Detecting spins

The resonance spectroscopy of electron and nuclear spins has numerous applications in a variety of fields but suffers from limited sensitivity. The use of superconducting quantum circuits at very low temperature is a means of significantly improving this sensitivity, up to the detection of a very small number of spins. At very low temperatures T, the energy k_B T, characteristic of thermal fluctuations, is well below the energy of a single photon, at the spin resonance frequency (4-8 GHz). This implies that at thermal equilibrium, the spins are completely polarised in their lowest energy state, which maximises the resonance signal. Moreover, the thermal noise is also eliminated, leaving only quantum fluctuations of the microwave field. In addition, quantum superconducting circuits offer many opportunities for detecting the weak microwave signals emitted by the spins, with great efficiency. An experiment based on bismuth atoms has shown that the spectrometer is capable of detecting about 65 spins in 1 second, which is a five order of magnitude improvement in sensitivity by comparison with the state of the art. It has also been shown that spin relaxation can be rapidly induced by the Purcell effect. (A.C.)