Physicists Demonstrate First Laser Made From a Cloud of Gas | MIT Technology Review

Physicists Demonstrate First Laser Made From a Cloud of Gas | MIT Technology Review:  In recent years, physicists have found an answer in the form of random lasers. These consist of some kind of disordered medium, such as semiconductor powder. The light that stimulates emission is not confined by mirrors but by the disordered state of the powder–the light simply bounces around inside it at random...




These guys have built their laser out of a small cloud of rubidium atoms confined in a magneto-optical trap. They excite the atoms and then zap them with a laser tuned close to the expected emission frequency of rubidium. This bounces around at random inside the cloud triggering the stimulated emission of light.

The first controllable atom SQUID

The first controllable atom SQUID: Campbell and colleagues in the Laser Cooling and Trapping Group have long been investigating analogous behavior in toroidal Bose-Einstein condensates (BECs) – ultracold, donut-shaped ensembles of atoms that are all in the same quantum state and form a superfluid.

To create rotation, which is the superfluid counterpart to external magnetic fields in a SQUID, the team introduces a green laser beam perpendicular to and penetrating the plane of the superfluid ring, and slowly rotates the beam around the ring. (See animation.) The beam acts as a sort of optical paddle, causing the superfluid BEC atoms to rotate.

Just as a superconducting ring admits flux when the current exceeds a critical value, the ring of superfluid admits a vortex, resulting in a change in the circulation of atoms around the ring. Like everything else in the quantum world, the properties of those vortices are quantized – that is, they occur only at discrete values, and lead to quantized circulation states in the BEC. Campbell's team was able to observe and measure those quantum increments and for the first time was able to control the onset of discrete circulation states by tuning the power and rotational speed of the green laser.

Tying the knot with computer-generated holograms: Winding optical path moves matter

Tying the knot with computer-generated holograms: Winding optical path moves matter: Knotted traps are made by imprinting a computer-generated hologram on the wavefronts of an otherwise ordinary beam of light. NYU undergraduate student Elisabeth Shanblatt and NYU physicist David Grier, the authors of the Optics Express paper, use a "liquid-crystal spatial light modulator" to project their holograms. This is essentially the first cousin of a conventional LCD television screen. The spatial light modulator imprints a calculated pattern of phase shifts onto the light. When the modified beam is brought to a focus with a high-power lens, the region of maximum intensity takes the form of a 3-D curve. This curve can cross over and through itself to trace out a knot. Moreover, the same hologram can redirect the light's radiation pressure to have a component along the curve, so that the total optical force "threads the knot."
When Shanblatt and Grier began this investigation, they thought that creating knots would be a compelling and aesthetically pleasing demonstration of their method's power. Once the knots actually worked, they realized that there are very few—if any—other practical ways to create knotted force fields. Previously reported knotted vortex fields have intensity minima along the knot, rather than the intensity maxima, or "bright knots" that can be created using the computer-generated holograms.