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07/18/95

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Physicists propose device to create "molecules" of light

STANFORD -- Particles of light, called photons, normally don't stick together. But Stanford researchers have proposed a device that, at least theoretically, can make them do just that.

If the researchers succeed in forcing groups of photons to act as single objects, then these de facto light molecules could be used to make ultrasensitive interferometers, instruments -- used in gyroscopes and accelerometers -- that measure very small distances by means of the interference between light rays. Such interferometers also would assist ongoing efforts to develop a new type of laser.

It is natural for ordinary subatomic particles to stick together to form atoms and molecules. That is the basis of everyday materials. But photons normally are not attracted to each other. Instead, they interact like waves that pass through each other without much effect.

Writing in the June 12 issue of Physical Review Letters, postdoctoral student Joseph Jacobson, graduate student Isaac Chuang, engineering research associate Gunnar Björk and applied physics Professor Yoshihisa Yamamoto describe a device -- a special kind of mirror called a quantum switch -- that they argue can force groups of photons to act as if they are bound together.

Researchers propose using this effect as the basis for a new kind of interferometer. Light molecules should have much shorter wavelengths than the photons that compose them. For example, a light molecule made up of 100 photons should generate waves that are one-hundredth the length of the wavelength of the individual photons.

(Ordinary interferometers have a sensitivity proportional to the square root of the number of photons, whereas the light molecule interferometer has a sensitivity directly proportional to the number of photons used. As a result, light molecule interferometers should be considerably more sensitive.)

A molecular light interferometer hasn't been built yet, but is right at the edge of current capabilities. A research group at the Ecole Normale Superieure in Paris led by Serge Haroche is working on the necessary quantum switches, and the Stanford team is building an entire light molecule interferometer with the help of a special kind of laser, called a squeezed-state laser, that is described below.

The quantum switch is based on one of the peculiarities of quantum mechanics, the theory that explains the behavior of particles in the sub-atomic realm.

Start with a very good mirror reflecting nearly 100 percent. If 100 photons hit the mirror, then almost certainly 100 photons will be reflected. Now comes the strange part. If a second, nearly 100 percent reflecting mirror is placed behind the first mirror at just the right distance, the two mirrors as a whole suddenly become totally transmitting. Now all 100 photons are almost certainly assured of passing through both mirrors.

Now take this one step further. Fill the cavity between the two mirrors with atoms that can exist in two states: normal and excited. If again the distance between the two mirrors is picked correctly, then when the atoms are in their normal state, the pair of mirrors continue to be totally transmitting. When all the atoms are electrically excited, however, all the photons are reflected.

What happens when only a portion of these atoms are excited?

"Then you get a very strange kind of beam splitter. Each group of 100 photons is either reflected or transmitted. You never see a case where 50 go one way and 50 go the other, as you would expect with an ordinary 50- 50 half-silvered mirror," Jacobson said.

In addition to the ability to measure extremely small distances, a molecular light interferometer could provide important details about the statistical state of the light from a given source. Such information is important for looking at the output of advanced lasers.

Current lasers have an important limitation. They do not produce a definite number of photons per second: The number fluctuates unpredictably. So researchers have been trying to develop lasers that produce a predictable number of photons per second. They call this a number- state laser.

Yamamoto has pioneered the development of so- called squeeze-state lasers that are a big step down the road toward number-state lasers. They have a significantly lower level of photon fluctuation than ordinary lasers, but are still far from constant.

Scientists currently are interested in number-state lasers for a wide range of applications, including communications, precision metrology, gravity wave detection and quantum computing.

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