On May 16, 2014, at 14:00, in the library of the Institute of Physics Belgrade, Dimitrije Stepanenko (Center for Solid State Physics and New Materials, Institute of Physics Belgrade) presents a seminar talk entitled:

"Superradiant phase transition and coherence in triangular spin clusters"

Abstract: Molecular antiferromagnets with broken inversion symmetry have well defined chirality of spin texture in the low-energy sector.  This degree of freedom shows some promising properties for applications in quantum information processing: long coherence times for quantum error correction, interaction with electric fields for design of quantum logic gates, and the control of ground-state field in cavity quantum electrodynamics.

Hyperfine-induced decoherence in a triangular spin cluster varies across independent two-level subsystems that encode a qubit.  Electrically controllable eigenstates of spin chirality show decoherence times that approach milliseconds, two orders of magnitude longer than those estimated for the eigenstates of the total spin projection and of the partial spin sums. The robustness of chirality is due to its decoupling from components of both the total spin and individual spins in the cluster. This results in a suppression of the effective interaction between chirality and the nuclear spin bath.

A crystal of triangular molecular antiferromagnets coupled to a resonant cavity shows superradiant phase transition.  The critical coupling strength for transition depends on the external magnetic field, in sharp contrast to the standard case of two-level emitters, where the critical coupling was set by the structure of emitter alone.  The source of modification is traced to the entanglement of spin and chirality in the low-energy states of the cluster. We propose that the number of photons in the cavity can be switched between zero and some macroscopic value.  This effect can be used to detect spin-electric coupling in molecules.