Techniques that use quantum interference effects are being
actively investigated to manipulate the optical properties of
quantum systems1. One such example is electromagnetically
induced transparency, a quantum effect that permits the propagation
of light pulses through an otherwise opaque medium2±5.Here
we report an experimental demonstration of electromagnetically
induced transparency in an ultracold gas of sodium atoms, in
which the optical pulses propagate at twenty million times slower
than the speed of light in a vacuum. The gas is cooled to
nanokelvin temperatures by laser and evaporative cooling6±10.
The quantum interference controlling the optical properties of
the medium is set up by a `coupling' laser beam propagating at a
right angle to the pulsed `probe' beam. At nanokelvin temperatures,
the variation of refractive index with probe frequency can
be made very steep. In conjunction with the high atomic density, this results in the exceptionally low light speeds observed. By
cooling the cloud below the transition temperature for Bose±
Einstein condensation11±13 (causing a macroscopic population of
alkali atoms in the quantum ground state of the con®ning
potential), we observe even lower pulse propagation velocities
(17ms-1) owing to the increased atom density. We report an
inferred nonlinear refractive index of 0.18 cm2W-1 and ®nd that
the system shows exceptionally large optical nonlinearities, which
are of potential fundamental and technological interest for quantum
optics.
Now let me talk about a possible use of the above work. As we know, there is interactions between photons, although such interactions are usually very weak. In vacuum, such interaction happens at about a frequency inversely proportional to the eighth power of bare (vacuum) light speed, c. The interactions can be understood in either quantum electrodynamics or classical ones (QED or CED). For simplicity, let's illustrate this with CED. According Maxwell equations, the EM fields are produced by sources, which are electrical charges and currents. These charges and currents come from of course matters (vacuum is also a kind of matter in the quantum sense, that vacuum can also interaction with other things). The whole system is thus a complex of both matter and EM fields. The laws governing the dynamics of this global system are expressed as two schools of equations: Maxwell ones and Newton's equations, the former telling EM fields how to behave while the latter dictating the matter how to behave. These two schools of equations are coupled: (1)Maxwell equations containing sources in terms of variables of matter; (2)Newton's equations containing forces coming from EM field strengths. Now if one express the matter variables in terms of EM fields through solving Newton's equations, one is able to obtain a highly nonlinear and anharmonic equations for EM fields by substituting the sources in terms of EM fields as obtained. The anharmonicity directly results in photon-photon interaction, which means, the superposition principle does not hold exactly true. Such anharmonicity is quite hard to capture in usual experiments, because it is of the order of 1/c^8 in the vacuum. On the other hand, if we can reduce light speed, such interaction shall increase. Now my idea is this: fire two light beams opposite to each other upon the sodium gas as described in [1], and these two beams shall linger in the gas due to slowing down and interact repeatedly, via generating electron-hole pairs, much the same way as in vacuum. The calculations can be easily done.
[1]Light speed reduction to 17 metres per second in an ultracold atomic gas
Lene Vestergaard Hau*², S. E. Harris³, Zachary Dutton*²
& Cyrus H. Behroozi*§
* Rowland Institute for Science, 100 Edwin H. Land Boulevard, Cambridge,
Massachusetts 02142, USA
² Department of Physics, § Division of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138, USA
³ Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305,
USA
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