2,242 research outputs found
Observation of quantum jumps in a superconducting artificial atom
A continuously monitored quantum system prepared in an excited state will
decay to its ground state with an abrupt jump. The jump occurs stochastically
on a characteristic time scale T1, the lifetime of the excited state. These
quantum jumps, originally envisioned by Bohr, have been observed in trapped
atoms and ions, single molecules, photons, and single electrons in cyclotrons.
Here we report the first observation of quantum jumps in a macroscopic quantum
system, in our case a superconducting "artificial atom" or quantum bit (qubit)
coupled to a superconducting microwave cavity. We use a fast, ultralow-noise
parametric amplifier to amplify the microwave photons used to probe the qubit
state, enabling continuous high-fidelity monitoring of the qubit. This
technique represents a major step forward for solid state quantum information
processing, potentially enabling quantum error correction and feedback, which
are essential for building a quantum computer. Our technology can also be
readily integrated into hybrid circuits involving molecular magnets, nitrogen
vacancies in diamond, or semiconductor quantum dots.Comment: Updated draft including supplementary information. 8 pages, 6
figures. Supplementary videos are available on our website at
http://physics.berkeley.edu/research/siddiqi/docs/supps
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
Cavity-assisted quantum bath engineering
We demonstrate quantum bath engineering for a superconducting artificial atom
coupled to a microwave cavity. By tailoring the spectrum of microwave photon
shot noise in the cavity, we create a dissipative environment that autonomously
relaxes the atom to an arbitrarily specified coherent superposition of the
ground and excited states. In the presence of background thermal excitations,
this mechanism increases the state purity and effectively cools the dressed
atom state to a low temperature
Heralded state preparation in a superconducting qubit
We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a
superconducting flux qubit in which the pointer state distributions can be
resolved to below one part in 1000. In the weak excitation regime, continuous
measurement permits the use of heralding to ensure initialization to a fiducial
state, such as the ground state. This procedure boosts readout fidelity to
93.9% by suppressing errors due to spurious thermal population. Furthermore,
heralding potentially enables a simple, fast qubit reset protocol without
changing the system parameters to induce Purcell relaxation.Comment: 5 pages, 5 figure
Resonance fluorescence from an artificial atom in squeezed vacuum
We present an experimental realization of resonance fluorescence in squeezed
vacuum. We strongly couple microwave-frequency squeezed light to a
superconducting artificial atom and detect the resulting fluorescence with high
resolution enabled by a broadband traveling-wave parametric amplifier. We
investigate the fluorescence spectra in the weak and strong driving regimes,
observing up to 3.1 dB of reduction of the fluorescence linewidth below the
ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum
on the relative phase of the driving and squeezed vacuum fields. Our results
are in excellent agreement with predictions for spectra produced by a two-level
atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)],
demonstrating that resonance fluorescence offers a resource-efficient means to
characterize squeezing in cryogenic environments
Improvement of isometric dorsiflexion protocol for assessment of tibialis anterior muscle
It is important to accurately estimate the electromyogram (EMG)/force relationship of triceps surae (TS) muscle for detecting strength deficit of tibalis anterior (TA) muscle. In literature, the protocol for recording EMG and force of dorsiflexion have been described, and the necessity for immobilizing the ankle has been explained. However, there is a significant variability of the results among researchers even though they report the fixation of the ankle. We have determined that toe extension can cause significant variation in the dorsiflexion force and EMG of TS and this can occur despite following the current guidelines which require immobilizing the ankle. The results also show that there was a large increase in the variability of the force and the RMS of EMG of TS when the toes were not strapped compared with when they were strapped. Thus, with the current guidelines, where there are no instructions regarding the necessity of strapping the toes, the EMG/force relationship of TS could be incorrect and give an inaccurate assessment of the dorsiflexor TA strength. In summary, - Current methodology to estimate the dorsiflexor TA strength with respect to the TS activity, emphasizing on ankle immobilization is insufficient to prevent large variability in the measurements. - Toe extension during dorsiflexion was found to be one source of variability in estimating the TA strength. - It is recommended that guidelines for recording force and EMG from TA and TS muscles should require the strapping of the toes along with the need for immobilizing the ankle
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
- …