Description du sujet de thèse

Optomechanics is a rapidly growing field of fundamental physics that aims to couple a quantum system with a mechanical system to create macroscopic non-classical states. In this context, so-called hybrid systems, where coupling is ensured through mechanical constraints, are particularly promising [Treutlein2014]. While some approaches in the microwave domain have achieved remarkable progress, the same is not true in the optical domain, where emitters generally have very short lifetimes (e.g., quantum dots).
In this setting, rare-earth ion-doped crystals (REICs) stand out as an exception. They combine an intrinsically hybrid nature (due to the coupling of electronic levels to the deformation of the crystal lattice) with exceptional low-temperature coherence properties for solid-state systems (with linewidths reaching a few kHz). The combination of these two properties makes them ideal candidates for exploring optomechanical coupling in previously inaccessible regimes, thus opening up new perspectives in the field.
Accessing the extremely narrow lines of rare-earth ions in a crystalline matrix will require spectral hole burning (SHB), a technique that creates a narrow transparency window within an inhomogeneous absorption profile. Initially designed for high-resolution spectroscopy, this method is now widely used in various quantum technology architectures (e.g., quantum memories [Afzelius2009] or broadband spectral analyzers [Berger2016, Louchet2020]). It allows for the selection of a fraction of ions and restores their quantum properties, which would otherwise be masked by the inhomogeneous broadening associated with crystal defects.

This PhD research will focus on leveraging optomechanical coupling in REICs functionalized by SHB. The work will be structured around three complementary axes:
Quantum Accelerometer Sensor Optimization : This axis will focus on the study and optimization of a quantum accelerometer sensor compatible with cryogenic temperatures. Currently, the best cryogenic accelerometers (MEMS) offer sensitivities in the range of ∼10 ng/sqrt(Hz) with a bandwidth of ∼1kHz. REICs could enable significant improvements in bandwidth while maintaining comparable sensitivities, thanks to the quasi-continuous interrogation of a spectral hole. The implementation of such a sensor could benefit quantum technology research, particularly for vibrational diagnostics of cryostats.
Decoherence Mechanisms in Doped Crystals : This research axis aims to deepen the understanding of decoherence mechanisms, particularly those associated with optomechanical coupling, which remain poorly understood. Factors such as sample geometry, mounting techniques, and excitation of a large fraction of ions will be examined for their impact on the spectroscopic properties of the ions.
Exploration of Novel Optomechanical Coupling Regimes : This fundamental and exploratory axis will exploit the narrow linewidths of REICs to investigate new optomechanical coupling regimes. Given the optical decoherence rates on the order of kHz, coupling to lattice deformation could operate in the strong coupling regime (g_0>Γ_opt) if sufficiently high quantum zero-point fluctuation levels are achieved. To access these regimes, we will require low-mass mechanical resonators prepared in doped single crystals. Thin crystalline slabs with a thickness of 300µm are already available and will be used for initial characterizations.

Contexte de travail

The PhD will be supervised by Anne Louchet-Chauvet (Institut Langevin) and co-supervised by Pierre Verlot (LuMIn). It is part of a collaborative research project funded by the ANR, involving five partner research laboratories: Institut Langevin, LuMIn, IRCP, LTE, and Institut Néel. This project funds a total of four PhD positions across these laboratories. The selected PhD candidates will benefit from the dynamic collaboration through regular consortium meetings and will have opportunities for research stays at partner laboratories.