Quantum forgetfulness of a Single Molecule Magnet

vendredi 30 avril 2010

A step critical to the implementation of a quantum computer based on complex magnetic molecules (Single Molecule Magnets) is control of the decoherence mechanisms. Decoherence is the process by which a quantum system “forgets” its quantum nature and become classical when it interacts, even very weakly, with the random fl uctuations of its environment. Simply speaking, a collective quantum system possesses an “internal clock” acting like a pendulum whose amplitude decreases progressively because of these environmental interactions. When the clock’s motion stops, the system is eff ectively at rest and collapses to the classical regime.

This loss of coherence is a serious obstacle in the development of a quantum computer : any quantum calculation requires that the internal clocks of each single element of the computer (the quantum bits or “qubits”) oscillate for a sufficiently long time for a quantum calculation to be carried out. This has turned out to be a particularly diffi cult problem with complex “Single Molecule Magnets” of large dimensions (each molecule contains several hundred atoms). In a Single Molecule Magnet, the electronic spins are coupled to the large number of nuclear spins present in their own environment, killing the quantum oscillations very rapidly. Researchers from Néel Institute and the Institute for Nanoscience and Cryogenics of CEA-Grenoble have now been able to monitor the Rabi oscillations driven by microwaves for the famous low-spin molecular magnet V15, see Fig. 1. This is a vanadium-based system containing 15 vanadium ions with their spins strongly coupled through oxygen atoms. The 15 spins ½ of vanadium are all coupled antiferromagnetically resulting in a mesoscopic spin ½ in the ground state. The use of big molecules such as V15 should facilitate individual addressing, especially since molecular chemistry methods can produce self-organization of such molecules in well ordered monomolecular layers, easy to address. From pulsed Electron Paramagnetic Resonance (“spinecho”) measurements of coherence times, the different possible of decoherence have been evaluated. In this system, the main source of decoherence comes from the vanadium nuclear spins. If it were not for the nuclear spins, this Single Molecule Magnet would stay “quantumly alive” for 100 μs, long enough to implement simple molecular quantum gates. Such long coherence times could be obtained in molecular magnets if nuclear-spin-free molecules are used. This first demonstration of the Rabi oscillations of a Single Molecule Magnet has already generated several follow-ups using the same technique. Besides its potential applications in quantum computing, this result shows how robust quantum mechanics can be : A molecule with 215 quantum eigenstates involving complex interactions (super-exchange, dipolar and Dzyaloshinsky-Moriya interactions), can “stay alive” for a fraction of a microsecond. This work was done in collaboration with T. Mitra and A. Müller of the Faculty of Chemistry of Bielefeld University (Germany) and B. Tsukernblat of the Chemistry Department, Ben- Gurion University, Beer-Sheva (Israel). Fig. 1 : Schematic representation of the Single Molecule Magnet V15 (green=Vanadium, orange=Arsenic, red=Oxygen (there are many other atoms that are not represented). The curve shows the observed Rabi oscillation of the magnetization, excited by a pulse of 9.7 GHz microwaves in an EPR cavity in a 0.35 Tesla fi eld at 2.5 K. The damping of the oscillation is caused mainly by interaction with the vanadium nuclear spins.

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