Members of the American Physical Society recently elected Williams professor of physics William Wootters to the position of fellow, an honor shared by only a handful of the society’s members. Wootters was elected for conceiving of quantum teleportation, a method of transmitting information that may some day allow us to build computers on the order of a trillion times faster than the fastest computers we have today. His achievement has even given him a cover-page article in Scientific American.
Quantum teleportation is not like the “beaming-up” seen in Star Trek. In fact, the term “teleportation” is a bit of a misnomer. The procedure is not the actual physical teleportation of a particle as the word might suggest, but rather the teleportation of the properties, or quantum states, of the particle to another particle.
Wootters and five other physicists first conceived of the idea of quantum teleportation at a conference at the University of Montreal in 1992. They developed it to solve the problem of attempting to send a photon of a specific polarization to a recipient at an unknown location. Polarization is a property of photons, tiny packets of light that is only somewhat understood today; let it suffice that a photon can have any polarization that is perpendicular to the direction of its motion.
Wooters explains the phenomenon with a hypothetical situation. Suppose that Alice has a photon of unknown polarization. She wishes Bob, whose location she does not know, to have a photon of the identical polarization. Says Wootters, “You might think, why doesn’t she just measure the polarization,” and broadcast that information everywhere with a label indicating that this information is the polarization of Alice’s photon? “Bob could then just construct a photon of that polarization in his lab – that would be easy enough – and then the job would be accomplished.
“The reason that’s impossible,” Wooters continued, “is that you can’t determine the polarization of a photon by measuring it.” Measurements of a photon’s polarization are quantum measurements, which differ from everyday, macroscopic measurements in that it must be binary (one cannot ask, “What is the polarization of a photon?” one must ask, “Is this photon polarized up or down?”). Even if the photon is not polarized up or down, it must report a polarization that satisfies the question asked in the act of measurement, either “I am up,” or “I am down.” The polarization of the photon becomes whatever it reported, and the information describing its former polarization is destroyed.
Since the idea of simply measuring the photon’s polarization and then broadcasting that information will not succeed, “another thing [Alice] might think of trying is to use some sort of amplifying device to make many copies of the photon.” She could then send out these copies of the photon and if she sent enough Bob would receive one. Unfortunately, as Wootters explains, “the copying idea doesn’t work either, because there’s a basic theorem of quantum mechanics…that you can’t make a copy of an unknown quantum state.”
How then, does quantum teleportation solve Alice and Bob’s problem? The first step is to create an “entangled” pair of photons. Quantum objects, such as photons and electrons, can be entangled with regard to a particular quantum property. Two photons entangled with respect to their polarization always are measured to have opposite polarizations. Thus, if one photon is measured to have up polarization, the other photon instantaneously becomes polarized down. Entangled photons do not actually have polarizations at all until one is measured, at which point one becomes polarized one direction and the other becomes polarized the opposite direction.
Producing entangled photons is simple—one only has to send a pulse of light through a crystal. Any photons that are deflected by the crystal become entangled with respect to their polarizations.
After creating this entangled pair of photons, one, photon A, is given to Alice, and the other, photon B, to Bob. Bob and Alice then go their separate ways each not knowing where the other is. The way Alice can now furnish Bob with information about a photon’s polarization without measuring it is by making a joint measurement between her entangled photon and the photon whose polarization she wishes Bob to have access to, photon C.
Wootters’ concept has resulted in the first quantum computer: a two-bit processor consisting of a single atom. Unlike present-day computers, which store information in a manner consistent with pre-quantum physics, quantum computers would store and transmit information through objects that behave according to the non-intuitive laws and non-Boolean logic of quantum mechanics. The advantage of quantum laws in computing is that quantum laws permit the processing of information far faster than classical laws that govern the behavior of today’s microchips.
Wootters says that if it took the fastest computer today billions of years to factor a very large number, it would only take a quantum computer a day. He expects these working quantum computers to appear in roughly twenty years, with primitive “quantum microchips” or “quantum networks” being developed in ten.
Wootters has spent much of his time and effort since the 1992 conference working on quantum computers through his work with entanglement, the foundation of quantum teleportation. As entanglement is not yet understood quantitatively, Wootters has been developing a theory of entanglement that will help us better understand it.