Science in Society Archive

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

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 vgof the 'wave-packet' of light is much smaller than the speed of light in vacuum, c; in fact, vgdepends 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.

Article first published 19/03/04


  1. Liu C, Dutton Z, Behroozi CH and Hau LV. Observation of coherent optical information storage in an atomic medium using halted light pulses. Nature 2001, 409, 490-3.
  2. Lukin M and Imamo¢glu A. Controlling photons using electromagnetically induced transparency Nature 2001, 413,273.
  3. Bajcsy M, Zibrov AS and Lukin MD. Stationary pulses of light in an atomic medium. Nature 2003, 426, 638-41.

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