M5L25o.txt

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Finally, the form of superradiance,
which is very important, is Raman superradiance.
So we don't have an excited state
whereby we put a lot of excitations
on, because the excited state would be very short-lived.
So what we instead do is we have Rabi frequency omega 1.
We have a large detuning delta.
And then the spontaneous emission
with the coupling constant, g, takes us down
to the excited state.
In the case that the Rabi frequency is much, much smaller
than delta, we can eliminate the excited state
from the description.
And what we obtain is now a system
which has an excited state.
The widths of this excited state,
this is pretty much the virtual state here,
is the scattering rate, which is the probability
to excite the atom is Rabi frequency over detuning
squared.
That's just perturbation theory.
And then we multiply with gamma or gamma over 2.
So this is the rate of spontaneous emission out
of the virtual state.
And from this virtual state, we go now to the ground state.
And the Rabi frequency, or the coupling
for this virtual state, is the original coupling, g,
between ground and excited state,
but now prorated by the amplitude
that we have mixed the excited state into the virtual state.
So therefore, we have now obtained a superradiant system.
And for instance, we did experiments
which became classic now because their concept is so clear:
We took a Bose-Einstein condensate,
we switched on one strong off-resonant laser beam,
and then we had a system which was 100% inverted
because we had no atoms in the final state.
The final state is a Bose-Einstein condensate,
but with a recoil kick.
So by just having a Bose-Einstein condensate
and shining this laser light on it, we had now in this picture
a 100% inverted system, which is the ideal realization
of a fully inverted Dicke state.
Everything is completely symmetric,
and then we observed superradiant emission
of light pulses.
So this has been the important realization.
So, important experiments have been
done with BECs in my group, and with cold atoms
in laser-cooled samples in Vladan Vuletic's group.

So why is superradiance so important?
If you have extended sample superradiance,
those samples are no longer coupling
to the electromagnetic field with the coupling constant, g.
The coupling constant g is now multiplied
by the optical density of your sample.
And there is a lot of interest for current research
for quantum computation, manipulation of photon states
and all that to do cavity QED.
And in cavity QED, we try to have
very good mirrors, very small mode volume
to have a very, very large g.
But this large g which we achieve in a cavity,
if you put many atoms in it, gets enhanced
by the optical density.
So the cavity enhancement and the superradiant enhancement
are multiplicative.
And often, it's very favorable for single photon manipulation
if you do both.
You get enhancement from the cavity,
and enhancement due to superradiance.
And the person who has really pioneered work along these direction is Vladan Vuletic here at MIT.