The population of the ground state of a complex system usually is achieved by freezing out thermal excitations, or by adiabatic loading of an already cold system into the desired potential. In contrast, photons can be directly cooled by radiative contact to a dye solution, allowing a direct population of the ground state. As photons are Bosons this is enhanced when crossing the phase transition to a Bose-Einstein condensate, where the ground state is macroscopically populated already at relatively large temperatures.
Using our home-made structuring method, we have imprinted micron-sized potentials for light, assembled in a square. superimposed by a parabolic potential to enable efficient cooling into the ground state. Using direct cooling we were able to macroscopically populate the ground state of the ring-like system, a toy model which also allows for ring currents. The emission was observed to be coherent, i.e. there is a well-defined phase relation between the micropotentials.
Perspectives of our work include the direct cooling into entangled states, as a resource for quantum information. Using larger systems one can study topological effects, as they e.g occur in the quantum Hall effect, or the optical analog of superconducting flux qubits.
Additional information:
Press release of the university