Exploiting Quantum 'Spookiness' to Create Secret Codes


5/1/2000

Exploiting quantum "spookiness" to create secret codes has been demonstrated for the first time (April 2000), 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 (artur.ekert@qubit.org) 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 otherwise 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, researcher Paul Kwiat and colleagues 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. (Courtesy American Institute of Physics, Physics News Update 480.)

The woodcut illustration shown above is taken from Blaise de Vigenere's Traicté des chiffres, published in Paris in 1586. In this scheme, the message is hidden among the stars and clouds and is decoded with a shared text.

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