http://www.aip.org/enews/physnews/2000/split/pnu480-1.htm Physics News Update The American Institute of Physics Bulletin of Physics News Number 480 (Story #1), April 24, 2000 by Phillip F. Schewe and Ben Stein EXPLOITING QUANTUM "SPOOKINESS" TO CREATE SECRET CODES has been demonstrated for the first time by three independent research groups, advancing hopes for eventually protecting sensitive data from any kind of computer attack. In the latest--and most foolproof--variation yet of the data-encryption scheme known as quantum cryptography, researchers employ pairs of "entangled" photons, particles that can be so intimately interlinked even when far apart that a perplexed Einstein once derided their behavior as "spooky action at a distance." Entanglement-based quantum cryptography has unique features for sending coded data at practical transmission rates and detecting eavesdroppers. In short, the entanglement process can generate a completely random sequence of 0s and 1s distributed exclusively to two users at remote locations. Any eavesdropper's attempt to intercept this sequence will alter the message in a detectable way, enabling the users to discard the appropriate parts of the data. This random sequence of digits, or "key," can then be plugged into a code scheme known as a "one-time pad cipher,"which converts the message into a completely random sequence of letters. This code scheme--mathematically proven to be unbreakable without knowledge of the key--actually dates back to World War I, but its main flaw had been that the key could be intercepted by an intermediary. In the 1990s, Oxford's Artur Ekert ([EMAIL PROTECTED]) proposed an entanglement-based version of this scheme, not realized until now. In the most basic version, a specially prepared crystal splits a single photon into a pair of entangled photons. Both the message sender (traditionally called Alice) and the receiver (called Bob) get one of the photons. Alice and Bob each have a detector for measuring their photon's polarization, the direction in which its electric field vibrates. Different polarizations could represent different digits, such as the 0 and 1 of binary code. But according to quantum mechanics, each photon can be in a combination (or superposition) of polarization states, and essentially be a 0 and 1 at the same time. Only when one of them is measured or other! ! ! ! ! ! ! wise disturbed does it "collapse" to a definite value of 0 and 1, in a random way. But once one particle collapses, its entangled partner is also forced to collapse into a specific digit correlated with the first digit. With the right combination of detector settings on each end, Alice and Bob will get the exact same digit. After receiving a string of entangled photons, Alice and Bob discuss which detector settings they used, rather than the actual readings they obtained, and they discard readings made with the incorrect settings. At that point, Alice and Bob have a random string of digits that can serve as a completely secure key for the mathematically unbreakable one-time pad cipher. In their demonstration, Los Alamos researchers (Paul Kwiat, 505-667-6173, [EMAIL PROTECTED]) simulated an eavesdropper (by passing the photons through a filter on their way to Alice and Bob) and readily detected disturbances in their transmissions (by employing what may be the first practical application of the quantum-mechanical test known as Bell's theorem), enabling them to discard the purloined information. In a separate demonstration of entangled cryptography for completely isolated Alice and Bob stations separated by 1 km of fiber optics, an Austrian research team (Thomas Jennewein, University of Vienna, 011-43-1-4277-51207, [EMAIL PROTECTED]) created a secret key and then securely transmitted an image of the "Venus" von Willendorf, one of the earliest known works of art. (See figures at www.quantum.at and Physics News Graphics.) Meanwhile, a University of Geneva group (Nicholas Gisin, [EMAIL PROTECTED], 011-41 22 702 65 97) demonstrates entangled cryptography over many kilometers of fiber using a photon frequency closest to what is used on real-life fiber optics lines. In these first experiments, the three groups demonstrated relatively slow data transmission rates. However, entanglement-based cryptography is potentially faster than non-entangled quantum cryptography, which requires single-photon sources (and therefore, faint light sources) to foil eavesdropping. Entangled cryptography also produces relatively small amounts of excess photons which an eavesdropper could conceivably skim for information.
