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Register a new userThe Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by Strong Pi Pulses
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In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as delta-functions. In NMR, this assumption leads to the prediction that pi pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one pi pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between pi pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments that set limits on possible extrinsic causes for the phenomena are reported. We find that the action of the systems internal Hamiltonian during any real pulse is sufficient to cause the effects. Exact numerical calculations, combined with average Hamiltonian theory, identify novel terms that are sensitive to parameters such as pulse phase, dipolar coupling, and system size. Visualization of the entire density matrix shows a unique flow of quantum coherence from non-observable to observable channels when applying repeated pi pulses.
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This submission has been withdrawn by arXiv administrators because it is a duplicate of 0705.0667.
NMR spin echo measurements of C-13 in C60, Y-89 in Y2O3, and Si-29 in silicon are shown to defy conventional expectations when more that one pi pulse is used. Multiple pi-pulse echo trains may either freeze our or accelerate the decay of the signal, depending on the pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.
We demonstrate that CPMG and XYXY decoupling sequences with non-ideal $pi$ pulses can reduce dipolar interactions between spins of the same species in solids. Our simulations of pulsed electron spin resonance (ESR) experiments show that $pi$ rotations with small ($<$~10%) imperfections refocus instantaneous diffusion. Here, the intractable N-body problem of interacting dipoles is approximated by the average evolution of a single spin in a changing mean field. These calculations agree well with experiments and do not require powerful hardware. Our results add to past attempts to explain similar phenomena in solid state nuclear magnetic resonance (NMR). Although the fundamental physics of NMR are similar to ESR, the larger linewidths in ESR and stronger dipolar interactions between electron spins compared to nuclear spins preclude drawing conclusions from NMR studies alone. For bulk spins, we also find that using XYXY results in less inflation of the deduced echo decay times as compared to decays obtained with CPMG.
We study finite-time Landau-Zener transitions at a singlet-triplet level crossing in a GaAs double quantum dot, both experimentally and theoretically. Sweeps across the anticrossing in the high driving speed limit result in oscillations with a small visibility. Here we demonstrate how to increase the oscillation visibility while keeping sweep times shorter than T2* using a tailored pulse with a detuning dependent level velocity. Our results show an improvement of a factor ~2.9 for the oscillation visibility. In particular, we were able to obtain a visibility of ~0.5 for Stuckelberg oscillations, which demonstrates the creation of an equally weighted superposition of the qubit states.
The inhomogeneity of an electron spin ensemble as well as fluctuating environment acting upon individual spins drastically shorten the spin coherence time $T_2$ and hinder coherent spin manipulation. We show that this problem can be solved by the simultaneous application of a radiofrequency (rf) field, which stimulates coherent spin precession decoupled from an inhomogeneous environment, and periodic optical pulses, which amplify this precession. The resulting resonance, taking place when the rf field frequency approaches the laser pulse repetition frequency, has a width determined by the spin coherence time $T_2$ that is free from the inhomogeneity effects. We measure a 50-Hz-narrow electron spin resonance and milliseconds-long $T_2$ for electrons in the ground state of Ce$^{3+}$ ions in the YAG lattice at low temperatures, while the inhomogeneous spin dephasing time $T_2^*$ is only 25 ns. This study paves the way to coherent optical manipulation in spin systems decoupled from their inhomogeneous environment.
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