FC: Quantum cryptography: Codemaking through quantum mechanics

[Declan McCullagh <declan () well com>]

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 (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 oth!
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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, 
kwiat () lanl gov) 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, thomas.jennewein () univie ac at) 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, 
Nicolas.Gisin () physics unige ch, 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.

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