Using lasers and ultracold atoms, physicists have found a way to stop and start a pulse of light. This magic trick may one day be used to store data in a quantum computer.
Imagine you are standing beside a railway track, waiting for the next express train. Stretched across the tracks in front of you is a sheet of some strange, iridescent fabric, as thin as silk, which the train is about to rip apart (a, above). But, to your amazement, when the front of the train hits the gossamer fabric it does not immediately break through — instead it appears to be 'soaked up' into the material (b). In just a few seconds the entire train has vanished into the fabric. The roar of the train is replaced by an eerie silence. Then, as suddenly as it disappeared, the train emerges from the other side of the fabric (c), resuming its original length and speed, and roars off out of sight.
This trainspotting fantasy helps us to appreciate the most recent accomplishments (presented on page 490 of this issue1) of Lene Hau and her colleagues. Two years ago, Hau's group shot a pulse of laser light 3 microseconds in duration and about 1 kilometre in length into a specially prepared sample of ultracold sodium gas2. The gas sample was about 0.2 mm in length and had the unusual property that the velocity of light within it was ten million times slower than in free space. When the leading edge of the pulse entered the sodium cloud, it immediately slowed to the unheard of speed of 30 m s −1. At this leisurely pace, the light pulse took so long to cross the sample that, long before it emerged from the other side, the tail end of the light pulse vanished into the sample as well. Squeezed to within one ten-millionth of its original length, the pulse crept across the sample until finally it emerged, restored to its original length, and accelerated to its customary speed of 3 × 108 m s−1 .
The key to slowing light is the presence of a second laser beam, the so-called 'coupling' pulse. Distinguishable from the propagating (or 'probe') pulse by its polarization, the coupling light delicately adjusts the internal energy levels of the atoms, suppressing their ability to absorb the probe light — in effect, a single absorption level is split into two levels that cancel each other out. This phenomenon is known as electromagnetically induced transparency3. At the same time, the 'refractive index' of the atomic cloud — in simple terms, how much it bends light — develops a steep dependence on the probe frequency. This in turn leads to a very slow 'group velocity' — the speed at which the envelope of light intensity moves through the sample.
Much of the excitement over the original experiment by Hau's group focused on the value of the speed of light obtained. And in the past two years, several other groups have achieved similar results4,5,6, using more conventional, room-temperature gases. In some cases, the speed of light was reduced to even slower than 30 m s−1. Casual observers of the fracas were left to wonder, why bother with the ultracold atoms? But a more careful reading of the 'hot-atom' papers shows that they all lacked the eerie quality that so perturbed our hapless hobbyist by the train tracks. The maximum delay that the room-temperature experiments could impose on a light pulse was much less than the total duration of the pulse itself. So there was no chance that a significant fraction of the probe pulse (let alone the entire pulse) could be tucked away inside the sample of atoms. From a technological viewpoint, if we regard the propagating light pulse as a 'bit' of optical information, then the hot-atom experiments displace each bit by only a small fraction of its width — not enough to turn a one into a zero, by any means.
The new results from Hau's group1 bring the sense of wonder back to slow-light experiments. In their latest caper, the group changes the rules of the game right in the middle of the observation. This time there is a magician standing beside the rail track. At the precise moment when the entire express train disappears into the fabric, the magician snaps her fingers. Click. And nothing happens. No train reappears. Finally, after a time long enough (had the magician not intervened) for the train to have gone another 200 km along the track, she snaps her fingers again and shazam! the train comes blasting out as if nothing had happened. Or, she can snap her fingers three times in quick succession and three separate carriages of the train appear.
What Hau and her colleagues actually do is cool a sample of sodium atoms to within one millionth of a degree above absolute zero. With the coupling-beam intensity tuned to provide a group velocity of 28 m s−1, they inject a probe pulse 2 km long into the sample. Then, just as the pulse disappears into the sample, but before it reappears, the coupling beam is turned off. As a result, the speed of the probe beam goes to zero as well, and the pulse comes to a dead stop in the middle of the sample. The pulse of light can be kept on ice in the sample for up to a millisecond (a virtual eternity on the scale of the original 6-microsecond pulse duration). At any time in that millisecond interval, the coupling beam can be turned back on, resulting in the probe beam promptly emerging from the sample and continuing along its way.
How can a photon be brought to a halt? Aren't these fundamental particles of light meant to travel, after all, at the speed of light? The key fact here is that, as the pulse of light penetrates into the dense region of the ultracold atomic cloud, it turns into a 'quantum coherence pattern' printed on the sodium atoms — the information in the light beam becomes stored within the quantum phase relationship within the internal atom states. In the final limit, when the pulse comes to a dead stop, all the photons have been 'imprinted' (absorbed in a fully reversible way) into the coherence pattern. Later, when the coupling light is turned back on, the information contained in the pattern is read out and converted back into propagating photons that accelerate to the conventional speed of light as they come to the edge of the atom sample.
While the probe pulse is in captivity, it is subject to the capricious powers of its jailers. By rapidly turning the group velocity up and down, Hau's group can let a small fraction of the pulse escape the sample, while keeping the remainder confined. In this way, they can chop up the incoming pulse into as many as three transmitted pulses, with an arbitrary delay between each pulse. The authors refer to this as 'multiple read-out' of the stored pulse. Moreover, if the probe pulse is 'written in' to the material at a certain group velocity, and then is 'read out' at a higher group velocity, the transmitted pulse will have a shorter duration, and correspondingly higher intensity, than the injected pulse. It is as if the fantasy express train is now shorter and wider than the original.
Hau and her colleagues suggest that their newly demonstrated ability to control the flow of optical information may have technological relevance to quantum computing. Whether this will turn out to be true is not clear. But for now it hardly matters — trainspotting just doesn't get any more interesting than this.
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Scientific Reports (2016)