Number 231 (Story #2), June 23, 1995 by Phillip F. Schewe and Ben Stein A "MOLECULE" OF LIGHT, a group of photons acting as a single bound object, can effectively be created with a device proposed by Joseph Jacobson and his colleagues at Stanford University. All objects, whether atoms or photons, can be thought of as waves spreading out in space, with a "deBroglie wavelength" proportional to the object's momentum. For instance, if one binds together two atoms having the same momentum, the deBroglie wavelength of the resulting molecule is half that of the individual atoms. By contrast, if one combines two photons each having a wavelength corresponding to blue light, the wavelength normally remains that corresponding to blue light. But Jacobson (415-725-7699) and colleagues hope to get around this dilemma by proposing the development of an interferometer (a device in which a wavetrain is split into two parts and later recombined to form an interference pattern) using an optical element known as a quantum switch, consisting of a specially prepared atom contained in a tiny resonant cavity. When illuminated by the light beam, the switch would permit either all or none of the photons to pass through. Jacobson suggests that if an ensemble of 100 photons were sent into the switch, the photons would take on a dual personality: considered as a form of light, the blue photons would indeed retain their blue wavelengths; but, if the photon wave train were made to interfere with itself, the group of photons would behave as if it were a composite object with an effective wavelength only 1/100th that of individual blue photons. (Think of a busy restaurant where a fussy maitre'd, addressing a waiting list of 10 unrelated strangers, tells them that they will be admitted only if they all sit together.) Why enforce such a togetherliness on photons? The much smaller wavelength of the photon "molecule" would result in a greater sensitivity than is possible with current interferometers which, exquisitely sensitive to small rotations or changes in the path lengths over which the light waves propagate, are used in such devices as accelerometers and gyroscopes. In addition, this sort of interferometry will permit the study of the extent to which the deBroglie wavelength depends on the internal structure of an object. (Joseph Jacobson et al., Physical Review Letters, 12 June 1995. Science journalists can obtain copies of the article or related figures by contacting AIP Public Information, 301-209-3091 or physnews@aip.org)