[E-Lang] Quantum computing and capabilities

Chris Hibbert hibbertc@pacbell.net
Mon, 05 Feb 2001 20:48:03 -0700

Ralph Hartley wrote:
> There is no reason in principle that photons cannot be stored. I
> recently read of experiment in which this was done. In any event, qbits
> can be converted into electron or proton spins, quantized currents in
> superconductors etc., and they can be stored for a long time.

I read about the result as well.  It's not really clear that techniques
like that will be developed to the point that it would make sense to try
to develop a system in which the receiver is, in any strong sense,
off-line from the transmission of bits.  In principle, however, your
point is correct.

> There can be as much mediation as you like as long as there is no
> measurement.  Qbits have to be treated gently, but they can be 
> stored, relayed, and even error corrected, but never copied.
> [if the qbits are meddled with], all security is lost - except 
> that Bob and Alice can tell that the qbits they were given are bad.

I think you're right about that, though it's a surprising result.  (I
know, surprise is not a measure of validity when dealing with QM.)

Your original claim was 

>Quantum cryptography is strictly stronger than one time pads, 
>because it does not require a trusted courier.

I guess I would have to agree, but continue to point out that
practicality may lead to different solutions than the theory would lead
you to believe.  Your suggestion to interpose a shared source for qbits
doesn't feel like it reduces costs over using a courier in any practical
sense, though I'll agree that it is theoretically secure.