Lasers Manipulate Condensate

Lamont Wood

Researchers at the National Institute of Standards and Technology have used lasers to write and then read quantum-mechanical phase patterns in a Bose-Einstein condensate -- an ultracold cloud of atoms that share the same quantum state.

They used laser-based techniques to generate and study solitons -- stable, localized density variations that propagate almost without dissipation because the atoms in the condensate act as a nonlinear medium, explained team member Johannes Denschlag, a postdoctorate Humboldt fellow from Germany.

The researchers used "optical phase imprinting" to write the phase patterns, employing a 20-mW liquid dye laser tuned to 589 nm, which is slightly below the atomic resonance of the condensate's sodium atoms. They projected the beam through a mask and focused it on the condensate. The mask creates a light pattern that alters the wave function that describes the condensate. But to create solitons, they only needed to block half the beam with a razor blade, Denschlag said.

Researchers at the National Institute of Standards and Technology used lasers to write "NIST" in Bose-Einstein condensates.

To read the resulting phase pattern, the researchers used laser-based matter-wave interferometry, he said. This involved two counterpropagating liquid dye laser beams of about 1 mW, detuned from the atomic resonance of the atoms by 2 GHz and from each other by 100 kHz. Pulsed into the condensate, the photons moved atoms along the beams. The atoms interfered with one another, and the interference patterns revealed the phase distribution of the condensate's new wave function, he explained.

Combining the two techniques, the researchers demonstrated that imprinting a phase step to the condensate's wave function generates solitons, as predicted.

Future use of the laser technique could include study of the nonlinear dynamics of solitons and attempts to create quantum vortices.