EFFECTS OF A SINGLE QUANTUM SPIN ON JOSEPHSON OSCILLATIONS

We consider a single localized spin-1/2 placed between singlet superconducting leads of a Josephson junction inserted in a superconducting ring, subject to a dc magnetic field B || z. Turning on the tunneling or a time-dependent flux in a superconducting ring, induces oscillations of the Josephson current, with an amplitude sensitive to the initial value of the z-component of the spin, Sz = ±1/2, which allows for a measurement of the initial spin state. At low temperatures when effects of quasiparticles are negligible, this procedure realizes a quantum-non-demolition (QND) measurement of Sz.

Coherent Response Signals of Dipolar-coupled Spin Systems

Recently, it has been demonstrated that long pulses of a weak radio-frequency field can generate long-lived coherent NMR signals in bulk liquid crystals, which are systems of dipolar-coupled spins with unresolved conventional spectra. Here we describe this phenomenon in more detail and present results of new experimental investigations and computer simulations. It is shown that such response signals can also be excited when the initial spin state of a system corresponds to dipolar ordering. In addition, results of the application of weak pulses on liquid crystalline systems with heteronuclear dipolar couplings are presented, and the role of overlapping peaks is explored.