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ISIS Press Release 19/03/04
Trapping Light
Remarkable things can happen when atoms and molecules interact with
light. Normally opaque matter can become transparent, and light can be slowed
down enormously, or come to a complete stop. The secret is quantum coherence,
and the promise, quantum communication. Dr. Mae-Wan Ho reports
A full version this article with illustration and sources is posted
on ISIS members’ website. Details here.
Seeing through a wall?
Normal light does not interact strongly with matter, which is why we can
see, and why it is so good for communicating information. But processing
information requires interaction between signal carriers, and that generally
involves strong light-matter interaction.
Normal light is incoherent light, vibrating in random directions
and phases over a wide range of frequencies. In contrast, coherent light
vibrates in phase in one direction and usually at a single frequency, as in a
monochromatic laser. Coherent light interacts with matter very strongly, and
that is when remarkable things can happen.
When atoms and molecules absorb light energy, they go from
‘ground’ states to ‘excited’ states; and different atoms
and molecules require energy at specific frequencies. When the frequency of the
coherent light matches the frequency of excitation, resonance occurs,
and the response of the particular atoms or molecules is greatly enhanced.
Light is strongly absorbed and dispersed, as the excited atoms fluoresce, i.e.,
decay back to the ground state by re-emitting light.
If the atoms have two lower energy states (1 and 2) to which the excited
state (3) can drop (Fig. 1), it is possible to modify the propagation of light
resonating with the 3→1 transition by applying a second
‘control’ or ‘coupling’ laser that resonates with the
3→2 transition. The combined effects of the two laser-light fields is to
get all the atoms in the medium into a coherent quantum
superposition of the states 1 and 2, with a definite phase relationship
between them. That means the atoms are in both states 1 and 2 at the
same time; as in the famous parable of Schrödinger’s cat that’s
in a superposition of being both dead and alive at the same time.
In this condition, the two possible pathways in which light can be
absorbed for the 1→3 and the 2→3 transitions interfere and cancel
each other, so no light is absorbed. In other words, the atomic medium becomes
totally transparent. This phenomenon is known as electromagnetically induced
transparency (EIT).
Figure 1 Atomic transitions that can be tuned to
coherent quantum superposition.
The same conditions give the possibility of trapping light or slowing
it down enormously, even to a standstill.
How to slow light down
EIT depends on a very good match between the frequencies of the two
atomic transitions to their respective laser frequencies, especially the
frequencies separating the states 1 and 2. If matching is not perfect the
interference is not ideal, and the medium becomes absorbing. Hence the window
of transparency is very narrow. It typically goes up steeply and drops steeply
on the other side. At its peak, the refractive index of the atomic
medium (a measure of how quickly light propagates through the medium) is very
nearly equal to 1; which means that the propagation velocity of light is equal
to that in vacuum. To either side, however, the refractive index drops sharply
with frequency. That means light is very much slowed down. The group velocity
vg of the
‘wave-packet’ of light is much smaller than the speed of light in
vacuum, c; in fact, vg depends on the
intensity of the control laser field and the density of atoms, decreasing the
control power or increasing the atom density makes vg slower.
What happens when a beam or pulse of light enters the medium is this.
At the front end, the light beam is rapidly slowed down while the back end is
still propagating at the speed of light in vacuum. Thus, the light beam is
effectively compressed by the ratio c/ vg, its energy is being
expended to establish coherent superposition between atomic states 1 and 2, or
in other words, to flip atomic spins. The wave of flipped spins now propagates
together with the light pulse, in a ‘dark-state polariton’. As the
light pulse goes out of the medium, it once again expands to its full length,
and the atoms return to their original ground state. The pulse has been delayed
as a whole by t = (1/ vg –
1/c)L, where L is the length of the medium.
In 1999, Lene Hau and colleagues at Harvard University shot a pulse of
laser, 3 microseconds in duration and about l kilometre in length, into
ultra-cold sodium gas, about 0.2mm in length, that slowed the speed of light
ten million fold. When the tip of the light arrow entered the sodium cloud, it
slowed to the speed of 30 meters per second. It took so long to get across the
sodium cloud that long before the tip could emerge, the tail end of the light
arrow had vanished into the sample as well. Squeezed to one ten-millionth of
its original length, the arrow crept across until it finally emerged, restore
to its original length and accelerated to its usual speed of 3 x 108 metres per second.
But ultra-cold temperature is not really necessary to slow light down. A
subsequent experiment slowed light down to less than 100 metres per second in a
hot gas of rubidium atoms.
In January 2001, Lene Hau’s team managed to make light come to a
complete halt in the cloud of ultracold sodium atoms simply by turning off the
coupling laser. The light beam is thereby frozen, or trapped within the medium
for up to one millisecond, before the coupling laser is switched on to
regenerate the light beam. The information, or the coherent light field of the
beam becomes stored or imprinted within the coherent quantum phase relationship
within the internal atom states. When the coupling light is turned back on, the
information contained in that pattern is read out and converted back into the
propagating beam again.
By rapidly turning the coupling beam on and off in succession, the
researchers can let a small fraction of the pulse escape the sample, while
keeping the remainder confined. In this way, they can chop the original light
pulse into as many as three transmitted pulses, with an arbitrary delay between
each pulse.
Bajcsy and coworkers, also at Harvard University, recently trapped a
light pulse inside hot gaseous rubidium atoms by a similar method.
But in their experiment, they put in forwards and backwards control
beams simultaneously to regenerate the signal. The forward and backwards fields
interfere, producing a spatially periodic variation of light intensity, which
in turn results in spatially varying atomic absorption in the medium. This acts
like a stack of mirrors, which reflect and contain the regenerated field within
the medium. Only after one of the control beams is turned off does the light
pulse leave the medium in the direction of the remaining control beam. The two
laser pulses play a dual role: they convert the stored state into a signal
pulse, and modulate the atomic absorption to localize or freeze the regenerated
field.
Frozen light naturally suggests several interesting applications.
First, as there is little or no loss, there will be little or no noise. This is
great for quantum informatics. Furthermore, the nonlinearity associated with
phase coherent matter is very large: in a stationary pulse, even tiny amounts
of light will cause large effects, which have long been sought in both
classical and quantum signal processing.
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