IP: Quantum cryptography: Codemaking through quantum mechanics

[Dave Farber <farber () cis upenn edu>]

> Date: Sat, 29 Apr 2000 09:43:50 -0700
> To: politech () vorlon mit edu
> From: 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!
> er!
> !
> !
> !
> 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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