2,894 research outputs found
Improving wafer-scale Josephson junction resistance variation in superconducting quantum coherent circuits
Quantum bits, or qubits, are an example of coherent circuits envisioned for
next-generation computers and detectors. A robust superconducting qubit with a
coherent lifetime of (100 s) is the transmon: a Josephson junction
functioning as a non-linear inductor shunted with a capacitor to form an
anharmonic oscillator. In a complex device with many such transmons, precise
control over each qubit frequency is often required, and thus variations of the
junction area and tunnel barrier thickness must be sufficiently minimized to
achieve optimal performance while avoiding spectral overlap between neighboring
circuits. Simply transplanting our recipe optimized for single, stand-alone
devices to wafer-scale (producing 64, 1x1 cm dies from a 150 mm wafer)
initially resulted in global drifts in room-temperature tunneling resistance of
30%. Inferring a critical current variation from this
resistance distribution, we present an optimized process developed from a
systematic 38 wafer study that results in 3.5% relative standard deviation
(RSD) in critical current () for 3000 Josephson junctions (both single-junctions and
asymmetric SQUIDs) across an area of 49 cm. Looking within a 1x1 cm moving
window across the substrate gives an estimate of the variation characteristic
of a given qubit chip. Our best process, utilizing ultrasonically assisted
development, uniform ashing, and dynamic oxidation has shown = 1.8% within 1x1 cm, on average,
with a few 1x1 cm areas having 1.0% (equivalent to 0.5%). Such stability would drastically improve the yield of
multi-junction chips with strict critical current requirements.Comment: 10 pages, 4 figures. Revision includes supplementary materia
Phase-locking transition in a chirped superconducting Josephson resonator
By coupling a harmonic oscillator to a quantum system it is possible to
perform a dispersive measurement that is quantum non-demolition (QND), with
minimal backaction. A non-linear oscillator has the advantage of measurement
gain, but what is the backaction? Experiments on superconducting quantum bits
(qubits) coupled to a non-linear Josephson oscillator have thus far utilized
the switching of the oscillator near a dynamical bifurcation for sensitivity,
and have demonstrated partial QND measurement. The detailed backaction
associated with the switching process is complex, and may ultimately limit the
degree to which such a measurement can be QND. Here we demonstrate a new
dynamical effect in Josephson oscillators by which the bifurcation can be
accessed without switching. When energized with a frequency chirped drive with
an amplitude close to a sharp, phase-locking threshold, the oscillator evolves
smoothly in one of two diverging trajectories - a pointer for the state of a
qubit. The observed critical behavior agrees well with theory and suggests a
new modality for quantum state measurement.Comment: 5 pages, 4 figure
1/f noise of Josephson-junction-embedded microwave resonators at single photon energies and millikelvin temperatures
We present measurements of 1/f frequency noise in both linear and
Josephson-junction-embedded superconducting aluminum resonators in the low
power, low temperature regime - typical operating conditions for
superconducting qubits. The addition of the Josephson junction does not result
in additional frequency noise, thereby placing an upper limit for fractional
critical current fluctuations of (Hz) at 1 Hz for
sub-micron, shadow evaporated junctions. These values imply a minimum dephasing
time for a superconducting qubit due to critical current noise of 40 -- 1400
s depending on qubit architecture. Occasionally, at temperatures above 50
mK, we observe the activation of individual fluctuators which increase the
level of noise significantly and exhibit Lorentzian spectra
Quantum Fluctuations in the Chirped Pendulum
An anharmonic oscillator when driven with a fast, frequency chirped voltage
pulse can oscillate with either small or large amplitude depending on whether
the drive voltage is below or above a critical value-a well studied classical
phenomenon known as autoresonance. Using a 6 GHz superconducting resonator
embedded with a Josephson tunnel junction, we have studied for the first time
the role of noise in this non-equilibrium system and find that the width of the
threshold for capture into autoresonance decreases as the square root of T, and
saturates below 150 mK due to zero point motion of the oscillator. This unique
scaling results from the non-equilibrium excitation where fluctuations, both
quantum and classical, only determine the initial oscillator motion and not its
subsequent dynamics. We have investigated this paradigm in an electrical
circuit but our findings are applicable to all out of equilibrium nonlinear
oscillators.Comment: 5 pages, 4 figure
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