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
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, email@example.com)
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
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
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.
al., Physical Review Letters, 14 May; text at Physics