458 research outputs found
Nuclear spin relaxation induced by a mechanical resonator
We report on measurements of the spin lifetime of nuclear spins strongly
coupled to a micromechanical cantilever as used in magnetic resonance force
microscopy. We find that the rotating-frame correlation time of the statistical
nuclear polarization is set by the magneto-mechanical noise originating from
the thermal motion of the cantilever. Evidence is based on the effect of three
parameters: (1) the magnetic field gradient (the coupling strength), (2) the
Rabi frequency of the spins (the transition energy), and (3) the temperature of
the low-frequency mechanical modes. Experimental results are compared to
relaxation rates calculated from the spectral density of the magneto-mechanical
noise.Comment: 4 pages, 4 figure
Quantum versus classical hyperfine-induced dynamics in a quantum dot
In this article we analyze spin dynamics for electrons confined to
semiconductor quantum dots due to the contact hyperfine interaction. We compare
mean-field (classical) evolution of an electron spin in the presence of a
nuclear field with the exact quantum evolution for the special case of uniform
hyperfine coupling constants. We find that (in this special case) the
zero-magnetic-field dynamics due to the mean-field approximation and quantum
evolution are similar. However, in a finite magnetic field, the quantum and
classical solutions agree only up to a certain time scale t<\tau_c, after which
they differ markedly.Comment: 6 pages, 1 figure, accepted for publication in the Journal of Applied
Physics (ICPS06 conference proceedings); v2: updated references, final
published versio
Phenomenological noise model for superconducting qubits: two-state fluctuators and 1=f noise
We present a general phenomenological model for superconducting qubits
subject to noise produced by two-state fluctuators whose couplings to the qubit
are all roughly the same. In flux qubit experiments where the working point can
be varied, it is possible to extract both the form of the noise spectrum and
the number of fluctuators. We find that the noise has a broad spectrum
consistent with 1=f noise and that the number of fluctuators with slow
switching rates is surprisingly small: less than 100. If the fluctuators are
interpreted as unpaired surface spins, then the size of their magnetic moments
is surprisingly large.Comment: 7 pages, 2 figures, 1 tabl
NMR Evidence for Charge Inhomogeneity in Stripe Ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4
We report ^{17}O Nuclear Magnetic Resonance (NMR) results in the stripe
ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4 system. Below a temperature T_q ~ 80K,
the local electric field gradient (EFG) and the absolute intensity of the NMR
signal of the planar O site exhibit a dramatic decrease. We interpret these
results as microscopic evidence for a spatially inhomogeneous charge
distribution, where the NMR signal from O sites in the domain walls of the spin
density modulation are wiped out due to large hyperfine fields, and the
remaining signal arises from the intervening Mott insulating regions.Comment: 4 pages, to appear in Phys. Rev. Let
Enhanced thermal stability and spin-lattice relaxation rate of N@C60 inside carbon nanotubes
We studied the temperature stability of the endohedral fullerene molecule,
N@C60, inside single-wall carbon nanotubes using electron spin resonance
spectroscopy. We found that the nitrogen escapes at higher temperatures in the
encapsulated material as compared to its pristine, crystalline form. The
temperature dependent spin-lattice relaxation time, T_1, of the encapsulated
molecule is significantly shorter than that of the crystalline material, which
is explained by the interaction of the nitrogen spin with the conduction
electrons of the nanotubes.Comment: 5 pages, 4 figures, 1 tabl
Rabi oscillations of a qubit coupled to a two-level system
The problem of Rabi oscillations in a qubit coupled to a fluctuator and in
contact with a heath bath is considered. A scheme is developed for taking into
account both phase and energy relaxation in a phenomenological way, while
taking full account of the quantum dynamics of the four-level system subject to
a driving AC field. Significant suppression of the Rabi oscillations is found
when the qubit and fluctuator are close to resonance. The effect of the
fluctuator state on the read-out signal is discussed. This effect is shown to
modify the observed signal significantly. This may be relevant to recent
experiments by Simmonds et al. [Phys. Rev. Lett. 93, 077003 (2004)].Comment: 4 pages, 4 figure
On the sensitivity of condensed-matter P- and T-violation experiments
Experiments searching for parity- and time-reversal-invariance-violating
effects that rely on measuring magnetization of a condensed-matter sample
induced by application of an electric field are considered. A limit on
statistical sensitivity arises due to random fluctuations of the spins in the
sample. The scaling of this limit with the number of spins and their relaxation
time is derived. Application to an experiment searching for nuclear Schiff
moment in a ferroelectric is discussed.Comment: 6 pages, no figure
Collective Decoherence of Nuclear Spin Clusters
The problem of dipole-dipole decoherence of nuclear spins is considered for
strongly entangled spin cluster. Our results show that its dynamics can be
described as the decoherence due to interaction with a composite bath
consisting of fully correlated and uncorrelated parts. The correlated term
causes the slower decay of coherence at larger times. The decoherence rate
scales up as a square root of the number of spins giving the linear scaling of
the resulting error. Our theory is consistent with recent experiment reported
in decoherence of correlated spin clusters.Comment: 4 pages, 4 figure
Randomized benchmarking and process tomography for gate errors in a solid-state qubit
We present measurements of single-qubit gate errors for a superconducting
qubit. Results from quantum process tomography and randomized benchmarking are
compared with gate errors obtained from a double pi pulse experiment.
Randomized benchmarking reveals a minimum average gate error of 1.1+/-0.3% and
a simple exponential dependence of fidelity on the number of gates. It shows
that the limits on gate fidelity are primarily imposed by qubit decoherence, in
agreement with theory.Comment: 4 pages, 4 figures, plus supplementary materia
Breakdown of the Korringa Law of Nuclear Spin Relaxation in Metallic GaAs
We present nuclear spin relaxation measurements in GaAs epilayers using a new
pump-probe technique in all-electrical, lateral spin-valve devices. The
measured T1 times agree very well with NMR data available for T > 1 K. However,
the nuclear spin relaxation rate clearly deviates from the well-established
Korringa law expected in metallic samples and follows a sub-linear temperature
dependence 1/T1 ~ T^0.6 for 0.1 K < T < 10 K. Further, we investigate nuclear
spin inhomogeneities.Comment: 5 pages, 4 (color) figures. arXiv admin note: text overlap with
arXiv:1109.633
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