In the framework of the European Training Network on Quantum Sensing using Optimal Control (QuSCO), we are looking for a motivated PhD student to work on "Few-spin magnetic resonance".
Most EPR spectrometers rely on the inductive detection of the small microwave signals emitted by the spins during their Larmor precession into a microwave resonator in which they are embedded.The best spectrometers so far are able to detect typically the presence of 10^7 spins in a single experimental sequence. Our group aims at improving enormously the sensitivity of EPR spectroscopy, using superconducting micro-resonators of very high quality factors and ultra-low-noise microwave amplifiers based on Josephson junctions (called Josephson Parametric Amplifiers or JPA),cooled at 10mK in a dilution refrigerator. A patent application is pending. Using bismuth donors in silicon coupled to an aluminum micro-resonator, with a JPA to amplify the signal,we have already demonstrated the detection of 1700 spins with unity signal-to-noise ratio, which constitutes a gain of 10^4 in sensitivity compared to the state-of-the-art. In these experiments,thermal or technical noise were fully suppressed, and the dominant remaining noise source was the quantum fluctuations of the electromagnetic field at microwave frequencies.
The goal of the PhD thesis is to go further than these first experiments,and to reach the sensitivity to detect ensemble of few spins, or even individual spins if possible. The extra 3 orders of magnitude in sensitivity that we need to gain will be obtained by scaling down even further the resonator transverse dimensions,by introducing a short constriction of length 200nm and width∼20nm for a 15nm height in the resonator. This will enhance the spin-resonator coupling by 2 orders of magnitude,and should enable us to achieve our goal.
The ESR will also investigate two parallel strategies to enhance even further the sensitivity, based on the use of concepts borrowed from quantum optics and quantum control. First, squeezed microwave radiation could reduce the noise of the measurement beyond the quantum limit. A proof-of-principle demonstration has already been brought recently in our group but further work is needed in order to turn squeezed microwaves into a practical way to improve sensitivity. Then, our measurements would benefit from the use of optimal control pulses,optimized for the specific parameters of the experiment.These ideas will be ideally pursued in the framework of QuSCO:the squeezing will be investigated in collaboration with Aarhus University and the optimal control in collaboration with Dominique Sugny at the Université de Bourgogne. In a first time, the method will be tested with two model systems: NV centers in diamond,and Bismuth donors in silicon. We then plan to apply the spectrometer to the detection of nanoscale samples of possible biological interest, such as gadolinium spin-labels or simple metallo-proteins.
The applicant is requested to be in possession of a Master of Physics or equivalent diploma. He or she whill need to satisfy to the following mobility criterion : that is to not have worked more than one year in France for the last 3 years.
 A. Bienfait et al., arxiv:1610.03329