Some light reading.

-----Original Message-----
From: [log in to unmask]
[mailto:[log in to unmask]]On Behalf Of Declan McCullough
Sent: Saturday, April 29, 2000 12:44 PM
To: [log in to unmask]
Subject: FC: Quantum cryptography: Codemaking through quantum mechanics


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 ([log in to unmask]) 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, Los Alamos researchers (Paul Kwiat,
505-667-6173,
[log in to unmask]) 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, [log in to unmask]) 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,
[log in to unmask], 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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