Linking particles, waves, and uncertainties.

At the heart of quantum mechanics lies the notion that quantum objects such as electrons, photons, and atoms behave both like waves and like particles. However, according to the complementarity principle formulated by Niels Bohr, it's impossible to observe both wave and particle characteristics at the same time. The act of observation itself causes the quantum object to assume either its wave or particle guise.

Quantum mechanics also incorporates Werner Heisenberg's uncertainty principle, which establishes that it is impossible to determine simultaneously both the momentum and the position of a quantum object with perfect precision. If one can measure an object's position with absolute certainty, then its momentum is completely unknown and vice versa.

In many instances, physicists have shown that the uncertainty principle is responsible for the difficulty of observing both particle and wave behavior at the same time. Measuring an object's position makes its momentum unpredictable, and such momentum changes wipe out the object's wave-like properties. Hence, complementarity is a consequence of the uncertainty principle.

But it hasn't been clear whether this is always the case. Now, physicist Daniel E Walls and his colleagues at the University of Auckland in New Zealand show theoretically that this relationship between the complementarity and uncertainty principles must hold in any experiment in which a quantum object passes through a closely spaced pair of slits in a barrier.

Walls and his coworkers report their findings in the Feb. 17 NATURE.

In the classic double-slit experiment, quantum objects such as electrons must pass through two slits to get to a screen (see diagram). Because electrons can act like waves, they create a striped interference pattern on the screen (corresponding to where the waves cancel and reinforce each other) instead of just a pair of stripes directly in line with the slits.

However, any attempt to put a detector along an electrons path to pinpoint which slit the electron travels through destroys the interference pattern. A researcher ends up observing either the interference pattern or the electron's path, but not both at once.

In 1991, Marian O. Scully of the University of New Mexico in Albuquerque and his collaborators suggested a type of double-slit experiment involving atoms and lasers in which the interference pattern disappears and an atom's path is detected, apparently without transferring momentum to the atom. On this basis, the researchers suggested that complementarity is an independent component of quantum mechanics. It need not always follow from the uncertainty principle.

Walls and his coworkers argue that in a double-slit experiment, the amount of momentum transferred by the detector to a quantum object depends on the distance between the two slits, in accordance with the uncertainty principle. If less momentum is transferred, the object's path cannot be determined with certainty, and the object retains wave-like characteristics.

Scully and his colleagues had overlooked the momentum kicks, which come from the repeated emission and absorption of photons by an atom, that are needed to wipe out the atom's wave-like behavior, Walls and his group conclude. The uncertainty principle still rules.