Quantum Hopping on a Lattice of Cold Rydberg Atoms

Opportunity for students

In this project you will arrange an encounter between two cold Rydberg atoms. Rydberg atoms are huge (order 1 µm) and very sensitive for their environment. In contrast to ground-state atoms they notice each other on a distance of tens of microns, via a process known as dipole-dipole interaction. The approach we envision is to make one Rydberg atom with its outer electron excited to the n=60 state (the "lower state") and another one to the n=61 state (the "upper state"). On the timescale of the experiment the atoms don’t move - they are cold - but through dipole-dipole interaction they exchange excitation. The excitation can oscillate from one to the other, with coherent superpositions in between. This process is called quantum hopping.

The experiment is performed with 100-µK rubidium atoms in a so called magneto-optical trap (MOT). Two pulsed dye lasers make the two Rydberg atoms. The laser beams have a slightly different color and are focused about 20 µm apart in the cold atom cloud. Rydberg atoms in an electric field can break up in a free electron and an ion; the higher the n-state, the weaker the required field. This idea is used for detection: a slowly ramped electric field ionizes the two Rydberg states subsequently and the electrons are detected. This approach is also position sensitive, because electrons that are released at different positions in the electric potential acquire different speeds. Therefore, after some free flight, they arrive on the detector at a different time. In this way we can follow the dynamics of our quantum system.

This is more spectacular than you might think at first sight: what is the difference between the coherent evolution of a man-made quantum system and a quantum computer? The answer is: complexity. But it is a first step and, in contrast to other first steps towards quantum computing, it allows more easily for a second step.

What can you, bachelor or master student, do now? At the moment we have just seen our first cold Rydberg atoms. Now, a few things have to become fancier. The position of the Rydberg production should be a bit more accurate (5 µm laser focus) to ensure a well defined distance between the two atoms. Furthermore both the state selectivity and the position resolution have to be cranked up. But then you can be the first to observe quantum hopping!

If you want to join us, please contact:

Richard Newell

Ben van Linden van den Heuvell