A Physics Lesson (or 'Making Good on Old Promises')

Churchill College's MCR is hosted a Conference on Everything this past Friday (6 May) and I put together some of my work this into a pretty display format to show off at the poster session.

A long time ago, I promised that I'd describe what I've been doing in lab. So, here's my poster--both in a pretty graphical version (not for slow connections!) and a slightly-modified, primarily text version (below). It's aimed at an audience comprised of graduate students from all disciplines, so--while I tried to keep it pretty general--there's some highfalutin physics for the parts of my audience who know more.

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We start with ultracold atoms at fractions of a degree above absolute zero. At these temperatures, the atoms move very slowly and their wave functions overlap, which enables us to study quantum effects and, more generally, basic physics.

After cooling our atoms, we load them into an optical lattice, a crystal of light created from the interference between coherent light. We work with potassium-40, which is a fermion (or spin ½ particle). There are theoretical predictions of phase transitions and atomic pairing for fermions within these 'artificial crystals'. We want to observe these phase transitions and understand how the system changes when we, for example, change the spacing between the atoms in our lattice.

One way to observe these phase transitions is to change the ‘spin’—the hyperfine level—of an atom and measure how many atoms' spins we are able to flip as a function of variables like the spacing between sites or how much energy we put into the atom.

This is where my work comes in. To change the ‘spin’ of an atom, we need to give it a precise energy ‘kick’ equal to the energy difference between the ↓ and ↑ states and any additional energy difference (which is incorporated into the motion of the atom). The laser system I built provides this energy as the frequency difference between two lasers with the same phase.


In my laser system (above), light is produced, then separated into light for the experiment and for feedback. For feedback, light from both lasers is coupled onto a fast photodiode.

Electronic feedback is derived from the beat signal (below). The RF power spectrum of the phase-locked system has the majority of power in a peak centered at the frequency difference (here, 200MHz, the 2 GHz difference –the 1.8GHz reference) The sidebands mark where the system is no longer able to provide adequate feedback.


To date, the laser system is working in stand-alone operation. Next steps include incorporating the laser system into the experimental apparatus, testing stability, observing transitions between 'spin' states and measuring established single-particle excitations.