Quantum Coherent Properties of Spins

Chemistry Department, Tulane University
New Orleans, LA
November 14-16, 2008

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A spin qubit flipping states also flips the field acting on nearby nuclear spins between two orientations (shown as arrows). The nuclear spin vector follows a path conditional on the specific trajectory of the qubit, so that they are entangled.

Early work on the dynamics of ensembles of tunneling magnetic molecules established the main features of their quantum relaxation; the role of inter-molecular dipolar interactions, the influence of nuclear spins, and the way in which an external applied field influences the tunneling of individual molecular spins. Some questions still remain open - there is no proper theory of the relaxation dynamics under fast sweeping of the external field, and many interesting features of the experimental results remain unexplained. At the same time there has been enormous progress in synthesizing and exploring the properties of many different molecules in the last decade.

However the frontier has now shifted to the understanding of the coherent quantum dynamics of these systems, either as individual molecules, or as interacting ensembles. Experiments must now examine the resonance properties of these systems, looking for coherent Rabi oscillations or, better still, spin echo results. Undoubtedly one of the reasons for interest in this is the possibility that these molecules might be good candidates for qubits in some quantum information processing system. The main theoretical problem to understand here is decoherence in the molecules spin dynamics, caused by nuclear spins, phonons, and dipolar interactions; this problem is very complex. Experiments are thus probing the coupled dynamics of pairs of molecules, which is quite intricate - direct probing of entanglement in their dynamics should soon be achieved in such experiments.

This shift in emphasis has imposed new demands on the kinds of molecules that are interesting. At the same time chemists have been making a wide variety of new molecular magnets, no longer based exclusively on transition metal ions but also incorporating rare earths. These new systems will certainly open the door to new kinds of experiment, requiring different theoretical perspectives.

This meeting will bring together chemists with experimental and theoretical physicists interested in various aspects of these remarkable systems, which have in the last few years suggested quite new lines of research in quantum nanoscience.

Examples of magnetic molecules. Top: Fe8. Eight s=5/2 FeIII ions are forming a total spin S=10 (six spins are parallel and antiparallel to the other two); middle: Ni12 with a total spin S=12; bottom: Mn12 with a total spin S=10.