The Mn2+ ion (S=5/2), subjected to the very weak crystalline field of a cubic matrix, shows remarkable coherent quantum dynamics properties. When the pumping microwave field has an amplitude of the same order as the crystalline field, nonlinear dynamics appears. Population inversion can be achieved between transitions forbidden through the coherent absorption and emission of several photons. While a qubit is the superposition of two levels (i.e., S=1/2), its extension to a higher number of levels is called a qudit. In the case of the Mn2+ ion (S=5/2), we have been able to coherently manipulate six electronic levels and 36 levels by associating nuclear levels. In a series of articles, we have been able to show coherent control of up to five photons of the electronic levels, control of electron-nuclear transitions, and an experimental protocol for probing all 630 transitions. These results pave the way for the implementation of the Grover algorithm (an unstructured list quantum algorithm).

The study of quantum coherence in spin ensembles is a central theme, both for purely fundamental reasons and for potential applications in the field of quantum technologies. Our team is particularly interested in unconventional systems. Thus, our team's observation of the coherence of the magnetic resonance of strongly correlated defects opens the way to a whole new theme in quantum information: the realization and control of soliton qubits. To do this, we will use systems of strongly correlated organic spin chains (provided by the team of M. Foumigué of the ISCR) in which we will induce and control defects either by irradiation or by electrical control. In addition, by using magnetic field gradients, we will be able to locally address qubits.

The quantum coherence of a set of electron spins is very dependent on its environment. Phonons, magnetic dipoles and nuclear spin baths tend to destroy the coherent superposition state of spins. There are instrumental techniques that reduce these effects such as dynamic decoupling which consists in rapidly inverting several times the spin populations acting as noise filters. We have proposed and realized a new experimental protocol using two coherent electromagnetic waves, but of slightly different frequencies. The idea is the following: during the resonance, in the rotating reference frame, the spin acts as a damped oscillator. If we add a force of the same frequency as the Rabi frequency we should be able to force the set of spins to continue to oscillate. In the reference frame of the laboratory, this corresponds to having a second pumping frequency slightly different from that of the main source. Generally, to obtain such a configuration, an arbitrary function generator at microwave frequency is needed, often very expensive. In our case, a simple un-balanced mixer allowed us to realize the protocol. This protocol has been successfully tested on several electron spin systems (diamond, transition metals, rare earths) and it is probably applicable to many others such as quantum dots, superconductors or cold atoms.