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Spatially resolved atom fluorescence imaging

Final Report Summary - SRAFI (Spatially resolved atom fluorescence imaging)

At the beginning of the SRAFI fellowship, Dr Perrin has concentrated his work on the implementation and the characterisation of a novel fluorescence imaging based on a light sheet. Such system is made of two counter-propagating, overlapping and strongly anisotropic light beams that cold Rubidium 87 gases cross when they fall under the influence of gravity. The interaction of the atoms with light is responsible of the scattering of a large number of photons. The latter are subsequently collected by an objective with a relatively high numerical aperture and imaged on an EMCCD camera. Studying the statistics of the fluorescence signal in a large number of pictures has allowed to demonstrate the sensitivity of this fluorescence imaging to single atoms. Moreover, by observing the density fluctuations of thermal Bose gases, a thorough characterisation of the spatial resolution of this imaging system has been achieved.

In a second step, the research fellow, together with the rest of his team, has investigated the physics of degenerate quasi-one-dimensional (1D) Bose gases. More specifically, they have characterised the density fluctuations of these systems, which naturally appear during time of flight after releasing the atoms from the trap. The presence of these fluctuations can be related to the multimode character of the source. In the framework of collaboration with theoreticians specialised in the physics of such systems, quantitative comparison of the results of the experiment with theoretical predictions has been possible. Moreover such collaboration has proven that these measurements could be used for thermometry.

More recently, the fellow participated to the characterisation of 1D Bosonic Josephson junctions in thermal equilibrium, relying on matter-wave interferometry. This technique allowed to measure phase correlations along the junction. Simulating a Josephson junction can be achieved using double-well potentials obtained using radio-frequency dressing fields. Such study is a good example of how cold atoms can be used to investigate physics questions outside atomic physics, as here condensed matter physics. The results of the experiment were compared to a theoretical model based on simulations relying on a stochastic process. This technique allowed the accurate measurement of the coupling energy of the Josephson junctions, which is generally very difficult to obtain precisely.

In the last months of the fellowship, Dr Perrin contributed to a project concerning the collisional relaxation of quasi-1D Bose gases. Starting with a degenerate Bose gas in an anharmonic trap obtained through the combination of static magnetic fields and rf-dressing fields, the system was brought to its first transverse excited state using an optimal-control modulation of the position of the trap centre. After such excitation the only relaxation process conserving parity involves two excited atoms, which collide and decay simultaneously to the transverse ground state into two longitudinal free-particle modes with large and opposite momenta, so that energy and momentum are conserved. Using the fluorescence detector it was possible to demonstrate reduced fluctuations of the relative population of the two modes. The observed process can be seen as an efficient source of 'twin atom' beams in analogy to twin photon beams that have been extensively used in quantum optics.