63 research outputs found
Tunable coupling scheme for flux qubits at the optimal point
We discuss a practical design for tunably coupling a pair of flux qubits via
the quantum inductance of a third high-frequency qubit. The design is
particularly well suited for realizing a recently proposed microwave-induced
parametric coupling scheme. This is attractive because the qubits can always
remain at their optimal points. Furthermore, we will show that the resulting
coupling also has an optimal point where it is insensitive to low-frequency
flux noise. This is an important feature for the coherence of coupled qubits.
The presented scheme is an experimentally realistic way of carrying out
two-qubit gates and should be easily extended to multiqubit systems.Comment: 8 pages, 6 figures, minor change
Noise properties of a Josephson parametric oscillator
We perform the noise spectroscopy of a Josephson parametric oscillator (JPO)
by implementing a microwave homodyne interferometric measurement scheme. We
observe the fluctuations in the self-oscillating output field of the JPO for a
long 10 s time interval in a single shot measurement and characterize the phase
and amplitude noise. Furthermore, we investigate the effects of the pump
strength on the output noise power spectra of the JPO. We found strong
fluctuations in the phase with a characteristics in the phase noise
power spectrum, which is suppressed by increasing the pump strength
Power-dependent internal loss in Josephson bifurcation amplifiers
We have studied nonlinear superconducting resonators: lambda/2
coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in
the middle and lambda/4 CPW resonators terminated by JJs, which can be used for
the qubit readout as "bifurcation amplifiers." The nonlinearity of the
resonators arises from the Josephson junctions, and because of the
nonlinearity, the resonators with appropriate parameters are expected to show a
hysteretic response to the frequency sweep, or "bifurcation," when they are
driven with a sufficiently large power. We designed and fabricated resonators
whose resonant frequencies were around 10 GHz. We characterized the resonators
at low temperatures, T<0.05 K, and confirmed that they indeed exhibited
hysteresis. The sizes of the hysteresis, however, are sometimes considerably
smaller than the predictions based on the loaded quality factor in the weak
drive regime. When the discrepancy appears, it is mostly explained by taking
into account the internal loss, which often increases in our resonators with
increasing drive power in the relevant power range. As a possible origin of the
power-dependent loss, the quasiparticle channel of conductance of the JJs is
discussed.Comment: 8 pages, 9 figure
Single microwave-photon detector using an artificial -type three-level system
Single photon detection is a requisite technique in quantum-optics
experiments in both the optical and the microwave domains. However, the energy
of microwave quanta are four to five orders of magnitude less than their
optical counterpart, making the efficient detection of single microwave photons
extremely challenging. Here, we demonstrate the detection of a single microwave
photon propagating through a waveguide. The detector is implemented with an
"impedance-matched" artificial system comprising the dressed states
of a driven superconducting qubit coupled to a microwave resonator. We attain a
single-photon detection efficiency of with a reset time of
~ns. This detector can be exploited for various applications in
quantum sensing, quantum communication and quantum information processing.Comment: 5 pages (4 figures) + 4 pages (5 figures
Ultrastrong tunable coupler between superconducting LC resonators
We investigate the ultrastrong tunable coupler for coupling of
superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the
wide range tunability is achieved both antiferromagnetics and ferromagnetics
from MHz to 604 MHz. The ultrastrong coupler is composed of rf-SQUID
and dc-SQUID as tunable junctions, which connected to resonators via shared
aluminum thin film meander lines enabling such a huge coupling constant. The
spectrum of the coupler obviously shows the breaking of the rotating wave
approximation, and our circuit model treating the Josephson junction as a
tunable inductance reproduces the experimental results well. The ultrastrong
coupler is expected to be utilized in quantum annealing circuits and/or NISQ
devices with dense connections between qubits.Comment: 12pages, 7 figure
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