[E-Lang] Quantum computing and capabilities

hal@finney.org hal@finney.org
Sat, 3 Feb 2001 11:51:44 -0800

Chris Hibbert writes:
> QC requires something even stronger than a trusted courier: it requires
> an on-line connection between the parties.

As I think you meant to say, it is not just an on-line connection
in the sense we think of it today, it is the ability to get quantum
information from one side to the other.  Typically this requires the
exchange of quantum particles, usually photons, unchanged (i.e. they
can't be amplified or altered in any way).  In current experiments this
is done with fiber optic cables and they can reach lengths of tens of
kilometers, I believe.

> In exchange you get an
> assurance that the bits weren't tampered with, but I can imagine lots of
> circumstances when a trusted courier is feasible, and a fiber link (or
> the equivalent) is not.  One is Vinge's: interstellar communication.  Or
> am I missing something about setting up a quantum cryptographic connection?

Actually there is no problem in theory with sending photons unchanged over
interstellar distances; the difficulty arises in aiming them accurately
enough that they can be detected with significant probability.  Maybe with
a really, really big telescope it would be feasible.  I haven't seen any
estimates of whether it would be practical.

Also, I believe quantum key exchange algorithms require a round trip
(or maybe two) before the sides can start sending data.  This would be
a big problem at interstellar distances.