Prof. Jonathan Friedman – Amherst College
“Single-Molecule Nanomagnets: Tunneling, Interference and Quantum Computing”
Single-molecule magnets are ideal systems in which to study quantum phenomena on the mesoscopic scale. Composed of a handful of magnetic ions, they straddle the fence between the classical and quantum worlds, showing hysteresis effects characteristic of classical bulk magnets as well as quantum tunneling of magnetization in which the magnetization vector reverses direction by passing through a classically forbidden region. In this talk, I will review some of the more interesting phenomena found in these systems by several research groups. These include resonant tunneling between degenerate magnetization states, interference between tunneling paths that can lead to a suppression of tunneling, and exchange biasing produced by coupled molecular magnets. I will conclude by exploring the prospects for these systems to become viable qubits, the processing elements of quantum computers.
One of the most interesting questions in quantum mechanics is whether macroscopic quantities representing the collective motion of many degrees of freedom (e.g. the center of mass of a baseball) can behave quantum mechanically. Can macroscopic coordinates tunnel through an energy barrier or be put into a superposition of macroscopically distinct states (like Schroedinger’s unfortunate cat)? Two examples of such coordinates are the magnetic moment of a magnetic molecule and the flux inside a SQUID (superconducting quantum interference device). I will review the experimental evidence for resonant tunneling of the macroscopic coordinate in both of these systems. Both systems can be modelled as double-well potentials with energy levels localized in each well. An external bias tilts the potential and when two levels in opposite wells line up, there is enhanced interwell relaxation, a phenomenon known as resonant tunneling. In the SQUID system, external microwaves can drive transitions between levels and produce population inversion effects and photon-assisted tunneling, allowing a spectroscopic study of the level structure. I will describe very recent experiments that have detected evidence of a superposition of macroscopically distinct flux states in the SQUID — Schroedinger’s SQUID!