Claude Cohen Tannoudji

Prix Nobel en 1997 pour le ralentissement et le piégeage des atomes par la lumière laser.

Ses travaux sont à la source des recherches actuelles de l'IFRAF.

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## « Thermal blurring of a coherent Fermi gas » : séminaire du groupe atomes froids du LKB

De 9h30 a 10h30 en salle de conference du 46 rue d’Ulm 75005 Paris

Résumé

It is generally assumed that a condensate of paired fermions at equilibrium is characterized by a macroscopic wavefunction with a well-defined, immutable phase. In reality, all systems have a finite size and are prepared at non-zero temperature ; the condensate then has a finite coherence time, even when the system is isolated in its evolution and the particle number $N$ is fixed. The loss of phase memory is due to interactions of the condensate with the excited modes that constitute a dephasing environment. This fundamental effect, crucial for applications using the condensate of pairs macroscopic coherence, was scarcely studied. In my presentation, I will link the coherence time to the condensate phase dynamics, and show with a microscopic theory that the time derivative of the condensate phase operator $\hat\theta_0$ is proportional to a chemical potential operator $\hat\mu$ that I will construct including both the pair-breaking and pair-motion excitation branches. In a single realization of energy $E$, $\hat\theta_0$ evolves at long times as $-2\mu_\rm mc(E)t/\hbar$ where $\mu_\rm mc(E)$ is the microcanonical chemical potential ; energy fluctuations from one realization to the other then lead to a ballistic spreading of the phase and to a Gaussian decay of the temporal coherence function with a characteristic time $\propto N^1/2$. In the absence of energy fluctuations, the coherence time scales as $N$ due to the diffusive motion of $\hat\theta_0$. I will also propose a method to measure the coherence time with ultracold atoms, which we predict to be tens of milliseconds for the canonical ensemble unitary Fermi gas.

Preprint : H. Kurkjian, Y. Castin, A. Sinatra, arXiv:1502.05644