Chaotic Two Photon Laser

In many lasers an ensemble of coherent photons is amplified through the process of stimulated emission: a photon with just the right energy can strike an excited atom, causing it to jump down one quantum level, in the process emitting another photon just like the one that stimulated the atom. The total number of photons thereby goes from one up to two.

In principle two-photon stimulation can also be achieved. In this process a population of atoms can be excited in such a way that stimulation requires the presence of two incoming photons, causing the atom to jump down two quantum levels, which results in two new photons. The net effect: two photons are amplified into four photons (see figure at Physics News Graphics).

The main obstacle to such a scheme, finding a suitable amplifying medium, is overcome by physicists at Duke University. Daniel Gauthier and his colleagues (919-660-2511, gauthier@phy.duke.edu) use as their medium a beam of potassium atoms which have been oriented (polarized) in a single direction by the fields of a separate control laser beam and triggered by yet another laser pulse.

The first continuous two-photon laser was first demonstrated in 1992 (Update 75), but only now are their properties being explored in detail. For example, a 2-photon laser not only produces double the photons per atom, but has another property that sets it apart from other lasers. The amount of amplification is not only proportional to the number of atoms involved but also to the number of photons present; this creates a nonlinear effect which can lead to an unusual laser output as time goes by.

The Duke physicists have now driven their laser to this nonlinear regime, and have found, unexpectedly, that the output is chaotic. Next the researchers hope to tame the chaos and exploit the nonlinear properties of their two-photon laser to create ultrabright entangled twin laser beams which would display correlations at the photon-by-photon level, a feature expected to benefit precision measurement and quantum cryptography. (Pfister et al., Physical Review Letters, 14 May; text at Physics News Select)