Travailler au LKB

Scientific context

Rydberg atoms, i.e., atoms excited to high-principal-quantum-number levels, are particularly well suited to the quantum simulation of condensed matter systems [1]. They can be prepared from arrays of single atoms, laser-trapped in optical tweezers, and their strong dipole-dipole interactions (in the MHz range, even at a few microns) combined with their few-100µs lifetimes enable the observation of quantum phase transitions and of quenched dynamics. 

Circular Rydberg atoms, i.e., Rydberg atoms with maximal angular momentum, have a 100-times longer lifetime. Their use would bring quantum simulations to unprecedented regimes, with the simulation of long-term dynamics, such as thermalization. They would also enable the simulation of large interacting spins, beyond spin-1/2 physics. 
However, their use has been so far limited because they have no optical transitions, hindering their individual detection and manipulation in an array. We propose to remove this limitation with a hybrid platform, combining an array of laser-trapped circular atoms of Rubidium with an auxiliary array of Rb ancilla atoms transiently excited to a regular low-angular momentum Rydberg level. We trap the circular Rydberg atoms in optical bottle beams [2] and the ancilla atoms in Gaussian optical tweezers. They interact through a resonant dipole-dipole interaction.
Using a resonant dipole-dipole interaction between the ancilla and circular atoms, we have recently demonstrated the quantum non-demolition measurement of the circular Rydberg atoms with the ancillae, as well as a spatially-resolved manipulation of their quantum states [3]. In addition, we have measured the dipole-dipole interaction between two circular Rydberg atoms, and observed the coupling between spin and motional degrees of freedom [4]. This now opens the way to genuine quantum simulation with circular Rydberg atoms.

M2 Internship
Our experiments are performed in a UHV chamber operated at room-temperature. To fully benefit from the long lifetimes of the circular Rydberg atoms, we need to transfer the existing setup to a cryogenic environment. The intern will actively participate to the development of the cryogenic platform. In particular, he/she will participate to the adaptation of the optics, microwave and radiofrequency setups to the cryogenic platform. His or her work will, thus, constitute, a decisive contribution to our quantum simulations project.
 
PhD Thesis
The first months of the PhD work will be devoted to the operation of the new cryostat and its optimization. We will benefit from the enhanced lifetimes of the Rydberg levels to improve the performances of the existing room-temperature setup. During the PhD work, the proposed quantum simulator will then be operated. Quantum simulation of spin-1/2 Hamiltonians will first be performed, with the observation of long-term dynamics, out of the reach of existing simulators. We will then focus on the simulation of the interaction between spin 1s or larger. We will in particular use the ancillae to tune the interaction between the circular Rydberg atoms, using techniques developed in on-going collaborations with theoretical teams

References
[1] T. L. Nguyen et al., PRX 8, 011032 (2018)
[2] B. Ravon et al., PRL 131, 093401 (2023)       
[3] P. Méhaignerie et al., PRX Quantum 6, 010353 (2025)                               
[4] Y. Machu et al., arXiv :2509.24691 (2025)

In absence of nonlocal quantum correlations, Bell’s local hidden-variable model puts constraints on the measurement statistics. These constraints are at the root of Bell inequalities, but, in principle, they also put limitations on the use of our quantum states. In this project, we will focus on continuous-variable systems, which means that we deal with a bosonic system (e.g. light) upon which we perform homodyne measurements. Within the context of such systems, the student will have the choice to explore one of the following applications: quantum state engineering or quantum metrology.  

In the context of quantum state engineering, we will consider heralding schemes. In such a setup, we consider a bipartite quantum state in which on part of the system is measured to project the other part of the system into a desired, often more exotic, quantum state. It was already shown that, in the heralding scenario, certain types of quantum correlations are necessary or sufficient to produce quantum properties such as negativity of the Wigner function [2,3]. However, it is not clear whether there are properties that can only be generated through nonlocality. The student will try to answer this question by classifying the types of states that can be generated under the assumption that the initial bipartite state can be described by a local hidden-variable model.
Alternative, the student might also choose to explore the context of quantum metrology, and more specifically multiparameter estimation. Here, we assume that a multipartite quantum system has several unknown parameters. The precision with which local parameters can be jointly estimate can be enhanced by quantum correlations. This idea was used in the past to derive witnesses for entanglement [4] and quantum steering [5]. However, this approach has never been extended to nonlocality. Yet, we know that local hidden-variable models constrain the measurement statistics, and thus also its capabilities to estimate parameters. By characterising these constraints, we ultimately seek to identify a metrological Bell inequality. 
 
References
[1] N. Brunner, D. Cavalcanti, S. Pironio, V. Scarani, and S. Wehner, Rev. Mod. Phys. 86, 419 (2014).
[2] M. Walschaers PRX Quantum 2, 030204 (2021)
[3] M. Walschaers Quantum 7, 1038 (2023).
[4] C. E. Lopetegui, M. Isoard, N. Treps, and M. Walschaers, Optica Quantum 3, 312-328 (2025)
[5] C. E. Lopetegui, M. Gessner, M. Fadel, N. Treps, and M. Walschaers, PRX Quantum 3, 030347 (2022).