Nondemolition Measurements of a Single Quantum Spin using Josephson Oscillations

We consider a single localized spin-$1/2$ between the singlet superconducting leads of a Josephson junction (e.g., a superconducting STM). For the spin subject to a dc magnetic field $\mathbf{B}\parallel z$, we study the spin dynamics and the possibility to measure the spin state via transport through the junction embedded in a dissipative circuit. Turning on the tunneling or a voltage bias induces oscillations of the Josephson current, with an amplitude sensitive to the initial value of the $z$ component of the spin, $S_z=\pm 1/2$. At low temperatures, when effects of quasiparticles are negligible, this procedure realizes a quantum nondemolition measurement of $S_z$.

Figure 1

The circuit containing a Josephson junction with a spin. The tip of the upper lead is of atomic area as in an STM setup.