Détection de spins individuels par comptage de photons micro-ondes 

Since several decades, single photon detection in the optical domain is a routine feature of many experiments. In contrast, the same type of detection in the microwave domain is a recent invention. The detection scheme is based on a four-wave mixing, converting the photon to be detected and a pump photon into another photon plus the excitation of a superconducting quantum bit. The frequency conversion event is recorded by the excitation of the quantum bit, so that one microwave photon is effectively detected by quantum bit state readout. The main goal of this thesis was to improve the sensitivity of this detector, acting on two parameters : on one hand, the enhancement of detection efficiency, and on the other hand, the reduction of thermal photons detection. Detection efficiency has been enhanced by several improvements of detector’s fabrication, yielding a longer quantum bit decay time (otherwise said, detection record is forgotten more slowly). Regarding thermal noise reduction, it has been obtained by filtering with a tunable detection bandwidth. All these improvements have yielded a sensitivity of 3.10^-23 W/Hz^1/2. We have taken advantage of this threefold-enhanced sensitivity (compared to previous detector generation) in spin detection experiments in calcium tungstate crystals. In these experiments, erbium ions electron spins, as impurities in the crystal, are subjected to a magnetic field, causing a Zeeman splitting, and are coupled to a resonator enhancing their radiative decay in the microwave domain. The Zeeman split spins are excited by microwave pulses sent through the very same resonator, and then relax back in it by microwave photons emission, each spin emitting one single photon. The exact frequency of each of these photons depends on the crystalline environment in the vicinity of the spins that emitted them. Moreover, the microwave photon detector has enough frequency resolution to differentiate these frequencies. Hence, this setup effectively detects single electron spins. Besides, each of these electron spins being sensitive to variations of its magnetic environment, which is mostly made of tungsten nuclear spins in the specific case of this experiment, some slight variations of the frequency of these electron spins can be witnessed with the single microwave photon detector, which amounts to measure the state of all these nuclear spins. This indirect detection of nuclear spins has become possible thanks to the enhanced sensitivity of the detector, dividing the detection time by almost ten. Besides, several workflows have been lead in order to switch from single chip to wafer-scale fabrication of this detector.