To See a World in a Grain of Sand

Thomas Doherty
Tuesday 28 July 2015

The physical laws governing the smallest scales of existence are vastly different from those we experience in real life. In peering into the microscopic world we truly are examining another philosophical universe. The internship I have undertaken has been provided by the university’s cold atoms research group. Here we employ Bose-Einstein condensates to illuminate and elucidate the mysteries of the counter-intuitive quantum world.

Through use of a vast repertoire of cooling techniques we are able to guide a sample of rubidium atoms over the boundary between the classical and quantum mechanical regimes. Here the laws of exclusion that give rise to our understanding of matter cease and they condense to form a single, shared wave function. We might imagine a bag of marbles which at room temperature are a collection of unique and quite distinct objects. However, if we remove almost all of their heat, they lose the strength of their separation and fall into the marble at the bottom of the bag. We are now left with a seemingly solitary marble, with all others nestled within. This is one manifestation of the strange quantum universe, and we use this matter, named a Bose-Einstein condensate, to further investigate the microcosmos.

Our cooling apparatus is still under construction so our ultracold atoms are, unfortunately, still at room temperature. As such, my work is to lay the theoretical groundwork for some experimentation to be performed once the system has been completed. The work I have been guided towards relies fundamentally on the ability of laser light to exert a force on a condensate.

If one allowed water to flow down a mountainside, its winding path would be defined by the shape of the land. Here gravity provides the force and the relief the direction. Thus if we could reshape the mountain as we saw fit, we would have complete control over the path of the water. This is analogous to our condensates illuminated in the light of a laser. Here their motions are not generated by the slope of their planet, but rather the gradients in laser intensity. Thus if we have the ability to sculpt the laser into any shape we see fit, we can create any playground we wish for the condensates.

The difficulty, of course, comes in exactly how one gains this level of control over the laser. In our group we use a spatial light modulator to alter the phasing in the strands of the beam, before allowing them to interact and interfere into the desired shape. However, the methods we use to program the spatial light modulator allow us only to have perfect control in a single two dimensional slice. To have the ideal situation of the mountainside, we require methods to extend this control. Thus problems of optimization, uniqueness and the low computational power of desktop computers have become an integral part of my life.

In addition to my work in St Andrews, I was given the wonderful opportunity of attending an “Introductory Course in Ultracold Quantum Gases” at the University of Innsbruck. The experience of summertime in the beauty of the Tyrolean Alps was an unexpected bonus of my Laidlaw internship. I became fully immersed in the scientific field while falling in love with the peace and civility of the town. We experienced high temperatures of 39 degrees Celsius while ironically being based in the department that could house one of the coldest objects in the universe. It was with a heavy heart that I flew out of Innsbruck, through the Alpine valleys and onward to Scotland.

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