| From: | Chris Innanen <cgi@*******.JHUAPL.EDU> |
|---|---|
| Subject: | Quantum Communication (RL!) |
| Date: | Sun, 19 Dec 1993 16:46:07 -0500 |
you about it, but it's gonna be difficult... Unless you happen to be one
of the few that can wrap their brain around quantum mechanics.
This is a form of untappable communications line and unbreakable code.
The neatest thing about it is that those two 'un's aren't the sort of
'un's you're used to from modern advertising. It really IS impossible
to tap the line to listen in on the conversation, and the code really
IS totally secure.
First, here's quantum mechanics in a nutshell (though due to the
uncertainty principle, now that we know where it IS, we don't know
where it is GOING... :)
Quantum interaction: All measurement requires interaction, and interacting
with an itsy-bitsy, teenie-weinie, sub atomic particle (IBTWSAP) changes
it. (See the uncertainty principle.) A photon is an IBTWSAP.
Uncertainty principle: You can't know both the position AND the velocity
of an IBTWSAP. Pin down it's location by hitting it with something, and
it goes off in some unknown direction. Find out where it's going by having
it bounce off something, and you don't know where it will end up.
Quantum wave functions: Here's a rather weird bit. Before an IBTWSAP
interacts with something, neither it's location nor its velocity is
truly defined. Each exists in a sort of limbo state until interaction
with another particle causes its wave function to 'collapse' into a
defined state. Then both its location and its velocity become actual
values, though because the U-principle, we can still only find out one
or the other.
Okay, that's quantum physics (cha, right!) for the layman (99.9999% of the
world's population). Now here's one of the really neat things we are
learning to do with it...
Generate two detectablly different protons (the IBTWSAPs), and randomly
send each in a different direction down a fibre optic line. This needs to
be set so precisely that each reaches it's destination at the same time,
and these suckers are traveling at light speed! Luckily, state-of-the-art
detectors can detect a passing photon before it can get an inch away!
Their wave forms don't collapse until they reach their destinations at the
same instant. At which point the devices monitoring the line cause each
photon to say "I'm a 1!" or "I'm a zero!" as the photons' waves
collapse.
The result is a random string of ones and zeros at each of the two stations
on either end of the fibre optic light-pipe as the randomly selected photon
reaches one and it's opposite reaches the other.
Untappable: If anything is attached to the line in order to record the
passage of the photons passing through it, the pair will lose their
coherence, and neither will set off the detectors at the ends of the line.
Your source of random ones and zeros will stop if someone is trying to
listen in.
Unbreakable: Now that you've got this, it's time to start sending the coded
messages. So far we've only got a matched set of random noise generators.
Let's say this is a sample of the output from our quantum line:
011100000001110001001111010001010100111010111011100100001010010101101110
And this is the message we want to send:
S H A D O W R U N
19 8 1 4 15 23 18 21 14
11001 00010 10000 00100 11110 11101 01001 10101 01110
Then the whole coding process goes like this:
a) 011100000001110001001111010001010100111010111011100100001010010101101110
b) 11 00 1 00010 1 0 000 0 0100111 1011101 0 100 1 10 101011 1 0
c) 011011000001001111101000101110101111111011111110010111001011011111101001
A) This is the shared source of random numbers.
B) This is the message to be sent, spaced out to matches the random source.
C) This is the list of which random numbers to use to make the message.
You then can send C to the other end any way you like. No one can decode
the message without the random number source it refers to. And it's
impossible to tap THAT signal.
Neat, huh? I thought so... :)
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Chris Innanen cgi@*******.jhuapl.edu
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