Month: May 2007

Decoherence of many-spin systems in NMR: From molecular characterization to an environmentally induced quantum dynamical phase transition | arXiv.org

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Decoherence of many-spin systems in NMR: From molecular characterization to an environmentally induced quantum dynamical phase transition

Gonzalo A. Alvarez

(Submitted on 15 May 2007)

The control of open quantum systems has a fundamental relevance for fields ranging from quantum information processing to nanotechnology. Typically, the system whose coherent dynamics one wants to manipulate, interacts with an environment that smoothly degrades its quantum dynamics. Thus, a precise understanding of the inner mechanisms of this process, called “decoherence”, is critical to develop strategies to control the quantum dynamics. In this thesis we solved the generalized Liouville-von Neumann quantum master equation to obtain the dynamics of many-spin systems interacting with a spin bath. We also solve the spin dynamics within the Keldysh formalism. Both methods lead to identical solutions and together gave us the possibility to obtain numerous physical predictions that contrast well with Nuclear Magnetic Resonance experiments. We applied these tools for molecular characterizations, development of new numerical methodologies and the control of quantum dynamics in experimental implementations. But, more important, these results contributed to fundamental physical interpretations of how quantum dynamics behaves in open systems. In particular, we found a manifestation of an environmentally induced quantum dynamical phase transition.

via [0705.2350] Decoherence of many-spin systems in NMR: From molecular characterization to an environmentally induced quantum dynamical phase transition.

(left panel) Raw experimental data of 13C polarization in Fe(C5H5)2 as a function of the contact time and spin-spin coupling b for a spin swapping dynamics in a 13C-1H system. (right panel) Projection plot in the b − t plane show a canyon where the oscillation period diverges indicating a Quantum Dynamical Phase Transition.
(left panel) Raw experimental data of 13C polarization in Fe(C5H5)2 as a function of the contact time and spin-spin coupling b for a spin swapping dynamics in a 13C-1H system. (right panel) Projection plot in the b − t plane show a canyon where the oscillation period diverges indicating a Quantum Dynamical Phase Transition.

Signatures of a quantum dynamical phase transition in a three-spin system in presence of a spin environment 10.1016/j.physb.2007.04.083 : Physica B: Condensed Matter | ScienceDirect.com

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Signatures of a quantum dynamical phase transition in a three-spin system in presence of a spin environment

Gonzalo A. Álvarez, Patricia R. Levstein, Horacio M. Pastawski

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

Available online 5 May 2007.

via Signatures of a quantum dynamical phase transition in a three-spin system in presence of a spin environment 10.1016/j.physb.2007.04.083 : Physica B: Condensed Matter | ScienceDirect.com.

Abstract

We have observed an environmentally induced quantum dynamical phase transition in the dynamics of a two-spin experimental swapping gate [G.A. Álvarez, E.P. Danieli, P.R. Levstein, H.M. Pastawski, J. Chem. Phys. 124 (2006) 194507]. There, the exchange of the coupled states |↑,↓〉 and |↓,↑〉 gives an oscillation with a Rabi frequency b/ℏ (the spin-spin coupling). The interaction, ℏ/τSE with a spin-bath degrades the oscillation with a characteristic decoherence time. We showed that the swapping regime is restricted only to bτSE≳ℏ. However, beyond a critical interaction with the environment the swapping freezes and the system enters to a Quantum Zeno dynamical phase where relaxation decreases as coupling with the environment increases. Here, we solve the quantum dynamics of a two-spin system coupled to a spin-bath within a Liouville–von Neumann quantum master equation and we compare the results with our previous work within the Keldysh formalism. Then, we extend the model to a three interacting spin system where only one is coupled to the environment. Beyond a critical interaction the two spins not coupled to the environment oscillate with the bare Rabi frequency and relax more slowly. This effect is more pronounced when the anisotropy of the system-environment (SE) interaction goes from a purely XY to an Ising interaction form.