3,264 research outputs found
Force-detected nuclear double resonance between statistical spin polarizations
We demonstrate nuclear double resonance for nanometer-scale volumes of spins
where random fluctuations rather than Boltzmann polarization dominate. When the
Hartmann-Hahn condition is met in a cross-polarization experiment, flip-flops
occur between two species of spins and their fluctuations become coupled. We
use magnetic resonance force microscopy to measure this effect between 1H and
13C spins in 13C-enriched stearic acid. The development of a cross-polarization
technique for statistical ensembles adds an important tool for generating
chemical contrast in nanometer-scale magnetic resonance.Comment: 14 pages, 4 figure
Unusual decoherence in qubit measurements with a Bose-Einstein condensate
We consider an electrostatic qubit located near a Bose-Einstein condensate
(BEC) of noninteracting bosons in a double-well potential, which is used for
qubit measurements. Tracing out the BEC variables we obtain a simple analytical
expression for the qubit's density-matrix. The qubit's evolution exhibits a
slow () damping of the qubit's coherence term, which however
turns to be a Gaussian one in the case of static qubit. This stays in contrast
to the exponential damping produced by most classical detectors. The
decoherence is, in general, incomplete and strongly depends on the initial
state of the qubit.Comment: 5 pages, additional explanations related to experimental realization
are added, typos corrected, Phys. Rev. A, in pres
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
How to Probe for Dynamical Structure in the Collapse of Entangled States Using Nuclear Magnetic Resonance
The spin state of two magnetically inequivalent protons in contiguous atoms
of a molecule becomes entangeled by the indirect spin-spin interaction
(j-coupling). The degree of entanglement oscillates at the beat frequency
resulting from the splitting of a degeneracy. This beating is manifest in NMR
spectroscopy as an envelope of the transverse magnetization and should be
visible in the free induction decay signal. The period (approximately 1 sec) is
long enough for interference between the linear dynamics and collapse of the
wave-function induced by a Stern-Gerlach inhomogeneity to significantly alter
the shape of that envelope. Various dynamical collapse theories can be
distinguished by their observably different predictions with respect to this
alteration. Adverse effects of detuning due to the Stern-Gerlach inhomogeneity
can be reduced to an acceptable level by having a sufficiently thin sample or a
strong rf field.Comment: 6 pages, 4 figures, PDF, submitted to PR
Gate-controlled nuclear magnetic resonance in an AlGaAs/GaAs quantum Hall device
We study the resistively detected nuclear magnetic resonance (NMR) in an
AlGaAs/GaAs quantum Hall device with a side gate. The strength of the hyperfine
interaction between electron and nuclear spins is modulated by tuning a
position of the two-dimensional electron systems with respect to the polarized
nuclear spins using the side-gate voltages. The NMR frequency is systematically
controlled by the gate-tuned technique in a semiconductor device.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
Backscattering Between Helical Edge States via Dynamic Nuclear Polarization
We show that that the non-equilibrium spin polarization of one dimensional
helical edge states at the boundary of a two dimensional topological insulator
can dynamically induce a polarization of nuclei via the hyperfine interaction.
When combined with a spatially inhomogeneous Rashba coupling, the steady state
polarization of the nuclei produces backscattering between the topologically
protected edge states leading to a reduction in the conductance which persists
to zero temperature. We study these effects in both short and long edges,
uncovering deviations from Ohmic transport at finite temperature and a current
noise spectrum which may hold the fingerprints for experimental verification of
the backscattering mechanism.Comment: 4+ pages, 4 figure
Smile: A Simple Diagnostic for Selection on Observables
This paper develops a simple diagnostic for the selection on observables assumption in the case of a binary treatment variable. I show that, under common assumptions, when selection on observables does not hold, designs based on selection on observables will estimate treatment effects approaching infinity or negative infinity among observations with propensity scores close to 0 or 1. Researchers can check for violations of selection on observables either informally by looking for a "smile" shape in a binned scatterplot, or with a simple formal test. When selection on observables fails, the researcher can detect the sign of the resulting bias
Smile: A Simple Diagnostic for Selection on Observables
This paper develops a simple diagnostic for the selection on observables assumption in the case of a binary treatment variable. I show that, under common assumptions, when selection on observables does not hold, designs based on selection on observables will estimate treatment effects approaching infinity or negative infinity among observations with propensity scores close to 0 or 1. Researchers can check for violations of selection on observables either informally by looking for a "smile" shape in a binned scatterplot, or with a simple formal test. When selection on observables fails, the researcher can detect the sign of the resulting bias
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