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Section 7: Converting Light to Matter and Back: Carrying Around Light Pulses

Turning light into matter with the creation of a perfect matter copy of the extinguished light pulse.

Figure 16: Turning light into matter with the creation of a perfect matter copy of the extinguished light pulse.

Source: © Reprinted by permission from Macmillan Publishers Ltd: Nature 445, 623-626 (8 February 2007). More info

Light and other electromagnetic waves carry energy, and that energy comes in quanta called "photons." The energy of a photon is proportional to the frequency of the light with a constant of proportionality called Planck's constant. We have already encountered photons on several occasions and seen that photons carry more than energy—they also carry momentum: an atom gets that little kick each time it absorbs or emits a photon.

When we slow, compress, stop, or extinguish a light pulse in a Bose-Einstein condensate, we end up with the holographic, dark state imprint where atoms are in states 1 and 2 at the same time. That part of an atom which is in state 2 got there by the absorption of a probe laser photon and the emission of a coupling laser photon. It has thus received two momentum kicks: one from each laser field. Therefore, the state 2 imprint starts to move slowly—at a few hundred feet per hour. It will eventually exit the condensate and move into free space.

The matter copy continues the journey.

Figure 17: The matter copy continues the journey.

Source: © Reprinted by permission from Macmillan Publishers Ltd: Nature 445, 623-626 (8 February 2007). More info

At this point, we have created, out in free space, a matter copy of the light pulse that was extinguished. This matter copy can be photographed as shown in Figure 16. It takes the matter copy roughly one-half millisecond to exit the condensate, and we note that it has a boomerang shape. Why? Because when the light pulse slows down in the condensate, it slows most along the centerline of the condensate where the density of atoms is highest. Therefore, the light pulse itself develops a boomerang shape, which is clearly reflected in its flying imprint.

It is also interesting to remember that the atoms are in a superposition of quantum states: An atom in the matter copy is actually both in the matter copy traveling along and at the same time stuck back in the original condensate. When we photograph the matter copy, however, we force the atoms to make a decision as to "Am I here, or am I there?" The wavefunction collapses, and the atom is forced into a single definite quantum state. If we indeed have at hand a perfect matter copy of the light pulse, it should carry exactly the same information as the light pulse did before extinction; so could we possibly turn the matter copy back into light? Yes.

Turning light into matter in one BEC and then matter back into light in a second BEC in a different location.

Figure 18: Turning light into matter in one BEC and then matter back into light in a second BEC in a different location.

Source: © Reprinted by permission from Macmillan Publishers Ltd: Nature 445, 623-626 (8 February 2007). More info

To test this, we form a whole different condensate of atoms in a different location, and we let the matter copy move into this second condensate. Two such condensates are shown in Figure 17. In these experiments, the light pulse comes in from the left and is stopped and extinguished in the first (leftmost) condensate. The generated matter copy travels into and across free space, and at some point it reaches the second (rightmost) condensate. If we don't do anything, the matter copy will just go through the condensate and come out on the other side at 2.5 ms and 3.0 ms (See Figure 18). On the other hand, once the matter copy is imbedded in the second condensate, if we turn on the coupling laser...out comes the light pulse! The regenerated light pulse is shown in Figure 18a.

So here, we have stopped and extinguished a light pulse in one condensate and revived it from a completely different condensate in a different location. Try to run the animation in Figure 19 and test this process for yourself.

Figure 19: A light pulse is extinguished in one part of space and then regenerated in a different location.

Source: © Sean Garner and Lene V. Hau. More info

Quantum puzzles

The more you think about this, the weirder it is. This is quantum mechanics seriously at play. The matter copy and the second receiver condensate consist of two sets of atoms that have never seen each other, so how can they together figure out to revive the light pulse? The secret is that we are dealing with Bose-Einstein condensates. When we illuminate the atoms in the matter copy with the coupling laser, they act as little radiating antennae. Under normal circumstances, these antennae would each do its own thing, and the emitted radiation would be completely random and contain no information. However, the lock-step nature of the receiver condensate will phase lock the antennae so they all act in unison, and together they regenerate the light pulse with its information preserved. When the light pulse is regenerated, the matter copy atoms are converted to state 1 and added as a bump on the receiver condensate at the revival location. The light pulse slowly leaves the region, exits the condensate, and speeds up.

This demonstrated ability to carry around light pulses in matter has many implications. When we have the matter copy isolated in free space, we can grab onto it—for example, with a laser beam—and put it "on the shelf" for a while. We can then bring it to a receiver condensate and convert it back into light. And while we are holding onto the matter copy, we can manipulate it, change its shape—its information content. Whatever changes we make to the matter copy will then be contained in the revived light pulse. In Figure 18b, you see an example of this: During the hold time, the pulse is changed from a single-hump to a double-hump pulse.

You might ask: How long can we hold on to a light pulse? The record so far is a few seconds. During this time, light can go from the Earth to the Moon! In these latest experiments, we let the probe and coupling beams propagate in the same direction; so there is basically no momentum kick to the matter imprint, and it stays in the atom condensate where it was created. When atoms in the matter imprint collide with the host condensate, they can scatter to states other than 1 and 2. This will lead to loss of atoms from the imprint and therefore to loss of information. By exposing the atom cloud to a magnetic field of just the right magnitude, we can minimize such undesired interactions between the matter imprint and the host condensate. Even more, we can bring the system into a phase-separating regime where the matter imprint wants to separate itself from the host condensate, much like an oil drop in water. The matter imprint digs a hole for itself in the host, and the imprint can snugly nestle there for extended time scales without suffering damaging collisions. Also, we can move the matter imprint to the opposite tip of the condensate from where it came in, and the imprint can now be converted to light that can immediately exit the condensate without losses. This shows some of the possibilities we have for manipulating light pulses in matter form.