Bragg spectroscopy of a strongly interacting Fermi gas

The main objective of the research described in this thesis is to probe the manybody quantum effects in a strongly interacting Fermi gas using Bragg spectroscopy. An enriched source of Lithium-6 atoms is used to probe these properties. The broad Feshbach resonance of the two lowest hyperfine levels of 6Li at a magnetic field of 834G is exploited to precisely control the interactions between the atoms. In our experiments, a beam of hot 6Li atoms produced by an oven is cooled using a σ− Zeeman slower in an ultra-high vacuum environment. From this slowed atomic beam, 4% of the atoms are cooled and trapped in a magneto-optical trap (MOT) created by six perpendicularly intersecting near-resonant laser beams. This Doppler cooled MOT consists of 108 atoms at a temperature of 280 μK. A small fraction (1%) of atoms is then transferred from the MOT to a single optical dipole trap created by a 100 W fibre laser. To obtain the temperatures of degeneracy (≈ 100 nK), the atomic sample is further evaporatively cooled in the presence of Feshbach magnetic fields, by reducing the intensity of the dipole trap laser with the help of a commercial PID controller. Once the degenerate gas is formed, the dipole trap is switched off and the gas is allowed to expand for a certain time of flight before taking an absorption image on a CCD camera for analysis. At magnetic fields below the Feshbach resonance, the collisional interactions become repulsive and a bound molecular state exists for fermionic atoms with opposite spins. These bound fermions form a composite boson which follows bosonic statistics to form a molecular Bose-Einstein Condensate (BEC). At magnetic fields above the Feshbach resonance, the interactions become attractive and no stable bound state exists. However at ultracold temperatures, a Bardeen-Cooper-Schrieffer (BCS) state is created through weak coupling between the fermions forming a 'Cooper pair'. When the field is tuned to the Feshbach resonance (unitarity), the scattering length of the atoms diverges and the interactions in the gas become universal. Under such conditions, correlated pairs are predicted to exist due to many-body quantum effects. The size of these pairs becomes comparable to the inter-particle distances, making this system an ideal test bed for studying superfluidity. Bragg spectroscopy is designed and implemented to probe the composition of the particles at unitarity as well as their correlation properties across the BEC-BCS crossover. A comprehensive study of these properties across the Feshbach resonance is presented for the first time via Bragg scattering of the strongly interacting Fermi gas. The Bragg spectra are analysed in terms of the centre-of-mass displacement of the cloud along the direction of the Bragg pulse. A smooth transition from molecular to atomic spectra is observed with a clear signature of pairing at and above unitarity. Furthermore, the two-body correlations are characterised and their density dependence is measured across the broad Feshbach resonance.