31 research outputs found
Nonlinear microwave response of aluminum weak-link Josephson oscillators
We present the driven response at T=30mK of 6 GHz superconducting resonators
constructed from capacitively-shunted three dimensional (3D) aluminum
nanobridge superconducting quantum interference devices (nanoSQUIDs). We
observe flux modulation of the resonant frequency in quantitative agreement
with numerical calculation and characteristic of near-ideal short weak link
junctions. Under strong microwave excitation, we observe stable bifurcation in
devices with coupled quality factor (Q) ranging from ~30-3500. Near this bias
point, parametric amplification with > 20dB gain, 40 MHz bandwidth, and near
quantum-limited noise performance is observed. Our results indicate that 3D
nanobridge junctions are attractive circuit elements to realize quantum bits.Comment: 3 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
Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers
Two distinct types of magnetoresistance oscillations are observed in two
electronic Fabry-Perot interferometers of different sizes in the integer
quantum Hall regime. Measuring these oscillations as a function of magnetic
field and gate voltages, we observe three signatures that distinguish the two
types. The oscillations observed in a 2.0 square micron device are understood
to arise from the Coulomb blockade mechanism, and those observed in an 18
square micron device from the Aharonov-Bohm mechanism. This work clarifies,
provides ways to distinguish, and demonstrates control over, these distinct
physical origins of resistance oscillations seen in electronic Fabry-Perot
interferometers.Comment: related papers at http://marcuslab.harvard.ed
Suppression of crosstalk in superconducting qubits using dynamical decoupling
Currently available superconducting quantum processors with interconnected
transmon qubits are noisy and prone to various errors. The errors can be
attributed to sources such as open quantum system effects and spurious
inter-qubit couplings (crosstalk). The ZZ-coupling between qubits in fixed
frequency transmon architectures is always present and contributes to both
coherent and incoherent crosstalk errors. Its suppression is therefore a key
step towards enhancing the fidelity of quantum computation using transmons.
Here we propose the use of dynamical decoupling to suppress the crosstalk, and
demonstrate the success of this scheme through experiments performed on several
IBM quantum cloud processors. In particular, we demonstrate improvements in
quantum memory as well as the performance of single-qubit and two-qubit gate
operations. We perform open quantum system simulations of the multi-qubit
processors and find good agreement with the experimental results. We analyze
the performance of the protocol based on a simple analytical model and
elucidate the importance of the qubit drive frequency in interpreting the
results. In particular, we demonstrate that the XY4 dynamical decoupling
sequence loses its universality if the drive frequency is not much larger than
the system-bath coupling strength. Our work demonstrates that dynamical
decoupling is an effective and practical way to suppress crosstalk and open
system effects, thus paving the way towards higher-fidelity logic gates in
transmon-based quantum computers.Comment: Updated version includes additional results on improving the
performance of single and two-qubit gates using dynamical decoupling. 22
pages and 12 figure
Edge-State Velocity and Coherence in a Quantum Hall Fabry-Perot Interferometer
We investigate nonlinear transport in electronic Fabry-Perot interferometers
in the integer quantum Hall regime. For interferometers sufficiently large that
Coulomb blockade effects are absent, a checkerboard-like pattern of conductance
oscillations as a function of dc bias and perpendicular magnetic field is
observed. Edge-state velocities extracted from the checkerboard data are
compared to model calculations and found to be consistent with a crossover from
skipping orbits at low fields to E x B drift at high fields. Suppression of
visibility as a function of bias and magnetic field is accounted for by
including energy- and field-dependent dephasing of edge electrons.Comment: related papers at http://marcuslab.harvard.ed