Two-photon laser could give rise to entangled twin beams

17 Jun 2002

Light amplification using the two-photon stimulation of polarized atoms is seen to offer potential for novel future applications.

Physicists at Duke University in the US have succeeded in producing a two-photon laser that exhibits nonlinear outputs with potential applications in quantum cryptography and precision measurement.

A two-photon laser amplifies light by pumping an atom with two photons instead of one, so that a total of four photons are emitted when the atom jumps down to its lower energy state. These have been created before, but the Duke laser is of particular interest because its amplification medium is an element in which the atoms have non-zero nuclear spin - potassium. This means that there is the possibility of energy-state coupling between the nucleus and the electrons, which splits the atomic energy states into yet finer levels. These states can allow different types of interaction between the atoms and the laser fields.

Daniel Gauthier and his colleagues at Duke used a potassium atom beam and created full spin-polarization using a separate, control laser. They then pumped the beam with a different laser. Potassium was chosen because it exhibits hyper-fine energy states at the appropriate levels - and also because pumping a two-photon potassium laser required the wavelength that was available from the Ti:sapphire laser that they had in their laboratory. "We used atoms that were essentially 100% spin-polarized," said Gauthier. "We needed to do this to allow the multiphoton Raman scattering process."

They observed that the laser's output with time varied nonlinearly, as expected, but also that it displayed variations in beam polarization and chaotic behaviour. "Because of the great richness in the types of interaction between the atoms and the laser fields, our two-photon laser can operate on different states of polarization," explained Gauthier. "This opens up the possibility of polarization instabilities and chaos (which is what we have observed)". The chaotic behaviour is not entirely explained, but the scientists' theories for the polarization instabilities suggest that future adjustments to the laser might enable photon entanglement. Ultimately they intend to "tame" the chaos and create polarization-entangled twin laser beams. Correlated photon sources such as these would have enormous potential for quantum cryptography and precision measurement, suggests Gauthier.

Their work will be reported in the 14 May issue of Physical Review Letters.