Quantum behaviour of molecule-based magnets: basic aspects (quantum tunneling and quantum coherence) and applications (hardware for quantum computers and magnetic refrigeration). A tutorial

Abstract

The question addressed in this paper is that of the circumstances under which one can observe quantum tunneling and coherence of the magnetic moment of molecule-based magnets. These mesoscopic magnets are the only spin systems allowing a comparison between theory and experiment free of fitting parameters. Being intermediate between micro and macro, they allow us to study the border between classical and quantum mechanics. The magnetic hysteresis in these systems results from the existence of metastable states that correspond to the 2 S+1 different projections of the spin S of each molecule on its easy axis. The occurrence of magnetic relaxation at temperatures at which the thermal fluctuations die out is due to spin resonant tunneling between different S z states. At zero magnetic field and very low temperature, the only possible tunneling process would occur between the lowest states S z = S and S z =− S, which may be denoted as the states ‘yes’ and ‘no’, respectively. The mixing of such states may make it possible to detect their quantum superposition, and hence, the symmetric and antisymmetric combinations of such states. This is the spin analogue of Schrodinger's cat paradox. To conclude, I will discuss two potential realisations of quantum bits based on these molecular clusters and the possibility of using these systems as magnetic refrigerants in the Kelvin regime.