The leading technique to probe electron spins is Electron Paramagnetic Resonance (EPR; see below), although its sensitivity is not sufficient for small numbers of spins.
Using their experience in the field of superconducting circuits, physicists from Iramis were able to lower the EPR detection sensitivity to about ten electrons for an integration time of one second. To go beyond this, it was necessary to overcome the quantum fluctuations of the lowest possible energy state (known as the zero point). The researchers have now achieved this feat by using photon counting.
For this, they couple a few thousand electron spins of impurities implanted in silicon to a resonant cavity. This configuration allows these quantum systems with two close energy levels to deexcite according to the Purcell effect, by emitting single microwave photons. This very small number of photons can then be detected thanks to an ingenious single microwave photon detector (see below).
This remarkable sensitivity, which stands in contrast to conventional EPR, opens the way to multiple applications in quantum information processing, as well as the analysis of individual biological objects such as cells or proteins.
A GUIDE TO EPR
EPR (Electron Paramagnetic Resonance) is to electron spins as NMR (Nuclear Magnetic Resonance) is to nuclear spins. Specifically, EPR involves probing the sample in the presence of a magnetic field with two successive radio frequency pulses and detecting the "spin echo" emitted in response. Because the frequency of the oscillations is modified by the interactions with the nearest nuclei, the EPR signal provides information on the chemical environment of the electrons probed in the material.
A SOPHISTICATED SINGLE MICROWAVE PHOTON DETECTOR
The fluorescence microwave photons emitted by the electron spins cause a superconducting quantum bit to transit in its excited state, triggering the possibility of detection with high precision and almost no noise.