We make use of Bose-Einstein condensates (BEC) of metastable helium atom (He*) to perform interferometry experiments inspired by quantum optics.
A remarkable feature of the metastable state of helium is its very large internal energy (20eV), allowing the use of a single-atom resolved detector, based on the electron amplification.
This detector is called a microchannel plate (MCP) and provides three-dimensional data in the momentum space of the atoms.
Thanks to the nonlinearities of BECs, we have also designed an atomic source of non-classical states, whose statistics and correlation properties can be probed with atom interferometry techniques.

We demonstrate the effect of pulse shaping in momentum selective atomic Bragg diffraction. We compare temporal square pulses, which produce sidelobes in momentum space, with other shapes which can produce more nearly square momentum distributions. We produce pulses that simultaneously address two sets of velocity classes (Alice and Bob on the left) and demonstrate that we can control the differential phase imprinted on them in a way that is insensitive to laser phase fluctuations.

The article is currently under review but the preprint is available both on HAL and Arxiv

We have used a variant of the Hong Ou Mandel setup described below to realize a two-particle interferometer with four input and four output ports as shown in the figure. The source generates atom pairs in a superposition of different momentum states:  |ψ) ~ |p,-p) + |p',-p').  When such a state is injected into the atom interferometer, the probability of detecting an atom at each output port is 1/4, independent of any interferometer phase. The correlations between the different output ports however do vary as a function of the relative phase of the two closed circuits (red and blue in the figure). We have demonstrated this effect in our apparatus. An improved version of this experiment can lead to the violation of a Bell inequality involving the motional degrees of freedom of freely falling massive particles.

Dussarrat, P. et al. Two-Particle Four-Mode Interferometer for Atoms. Phys. Rev. Lett. 119, 173202 (2017).

The Hong Ou Mandel (HOM) effect is a remarkable illustration of 2 particle interference. Two identical particles arrive at the input ports of a beam splitter. If they are perfectly overlapped, they never exit in opposite output ports. The effect is well known for photons. We have recently performed the analogous experiment for atoms.

Lopes, R. et al. Atomic Hong–Ou–Mandel experiment. Nature 520, 66–68 (2015).

A BEC has sufficient optical thickness to act as a gain medium. When an elongated BEC is excited by a laser beam, the gain causes the spontaneous emission to be preferentially directed in the elongated direction, in so called endfire modes. Since it involves considerable gain and is highly directional, this emission resembles laser emission. We have measured the intensity (2nd order) correlation function of this emission and found that its statistical properties resemble those of a thermal source rather than those of a laser. We do this by observing the recoil of each atom undergoing a superradiant scattering process. Our detector allows us to construct the 2-atom correlation function and thereby infer the correlation function of the emitted light.

Lopes, R. et al. Second-order coherence of superradiance from a Bose-Einstein condensate. Phys. Rev. A 90, 013615 (2014).