Gareth Roberts Memorial Lecture - Cool things to do with lasers
18 January 2013
Following the untimely death of our Branch Secretary, Gareth Roberts, who sadly died at the start of 2012 after a long period of illness, it was decided that the Branch would hold a lecture in his memory, focusing on one of the areas of research in which he worked.
The lecture was held on 13 December 2012, at Newcastle University, and began with a speech by Prof. Albert Crowe, who worked with Gareth at Newcastle University, and was formerly Chair of the Branch.
Prof. Crowe gave a heartwarming speech about his memories of Gareth, before introducing our main speaker, Ifan G. Hughes, from Durham University. Prof. Hughes had previously worked with Gareth on Laser Cooling, and his talk included some of the projects on which they had worked together.
Prof Hughes first showed us a slide showing how lasers can be used for heating (such as in high power laser welding) and how lasers are traditionally thought of as a type of beam weapon that one might see in Star Trek. Hence, the idea that lasers might be able to cool things would seem counter-intuitive to many. He then showed that momentum transfer can exist between a photon and an atom by absorption of the atom and causing the atom to enter an excited state. The atom then relaxes by emitting another photon.
The initial photon absorption always causes a momentum transfer in the direction of the initial photon whereas the emission of the photon upon relaxation is in a random direction.
By using a laser beam of sufficient intensity, a beam of atoms being emitted into a vacuum from an oven may be brought to a ‘standstill’ in a distance on the order of 1 metre.
Of course there is a residual motion of the atoms. This gives a temperature of the laser cooled atoms around the microKelvin region.
Prof Hughes then described a magnetic device made from nanowires. The magnetic flux within the wires was parallel to the direction of the wire. The magnetic domains within the wire opposed each other at regular intervals.
At the regions where the magnetic domains met (the domain walls), the magnetic flux pointed out of the wire. In this way a sinusoidally varying magnetic flux could be created.
Prof Hughes showed that this form of magnetic field was what was required in order to make a magnetic mirror for the cooled atoms.
In order to demonstrate the magnetic mirror, Prof Hughes mounted it below the cloud of atoms.
The laser was then switched off and the atoms allowed to fall under gravity. It could be seen that the atoms bounced off the magnetic mirror when it was in place, but otherwise simply fell to the bottom of the chamber.
Among the potential technological applications of laser cooling, Prof Hughes mentioned highly accurate atomic clocks and quantum computation.
The talk was excellently delivered and provided the audience with a very well explained overview of the field of laser cooling of atoms.