There might be other uses for quantum superpositions, though. Stroud proposes data storage on an atom, because an electron in a Rydberg atom could be made to inhabit a superposition of 2,500 different energy levels. “That means that the electron’s wave function can be quite complex, encoding a great deal of information,” Stroud expounds. He demonstrated the possibility theoretically by writing “OPTICS” on an atom. Other uses for quantum superposition, such as in cryptography, chemistry and even teleportation, have been demonstrated. Schrödinger’s boxed cat may have outwitted the best philosophical minds so far, but it seems to have found plenty of technological reasons to stay put.
Jobs for Quantum Cats
Researchers have proposed and demonstrated several technologies exploiting entangled and superposed quantum states, such as quantum computing. A few other schemes include the following:
Using lasers, researchers can place molecules in a superposition of reaction pathways; then they can control the chemical process by adjusting the degree of interference. Last December workers separated isotopes with a similar technique. Obstacles include less than practical efficiency levels and difficulty in controlling phase characteristics of the laser.
Quantum Key Cryptography
A much better prospect than quantum computing is quantum key cryptography. Legitimate communicators create shared keys using the polarization of photons. Eavesdropping on these keys would immediately be noticed, because it would disrupt the key photons’ states. Quantum cryptography has been shown to function over several kilometers in optical fibers.
The idea has less to do with Star Trek than with reconstructing destroyed information. The crux is the Einstein-Podolsky-Rosen effect, which shows that two photons can remain entangled, no matter how far apart they are, until a measurement is made (which instantaneously puts both in a definite state). Alice takes one EPR photon, Bob the other. Later, Alice measures her EPR photon with respect to a third photon. Bob can use the relational measurement to re-create Alice’s non-EPR photon. Whether Bob truly rematerialized the photon or just created an indistinguishable clone is unclear. Researchers at the University of Innsbruck reportedly demonstrated the phenomenon, which might have use in quantum cryptography.
Quantum Laser Optics
Lasers ordinarily require a population inversion, a condition in which atoms in an excited state outnumber those in the ground state; the excited atoms emit laser photons as they drop to the ground state. In 1995 researchers sidestepped this requirement. In lasing without inversion, two coupling lasers give ground-state atoms two paths to one higher energy level. Interference between the paths renders the ground-state atoms invisible, and so fewer excited atoms are needed. Such lasers do not require as much power and in principle could emit light in the desirable x-ray region.