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Rubbed out with the quantum eraser Scientific American Jan 1996

Atoms, photons and other puny particles of the quantum world have long been known to behave in ways that defy common sense. In the latest demonstration of quantum weirdness, Thomas J. Herzog, Paul G. Kwiat and others at the University of Innsbruck in Austria have verified another prediction: that one can "erase" quantum information and recover a previously lost pattern. Quantum erasure stems from the standard "two-slit" experiment. Send a laser beam through two narrow slits, and the waves emanating from each slit interfere with each other. A screen a short distance away reveals this interference as light and dark bands. Even particles such as atoms interfere in this way, for they, too, have a wave nature.

QUANTUM ERASURE relies on a special crystal, which makes pairs of photons (red) from a laser beam
(purple) in two ways: either when the beam goes through the crystal directly (lop) or after reflection by a
mirror (bottom). Devices that rotate polarization indicate-and can subsequently erase-a photon's path
information.

But something strange happens when you try to determine through which slit each particle passed: the interference pattern disappears. imagine using excited atoms as interfering objects and, directly in front of each slit, having a special box that permits the atoms to travel through them. Each atom therefore has a choice of entering one of the boxes before passing through a slit. it would enter a box, drop to a lower energy state and in so doing leave behind a photon (the particle version of light). The box that contains a photon indicates the slit through which the atom passed. Obtaining this "which-way" information, however, eliminates any possibility of forming an interference pattern on the screen. The screen instead displays a random series of dots, as it sprayed by shotgun pellets. The Danish physicist Niels Bohr, a founder of quantum theory, summarized this kind of action under the term "complementarity": there is no way to have both which-way information and an interference pattern (or equivalently, to see an object's wave and particle natures simultaneously. But what if you could "erase" that telltale photon, say, by absorbing it? Would the interference pattern come back? Yes, predicted Marlan 0. Scully of the University of Texas and his co-workers in the 1980s, as long as one examines only those atoms whose photons disappeared (see "The Duality in Matter and Light," by Berthold-Georg Englert, Marlan 0. Scully and Herbert Walther; Sci. Am December 19941. Realizing quantum erasure in an experiment, however, has been difficult for many reasons (even though Scully offered a pizza for a connrincing demonstration). Excited atoms are fragile and easily destroyed. Moreover, some th orists raised certain technical objections, namely, that the release of a photon can disrupt an atom's iorward momentum (Scully argues it does not). The Innsbruck researchers sidestepped the issues by using photons rather than atoms as interfering objects. In a complicated setup, the experimenters passed a laser photon through a crystal that could produce identical photon pairs, with each part of the pair having about half the frequency of the original photon (an ultra-violet photon became two red ones). A mirror behind the crystal reflected the laser beam back through the crystal, giving it another opportunity to create photon pairs. Each photon of the pair went off in separate directions, where both were ultimately recorded by a detector. Interference comes about because of the two possible ways photon pairs can be created by the crystal: either when the laser passes directly through the crystal, or after the laser reflects off the mirror and back into it. Strategically placed mirrors reflect the photons in such a way that it is impossible to tell whether the direct or reflected laser beam created them. These two birthing possibilities are the "objects" that interfere. They correspond to the two paths that an atom traversing a double slit can take. Indeed, an interference pattern emerges at each detector. Specifically, it stems from the "phase difference" between photons at the two detectors. The phase essentially refers to slightly different travel times through the apparatus (accomplished by moving the mirrors). Photons arriving in phase at the detectors can be considered to be the bright fringes of an interference pattern; those out of phase can correspond to th(@ dark bands. To transform their experiment into the quantum eraser, the researchers tagged one of the photons of the pair (specifically, the one created by the laser's direct passage through the crystal). That way, they knew how the photon was created, which is equivalent to knowing through which slit an atom passed. The tag consisted of a rotation in polarization, which does not affect the momentum of the photon. (In Scully's thought experiment, the tag was the photon left behind in the box by the atom.) Tagging provides which-way information, so the interference pattern disappeared, as demanded by Bohr's complementarily. The researchers then erased the tag by rotating the polarization again at a subsequent point in the tagged photon's path. When they compared the photon hits on both detectors (using a so-called coincidence counter) and correlated their arrival times and phases, they found the interference pattern had retumed. Two other, more complicated variations produced similar results. "I think the present work is beautiful," remarks Scully, who had misgivings about a previous claim of a quantum-eraser experiment performed a few years ago. More than just satisfying academic curiosity, the results could have some practical use. Quantum cryptography and quantum computing rely on the idea that a particle must exist in two states say, excited or not-simultaneously. In other words, the two states must interfere with each other. The problem is that it is hard to keep the particle in such a superposed state until needed. Quantum erasure might solve that problem by helping to maintain the integrity of the interference. "You can still lose the particle, but you can lose it in such a way that you cannot tell which of two states it was in" and thus preserve the interference pattern, Kwiat remarks. But even if quantum computing never proves practical, the researchers still get Scully's pizza. -Philip Yam