146 research outputs found
Narrow band microwave radiation from a biased single-Cooper-pair transistor
We show that a single-Cooper-pair transistor (SCPT) electrometer emits
narrow-band microwave radiation when biased in its sub-gap region. Photo
activation of quasiparticle tunneling in a nearby SCPT is used to
spectroscopically detect this radiation, in a configuration that closely mimics
a qubit-electrometer integrated circuit. We identify emission lines due to
Josephson radiation and radiative transport processes in the electrometer, and
argue that a dissipative superconducting electrometer can severely disrupt the
system it attempts to measure.Comment: 4 pages, 3 figure
Josephson Effect in Pb/I/NbSe2 Scanning Tunneling Microscope Junctions
We have developed a method for the reproducible fabrication of
superconducting scanning tunneling microscope (STM) tips. We use these tips to
form superconductor/insulator/superconductor tunnel junctions with the STM tip
as one of the electrodes. We show that such junctions exhibit fluctuation
dominated Josephson effects, and describe how the Josephson product IcRn can be
inferred from the junctions' tunneling characteristics in this regime. This is
first demonstrated for tunneling into Pb films, and then applied in studies of
single crystals of NbSe2. We find that in NbSe2, IcRn is lower than expected,
which could be attributed to the interplay between superconductivity and the
coexisting charge density wave in this material.Comment: 3 pages, 2 figures. Presented at the New3SC-4 meeting, San Diego,
Jan. 16-21 200
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
Millikelvin thermal and electrical performance of lossy transmission line filters
We report on the scattering parameters and Johnson noise emission of low-pass
stripline filters employing a magnetically loaded silicone dielectric down to
25 mK. The transmission characteristic of a device with =1.3 GHz
remains essentially unchanged upon cooling. Another device with =0.4
GHz, measured in its stopband, exhibits a steady state noise power emission
consistent with a temperature difference of a few mK relative to a
well-anchored cryogenic microwave attenuator at temperatures down to 25 mK,
thus presenting a matched thermal load.Comment: 4 pages, 4 figure
Universal transport in 2D granular superconductors
The transport properties of quench condensed granular superconductors are
presented and analyzed. These systems exhibit transitions from insulating to
superconducting behavior as a function of inter-grain spacing.
Superconductivity is characterized by broad transitions in which the resistance
drops exponentially with reducing temperature. The slope of the log R versus T
curves turns out to be universaly dependent on the normal state film resistance
for all measured granular systems. It does not depend on the material, critical
temperature, geometry, or experimental set-up. We discuss possible physical
scenarios to explain these findings.Comment: 4 pages, 3 figure
The fabrication of reproducible superconducting scanning tunneling microscope tips
Superconducting scanning tunneling microscope tips have been fabricated with
a high degree of reproducibility. The fabrication process relies on sequential
deposition of superconducting Pb and a proximity-coupled Ag capping layer onto
a Pt/Ir tip. The tips were characterized by tunneling into both normal-metal
and superconducting films. The simplicity of the fabrication process, along
with the stability and reproducibility of the tips, clear the way for tunneling
studies with a well-characterized, scannable superconducting electrode.Comment: 4 pages, 3 figures, REVTeX. Submitted to Rev. Sci. Instru
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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