Last updated 07/11/2024

PhD and (master) internship proposal on the degenerate strontium Fermi gases experiment :

Spin manipulations in a degenerate Fermi gas of strontium atoms

Ultracold atoms, produced by laser cooling techniques, offer a platform to explore quantum collective effects in the regime of quantum degeneracy. We perform experiments with degenerate Fermi gases of strontium 87 atoms – an exotic fermionic system, in that its spin-9/2 degree of freedom encompasses a large number (10) of Zeeman sublevels. This is both an opportunity to explore novel many-body effects (for example, antiferromagnets with a novel mechanism for frustration), and a “technological” opportunity to use quantum objects with a large internal Hilbert space, as a resource for quantum simulation, computation, or sensing.

We have developed original methods to manipulate and measure the atomic spins. Our experiment will now enter a new phase, where collective effects are evidenced. We want to demonstrate the production of quantum correlated states by engineering either Hamiltonian or dissipative terms acting on the atoms. The Hamiltonian terms are best described by the Fermi-Hubbard model, for which the ground state is a quantum antiferromagnet. The dissipative control is counter-intuitive: it is indeed a novel insight that couplings to an environment, typically destroying the manifestations of quantum physics, will in specific cases actually produce and stabilize quantum states with many-body correlations. This exciting point means that quantum phenomena may be harvested for quantum simulation or quantum sensing (clocks, atom interferometers) in a more robust manner than formerly thought.

The implementation of our ideas will rely on the original spectroscopic properties of strontium: narrow optical lines, relevant to optical atomic clocks, and that in our case we use to engineer highly selective spin manipulations.  In particular, we will in the short term introduce a dissipation that selectively extracts pairs of atoms in spin-antisymmetric two-body wavefunctions. This results from photoassociation, controlled by laser, and the Pauli principle, that prevents identical fermions from being in the vicinity of each other. The effect is expected to pump the remaining atomic ensemble towards spin-symmetric entangled states.

Thanks to the use of an atom with a large spin F=9/2, exotic collective states will be at reach beyond those usually drawn on a Bloch sphere. Our objectives in the years of this PhD will be to characterize these states, test their interest for metrology (e.g. optical clocks desensitized to interaction shifts), and explore new schemes to manipulate the quantum correlations and symmetries of the collective spin state.
The project is built in strong connection with two other experiments in our group (quantum magnetism with dipolar chromium atoms; superradiance with strontium atoms), and in-house theory activities (P. Pedri). We are furthermore closely collaborating with theoretical groups, in particular L. Mazza, LPTMS, on dissipative dynamics, and T. Roscilde, ENS Lyon, on Hamiltonian dynamics.  

Project webpage : https://gqm.lpl.univ-paris13.fr/AF/StrontiumProject.htm

Ultracold Strontium team:
Benjamin Pasquiou, CNRS research engineer    
Bruno Laburthe-Tolra, CNRS researcher           

Martin Robert-de-Saint-Vincent, CNRS researcher

Contact: martin.rdsv (at) univ-paris13.fr


PhD and (master) Internship proposal on the Superradiant laser experiment

Continuous superradiant laser with a laser-cooled atomic beam

Atomic clocks are vital components for many applications in our modern society, such as the operation of GPS and the synchronization of telecommunication networks. Clocks are also used to bolster investigations of fundamental physical phenomena, such as the detection of low-frequency gravitational waves. Recently, a new type of clock has been proposed: the active clock using superradiant lasing. Instead of shining a very stable laser onto ultracold atoms to probe the atom resonance frequency (and thus measure time), the clock would operate by letting the atoms themselves emit light. Much like in a laser, cold atoms would be prepared in an excited state, then placed between two mirrors forming a cavity. The atoms then coherently emit light into the cavity mode. However, unlike a traditional laser, the light frequency will mostly be set by the atoms themselves, and not by the cavity. The light coherence will be set by a collective synchronization of the atomic dipoles with each other - a process called superradiance. Thus, in addition to its significance as a new clock architecture, this system is interesting from a fundamental point of view: it is an example of an open-dissipative system in which correlations of quantum nature may naturally arise. 

We have built a prototype for such a cold-atom-based superradiant laser. We want to tackle the unresolved issue of sustaining continuously a superradiant emission, thus harnessing its full potential as a clock. Our design is based on an effusive beam of strontium atoms inside a vacuum chamber, slowed, cooled, guided continuously up to an optical cavity, there to emit light in a superradiant fashion. The construction of the apparatus is completed, and we expect to acquire full control over the atomic velocity distribution in the next few months. The internship will thus be devoted to characterizing the signs of collective interaction between atoms and cavity (i.e., performing cavity-enhanced spectroscopy), and searching for superradiance signals in beat note spectroscopy. Throughout the PhD project, we will investigate the light properties to understand how the emitters synchronize their oscillations, and how the light coherence is related to correlations between all atomic emitters. Our experiment will have the unique capability to explore several distinct superradiant emission regimes, that will be identified through the spectral and correlation properties of the light and of the atoms. In collaboration with metrology experts, we will contribute to assessing the metrological interest (i.e., “performance” criteria to act as a clock) of atomic-beam continuous superradiant lasers.

Our group runs three experiments dedicated to the study of collective phenomena between atomic spins or dipoles. The two other experiments study quantum degenerate gases of interacting spinful atoms. The new team member will develop his work in connection with the entire team, developing a general culture in atomic physics and many-body physics.

References:
[1] H. Liu et. al., Rugged mHz-Linewidth Superradiant Laser Driven by a Hot Atomic Beam, Phys. Rev. Lett. 125, 253602 (2020).
https://arxiv.org/abs/2009.05717

[2] Laburthe-Tolra et al, Correlations and linewidth of the atomic beam continuous superradiant laser, SciPost Phys. Core 6, 015 (2023)

Project webpage : https://gqm.lpl.univ-paris13.fr/AF/SuperRadiantLaserProject.htm

Superradiant laser team:
Benjamin Pasquiou, CNRS research engineer
Bruno Laburthe-Tolra, CNRS researcher
Martin Robert-de-Saint-Vincent, CNRS researcher

Contact: martin.rdsv (at) univ-paris13.fr


Post-Doctoral fellowship proposal on the Superradiant laser experiment


We advertise an opening for a 15 months postdoctoral position at the Laser Physics Laboratory (Université Sorbonne Paris Nord), aiming at realizing our first studies with a continuous super-radiant laser experiment.

Super-radiant lasers are a kind of “active optical clocks”, where emitters (atoms) with a narrow spectral line emit spontaneously in the mode of an optical cavity with comparatively broad spectral line. This produces a self-referenced light source, inherently robust to the effect of vibrations of the optical cavity – a key technological advantage compared to standard optical atomic clocks. Remarkably, the emission process itself, on a narrow line and thus from a somehow metastable state, is enforced by a collective effect stimulating intensive research: superradiance, i.e. spontaneous emission enhanced by inter-atomic correlations or synchronization. Thus, two research interests meet in the engineering of superradiant lasers: the demonstration and characterization of a new technology for frequency metrology, and the study of many-body physics in a dissipative quantum system.

Nowadays, a challenge is to reach the continuous superradiant regime. We have chosen a comparatively simple architecture, in which the use of a moderately narrow line of strontium (7.5 kHz) should enable superradiance from an effusive thermal beam, simply laser cooled and collimated before it crosses the mode of a high-finesse Fabry-Perot cavity. Today, the construction of the experimental apparatus is nearing completion. The post-doctoral researcher will be in charge of demonstrating the channelling of atoms from the oven into the Fabry-Perot cavity mode, and the laser excitation to reach population inversion. Then she/he will characterize the emission of light into the cavity, in order to demonstrate, for the first time, a continuous super-radiant laser. We should be in an ideal setting to verify whether the linewidth can reach below the natural linewidth of the atom, as a result of the synchronization of the atomic dipoles.

This work will be realized within the Magnetic Quantum Gases group at LPL, which, next to this project, operates two experiments on quantum magnetism with degenerate gases of chromium and strontium. It develops in collaboration with experimentalists at FEMTO-ST (M. Delehaye, clock-line continuous superradiant laser) and LCFIO (I.F. Barbut, free-space superradiance).

Project webpage : https://gqm.lpl.univ-paris13.fr/AF/SuperRadiantLaserProject.htm

Superradiant laser team:
Benjamin Pasquiou, CNRS research engineer
Bruno Laburthe-Tolra, CNRS researcher
Martin Robert-de-Saint-Vincent, CNRS researcher

Contact: martin.rdsv (at) univ-paris13.fr



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