44 research outputs found
Superconducting microstrip amplifiers with sub-Kelvin noise temperature near 4 GHz
We present measurements of an amplifier operating at 3.8 GHz with 150 MHz of
bandwidth based on the microstrip input-coil resonance of a dc superconducting
quantum interference device (SQUID) with submicron Josephson junctions. The
noise temperature is measured using two methods: comparing the signal-to-noise
ratio of the system with and without the SQUID in the amplifier chain, and
using a modified Y-factor technique where calibrated narrowband noise is mixed
up to the SQUID amplifier operating frequency. With the SQUID cooled to 0.35 K
we observe a minimum system noise temperature of 0.55 K, dominated
by the contribution from the SQUID amplifier
High fidelity single-shot readout of a transmon qubit using a SLUG {\mu}wave amplifier
We report high-fidelity, quantum nondemolition, single-shot readout of a
superconducting transmon qubit using a DC-biased superconducting low-inductance
undulatory galvanometer(SLUG) amplifier. The SLUG improves the system
signal-to-noise ratio by 7 dB in a 20 MHz window compared with a bare HEMT
amplifier. An optimal cavity drive pulse is chosen using a genetic search
algorithm, leading to a maximum combined readout and preparation fidelity of
91.9% with a measurement time of Tmeas = 200ns. Using post-selection to remove
preparation errors caused by heating, we realize a combined preparation and
readout fidelity of 94.3%.Comment: 4 pages and 3 figure
Transient dynamics of a superconducting nonlinear oscillator
We investigate the transient dynamics of a lumped-element oscillator based on
a dc superconducting quantum interference device (SQUID). The SQUID is shunted
with a capacitor forming a nonlinear oscillator with resonance frequency in the
range of several GHz. The resonance frequency is varied by tuning the Josephson
inductance of the SQUID with on-chip flux lines. We report measurements of
decaying oscillations in the time domain following a brief excitation with a
microwave pulse. The nonlinearity of the SQUID oscillator is probed by
observing the ringdown response for different excitation amplitudes while the
SQUID potential is varied by adjusting the flux bias. Simulations are performed
on a model circuit by numerically solving the corresponding Langevin equations
incorporating the SQUID potential at the experimental temperature and using
parameters obtained from separate measurements characterizing the SQUID
oscillator. Simulations are in good agreement with the experimental
observations of the ringdowns as a function of applied magnetic flux and pulse
amplitude. We observe a crossover between the occurrence of ringdowns close to
resonance and adiabatic following at larger detuning from the resonance. We
also discuss the occurrence of phase jumps at large amplitude drive. Finally,
we briefly outline prospects for a readout scheme for superconducting flux
qubits based on the discrimination between ringdown signals for different
levels of magnetic flux coupled to the SQUID.Comment: 15 pages, 9 figure