Night lights: seeing in quantum darkness.

It sounds like an impossible mission: Safely detect the presence of a bomb so sensitive that even a single photon triggers it.

Although no such bomb exists, it serves as a convenient target for the probes of physicists exploring the fringes of quantum mechanics. Researchers have now shown that, in principle, they can shine a photon at a bomb's suspected position in such a way that the bomb neither absorbs nor reflects the photon, yet still reveals its presence.

Paul G. Kwiat, Harald Weinfurter, Thomas Herzog, and Anton Zeilinger of the University of Innsbruck, Austria, and Mark A. Kasevich of Stanford University describe their interactionfree quantum measurements in the June 12 Physical Review Letters.

These researchers have come "closer than any before to inventing a way of seeing in total darkness," comments Charles H. Bennett of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y.

In the realm of quantum physics, any measurement typically disturbs the system being measured, whether an electron orbiting an atom or a light beam passing through a pair of narrow slits. But this isn't the full story. In 1993, Avshalom C. Elitzur and Lev Vaidman of Tel Aviv University in Ramat Aviv, Israel, developed a scheme in which a partially silvered mirror splits a photon so it travels along two separate optical paths before recombining. If no obstacle is present along either path and no measurement is made to determine along which path the photon travels, the photon behaves like a wave, traversing both paths simultaneously and exhibiting interference at its destination.

Elitzur and Vaidman set up their interferometer so that if no bomb is present, the two photon "halves" interfere constructively to give a signal at one detector and destructively to give no signal at a second detector. By checking which detector picks up a photon, they could sometimes ascertain the presence of a bomb without exploding it. In these cases, the bomb modifies the photon interference pattern without interacting with the photon.

Kwiat and his coworkers improved on this technique. Using a more complicated measurement scheme, they can detect whether a bomb is present, while reducing the chance of exploding it as close to zero as desired. Their strategy hinges on the idea that continued observation of a quantum system can suppress spontaneous changes in behavior that would naturally occur in the undisturbed system. Thus, by repeatedly checking whether a system is in its initial state, one can prevent it from ever leaving that state.

The researchers take advantage of this effect in the passage of polarized photons through a medium that rotates a photon's direction of polarization. They set up an apparatus in which a photon emerging with its original polarization signifies the bomb's presence (without exploding it), while a new polarization indicates the bomb's absence.