For the Danish physicist Niels Bohr, a founder of quantum theory (and to whom Schrödinger’s regretful comment was directed), the answer was that measurements must be made with a classical apparatus. In what has come to be called the standard, or Copenhagen, interpretation of quantum mechanics, Bohr postulated that macroscopic detectors never achieve any fuzzy superposition, but he did not explain exactly why not. “He wanted to mandate ‘classical’ by hand,” says Wojciech Zurek of Los Alamos National Laboratory. “Measurements simply became.” Bohr also recognized that the boundary between the classical and the quantum can shift depending on how the experiment is arranged. Furthermore, size doesn’t necessarily matter: superpositions can persist on scales much larger than the atomic.
In November 1995 Pritchard and his M.I.T. colleagues crystallized the fuzziness of measurement. The team sent a narrow stream of sodium atoms through an interferometer, a device that gives a particle two paths to travel. The paths recombined, and each atom, acting as a wave, “interfered” with itself, producing a pattern of light and dark fringes on an observing screen (identical to what is seen when a laser shines through two slits). The standard formulation of quantum mechanics states that the atom took both paths simultaneously, so that the atom’s entire movement from source to screen was a superposition of an atom moving through two paths.
The team then directed a laser at one of the paths. This process destroyed the interference fringes, because a laser photon scattering off the atom would indicate which path the atom took. (Quantum rules forbid “which-way” information and interference from coexisting.)
On the surface, this scattering would seem to constitute a measurement that destroys the coherence. Yet the team showed that the coherence could be “recovered”— that is, the interference pattern restored—by changing the separation between the paths to some quarter multiple of the laser photon’s wavelength. At those fractions, it was not possible to tell from which path the photon scattered. “Coherence is not really lost,” Pritchard elucidates. “The atom became entangled with a larger system.” That is, the quantum state of the atom became coupled with the measuring device, which in this case was the photon.
Like many previous experiments, Pritchard’s work, which is a realization of a proposal made by the late Richard Feynman many years ago, deepens the mysteries underlying quantum physics rather than resolving them. It demonstrates that the measuring apparatus can have an ambiguous definition. In the case of Schrödinger’s cat, then, is the measurement the lifting of the lid? Or when light reaches the eye and is processed by the mind? Or a discharge of static from the cat’s fur?
A recent spate of Schrödinger’s cat experiments have begun to address these questions. Not all physicists concur that they are looking at bona fide quantum cats—“kitten” is the term often used, depending on the desired level of cuteness. In any event, the attempts do indicate that the quantum-classical changeover— sometimes called the collapse of the wave function or the state-vector reduction— has finally begun to move out of the realm of thought experiments and into real-world study.
Here, Kitty, Kitty
In 1991 Carlos Stroud and John Yeazell of the University of Rochester were experimenting with what are called Rydberg atoms, after the Swedish spectroscopist Johannes Rydberg, discoverer of the binding-energy relation between an electron and a nucleus. Ordinarily, electrons orbit the nucleus at a distance of less than a nanometer; in Rydberg atoms the outer electron’s orbit has swollen several 1,000-fold. This bloating can be accomplished with brief bursts of laser light, which effectively put the electron in many outer orbitals simultaneously. Physically, the superposition of energy levels manifests itself as a “wave packet” that circles the nucleus at an atomically huge distance of about half a micron. The packet represents the probability of the excited electron’s location.