[E-Lang] Quantum computing and capabilities
Mon, 05 Feb 2001 14:04:14 -0500
Chris Hibbert wrote:
> Ralph Hartley wrote:
>> But quantum cryptography is strictly stronger than one time pads,
>> because it does not require a trusted courier.
> QC requires something even stronger than a trusted courier: it requires
> an on-line connection between the parties.
Does it? In what sense? The distribution of the EPR pairs certainly
doesn't need to be on line. The parties don't need to exchange pairs,
they only need to obtain pairs from a common source. Because the source
is not trusted, it can be part of a common infrastructure. If Bob wants
to communicate with Alice he can obtain a bunch of Alice qbits from a
public repository of bits that Alice has the mates to. Every thing else
can be transmitted through adds in the Times. Of course he only knows
that he is talking to whoever has the other half of the pairs, but the
same thing is true of a public key.
In many of the protocols for quantum cryptography, the parties do
interact on-line, but I don't know any reason that is required.
> 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
A qbit can be transmitted as a polarized photon. Fiber optics is a cost
effective way to move photons around, but it's not the only way. They
can also be transmitted trough empty space. The large mirrors needed to
send and receive photons over interstellar distances are certainly MUCH
cheaper than a star ship. You need to be able to send photons for
unencrypted transmissions anyway. Quantum transmissions would favor
short wavelengths, so that there are fewer high energy photons. Other
considerations (bandwidth, antenna size) favor that too.
Theoretically you could send qbits through light bounced off a reflector
on the moon or even HF radio bouncing off the ionosphere, but you would
need very special transmitters/recievers, and it would be VERY inefficient.