Month: June 2010

Perfect state transfers by selective quantum interferences within complex spin networks | Phys. Rev. A 81, 060302 (2010)

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Perfect state transfers by selective quantum interferences within complex spin networks

Gonzalo A. Álvarez1,*, Mor Mishkovsky2,†, Ernesto P. Danieli1,‡, Patricia R. Levstein1, Horacio M. Pastawski1, and Lucio Frydman2,§

1Facultad de Matemática, Astronomía y Física, Universidad Nacional de Córdoba, 5000 Córdoba, Argentina

2Department of Chemical Physics, Weizmann Institute of Sciences, 76100 Rehovot, Israel

[Rapid Communication] Received 14 August 2009; published 15 June 2010

We present a method that implements directional, perfect state transfers within a branched spin network by exploiting quantum interferences in the time domain. This method provides a tool for isolating subsystems from a large and complex one. Directionality is achieved by interrupting the spin-spin coupled evolution with periods of free Zeeman evolutions, whose timing is tuned to be commensurate with the relative phases accrued by specific spin pairs. This leads to a resonant transfer between the chosen qubits and to a detuning of all remaining pathways in the network, using only global manipulations. Since the transfer is perfect when the selected pathway is mediated by two or three spins, distant state transfers over complex networks can be achieved by successive recouplings among specific pairs or triads of spins. These effects are illustrated with a quantum simulator involving 13C NMR on leucine’s backbone; a six-spin network.

© 2010 The American Physical Society

via Phys. Rev. A 81, 060302 (2010): Perfect state transfers by selective quantum interferences within complex spin networks.

NMR experiments (points) and numerical expectations (lines) for selective state transfers implemented for different situations by using the NMR pulse sequence illustrated in the lower right corner, derived in turn from Fig. 1(d). (a) An initial excitation on α is transferred to the β spin. (b) An initial Cβ excitation is optimally transferred to δ2 via the β − γ − δ2 pathway. (c) An initial Cβ excitation is selectively transferred to δ1 via the β − γ − δ1 pathway. In all cases, the rf carrier frequency was set on-resonance with Cβ ; all polarizations thus appeared evolving in a rotating frame that precessed with the Cβ frequency.
NMR experiments (points) and numerical expectations (lines) for selective state transfers implemented for different situations by using the NMR pulse sequence illustrated in the lower right corner, derived in turn from Fig. 1(d). (a) An initial excitation on α is transferred to the β spin. (b) An initial Cβ excitation is optimally transferred to δ2 via the β − γ − δ2 pathway. (c) An initial Cβ excitation is selectively transferred to δ1 via the β − γ − δ1 pathway. In all cases, the rf carrier frequency was set on-resonance with Cβ ; all polarizations thus appeared evolving in a rotating frame that precessed with the Cβ frequency.
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NMR Quantum Simulation of Localization Effects Induced by Decoherence | Phys. Rev. Lett. 104, 230403 (2010)

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NMR Quantum Simulation of Localization Effects Induced by Decoherence

Gonzalo A. Álvarez* and Dieter Suter†

Fakultät Physik, Universität Dortmund, Otto-Hahn-Strasse 4, D-44221 Dortmund, Germany

Received 21 January 2010; published 9 June 2010

The loss of coherence in quantum mechanical superposition states limits the time for which quantum information remains useful. Similarly, it limits the distance over which quantum information can be transmitted. Here, we investigate in a nuclear spin-based quantum simulator, the localization of the size of spin clusters that are generated by a Hamiltonian driving the transmission of information, while a variable-strength perturbation counteracts the spreading. We find that the system reaches a dynamic equilibrium size, which decreases with the square of the perturbation strength.

© 2010 The American Physical Society

via Phys. Rev. Lett. 104, 230403 (2010): NMR Quantum Simulation of Localization Effects Induced by Decoherence.

(a) Time evolution of the cluster size. The black squares represent the unperturbed time evolution and the other symbols correspond to different perturbation strengths according to the legend. (b),(c) Distributions of the amplitudes of the multiple quantum coherences of the density matrix for unperturbed dynamics ((b), p=0) and a perturbed evolution ((c), p=0.108), respectively. The perturbed evolution in (c) shows localization at a cluster size Kloc=56 spins.
(a) Time evolution of the cluster size. The black squares represent the unperturbed time evolution and the other symbols correspond to different perturbation strengths according to the legend. (b),(c) Distributions of the amplitudes of the multiple quantum coherences of the density matrix for unperturbed dynamics ((b), p=0) and a perturbed evolution ((c), p=0.108), respectively. The perturbed evolution in (c) shows localization at a cluster size Kloc=56 spins.