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Interacting two-component quantum gases in micro-magnetic traps

Final Report Summary - 2MODEACHIP (Interacting two-component quantum gases in micro-magnetic traps)

Dr Whitlock's MC-IIF fellowship has been extremely productive. He has been the leading postdoc on two experimental setups, and co-supervised four PhD students, resulting in a number of scientific achievements advancing cold atom technology and motivating new research directions. This has been recognised by numerous invited seminars at world leading research institutes and contributions to many international conferences.

In one set of experiments we have realised a vast two-dimensional array of trapped ultracold atom clouds, using a magnetic-film atom chip. We have loaded atoms into hundreds of tightly confining and optically resolved array sites and cooled the atoms to quantum degeneracy. We have shown that rapid density-dependent three-body atom loss in these microtraps is a robust way to prepare small ensembles comprising tens to hundreds of atoms each. Arrays of trapped atoms are the ideal starting points for developing scalable quantum registers for storing and processing quantum information.

This is also an ideal environment to investigate collective excitations via laser excited Rydberg states to engineer many-body interactions. We first demonstrated spatially resolved, coherent excitation of Rydberg atoms on an atom chip. Ultracold atoms excited to high-lying Rydberg states allow for long-range atom-atom interactions and can greatly enhance atom-surface interactions. We measure distance-dependent shifts of the Rydberg energy levels caused by local electric fields near the gold-coated chip surface. This indicates the presence of a localised patch of Rb adsorbates on the chip surface. Our results will allow for further studies of atom-surface interactions, many-body physics and quantum information science involving interacting Rydberg excited atoms on atom chips.

In the same experiment, we have improved the detection of trapped atom ensembles through advanced postprocessing and optimal analysis of laser-illuminated absorption images. We show how to maximise the information extracted from typical absorption images which can significantly improve the readout of trapped atom interferometers, or better resolve number squeesing and entanglement between atomic ensembles. The methods were also applied to probe sub-Poissonian number statistics in a lattice of small atomic ensembles. A number of other groups have shown great interest in our results and have started to implement some of our algorithms.

Another set of experiments were performed using an apparatus for the study of one-dimensional (1D) quantum gases on an atom chip. We introduced radio-frequency (rf) dressed potentials to enable additional flexibility for manipulating quantum gases. We demonstrated that state-dependent rf dressing is a new way to control interactions and non-equilibrium spin-dynamics in two-component 1D quantum gases. We have prepared coherent superpositions of both spin and motional degrees of freedom in a weakly interacting 1D Bose gas. We directly image non-equilibrium spin dynamics following a sudden change in the system parameters. The experimental control over the spin dynamics includes access to the point of spin-independent interactions where exact quantum many-body solutions are available. These results are relevant for our understanding of complex many-body phenomena such as spin waves, spin-charge separation and the relation between superfluidity and magnetism. Tunable interactions in two-component quantum gases have important applications in the areas of spin squeezing and quantum metrology, and the control of spin dynamics opens new avenues for studies of quantum coherence in interacting quantum systems.

In addition to the main research program Dr Whitlock also initiated a successful ongoing international research collaboration (Roman Schmied, MPQ Garching; now University Basel), signing the corresponding paper as the senior author.