Month: April 2012

Shift-driven modulations of spin-echo signals | Proc. Natl. Acad. Sci. U. S. A. 109, 5958 (2012).

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Pieter E. S. Smith, Guy Bensky, Gonzalo A. Álvarez, Gershon Kurizki, and Lucio Frydman

Abstract:

Since the pioneering works of Carr-Purcell and Meiboom-Gill [Carr HY, Purcell EM (1954) Phys Rev 94:630; Meiboom S, Gill D (1985) Rev Sci Instrum 29:688], trains of π-pulses have featured amongst the main tools of quantum control. Echo trains find widespread use in nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI), thanks to their ability to free the evolution of a spin-1/2 from several sources of decoherence. Spin echoes have also been researched in dynamic decoupling scenarios, for prolonging the lifetimes of quantum states or coherences. Inspired by this search we introduce a family of spin-echo sequences, which can still detect site-specific interactions like the chemical shift. This is achieved thanks to the presence of weak environmental fluctuations of common occurrence in high-field NMR—such as homonuclear spin-spin couplings or chemical/biochemical exchanges. Both intuitive and rigorous derivations of the resulting “selective dynamical recoupling” sequences are provided. Applications of these novel experiments are given for a variety of NMR scenarios including determinations of shift effects under inhomogeneities overwhelming individual chemical identities, and model-free characterizations of chemically exchanging partners.chemical exchange dynamic decoupling magnetic field inhomogeneity magnetic resonance quantum control

via Proc. Natl. Acad. Sci. U. S. A. 109, 5958 (2012): Shift-driven modulations of spin-echo signals.

Behavior observed for the illustrated compound (Cynnamic acid) upon implementing the proposed selective dynamical recoupling (SDR) sequence, as mediated by homonuclear 1H-1H couplings, for the indicated parameters. Experiments (black traces) are compared against simulations (red) using the indicated parameters, and analytical curves (blue) arise from the two-site modulation predicted by our results. (A) x-dependence observed for the isolated olefinic proton pair of Cynnamic acid at high-resolution. (B) Idem but for the Cynnamic acid placed in a grossly inhomogeneous magnetic field (shimming coils off), illustrating SDR’s ability to retrieve high resolution shift modulations. While it relies on fully refocused π-pulse trains that normally cancel also the shift modulations, by the assistance of slowly fluctuations due to the homonuclear 1H-1H couplings the shift modulation are selectively reintroduced.
Behavior observed for the illustrated compound (Cynnamic acid) upon implementing the proposed selective dynamical recoupling (SDR) sequence, as mediated by homonuclear 1H-1H couplings, for the indicated parameters. Experiments (black traces) are compared against simulations (red) using the indicated parameters, and analytical curves (blue) arise from the two-site modulation predicted by our results. (A) x-dependence observed for the isolated olefinic proton pair of Cynnamic acid at high-resolution. (B) Idem but for the Cynnamic acid placed in a grossly inhomogeneous magnetic field (shimming coils off), illustrating SDR’s ability to retrieve high resolution shift modulations. While it relies on fully refocused π-pulse trains that normally cancel also the shift modulations, by the assistance of slowly fluctuations due to the homonuclear 1H-1H couplings the shift modulation are selectively reintroduced.

Copyright ©2012 by the National Academy of Sciences

Controlling Spin-Spin Network Dynamics by Repeated Projective | Phys. Rev. Lett. 108, 140403 (2012)

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Christian O. Bretschneider (1), Gonzalo A. Álvarez (2), Gershon Kurizki (1), and Lucio Frydman (1)

(1) Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel

(2) Fakultät Physik, Technische Universität Dortmund, D-44221 Dortmund, Germany

Received 13 September 2011; published 3 April 2012

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network’s evolution, the strategy hereby presented derives from the “quantum Zeno effect” and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.

© 2012 American Physical Society

via Phys. Rev. Lett. 108, 140403 (2012): Controlling Spin-Spin Network Dynamics by Repeated Projective Measurements.

Polarization evolutions in Pyridine. Different symbols are different spins. a) Free evolution (symbols: experiments, lines: simulations). b) Repetitively measured evolution for different delays between measurements. This experimental data  illustrate the switch of the dynamics shown in (a) to quasimonotonic polarization transfers, as a result of introducing repeated projective measurements that involve instantaneous erasements of the off-diagonal density-matrix terms at intervals tau. This tailored dynamics allows for a simple determination of the spin-spin coupling network topology.
Polarization evolutions in Pyridine. Different symbols are different spins. a) Free evolution (symbols: experiments, lines: simulations). b) Repetitively measured evolution for different delays between measurements. This experimental data illustrate the switch of the dynamics shown in (a) to quasimonotonic polarization transfers, as a result of introducing repeated projective measurements that involve instantaneous erasements of the off-diagonal density-matrix terms at intervals tau. This tailored dynamics allows for a simple determination of the spin-spin coupling network topology.