement in qubit readout fidelity when operating in, rather than
outside of, the straddling regime. High-fidelity measurements can be obtained
at much smaller qubit-resonator couplings than current typical experimental
realizations, reducing spectral crowding and potentially simplifying the
implementation of multi-qubit devices.Comment: 9 pages, 6 figure
Relaxation and Dephasing in a Flux-qubit
We report detailed measurements of the relaxation and dephasing time in a
flux-qubit measured by a switching DC SQUID. We studied their dependence on the
two important circuit bias parameters: the externally applied magnetic flux and
the bias current through the SQUID in two samples. We demonstrate two
complementary strategies to protect the qubit from these decoherence sources.
One consists in biasing the qubit so that its resonance frequency is stationary
with respect to the control parameters ({\it optimal point}) ; the second
consists in {\it decoupling} the qubit from current noise by chosing a proper
bias current through the SQUID. At the decoupled optimal point, we measured
long spin-echo decay times of up to .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Letter
Dephasing of a superconducting qubit induced by photon noise
We have studied the dephasing of a superconducting flux-qubit coupled to a
DC-SQUID based oscillator. By varying the bias conditions of both circuits we
were able to tune their effective coupling strength. This allowed us to measure
the effect of such a controllable and well-characterized environment on the
qubit coherence. We can quantitatively account for our data with a simple model
in which thermal fluctuations of the photon number in the oscillator are the
limiting factor. In particular, we observe a strong reduction of the dephasing
rate whenever the coupling is tuned to zero. At the optimal point we find a
large spin-echo decay time of .Comment: New version of earlier paper arXiv/0507290 after in-depth rewritin
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