[E-Lang] Quantum computing and capabilities

hal@finney.org hal@finney.org
Tue, 6 Feb 2001 10:13:12 -0800

Ralph Hartley, <hartley@aic.nrl.navy.mil>, writes:
> hal@finney.org wrote:
> > 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.
> No.
> Suppose Bob wants to send a private message to Alice.
> For each bit of his key he does the following:
> Obtains an "Alice qbit" (a member of an EPR pair Alice has the other 
> half of).
> Selects an one of his own qbits such that the corresponding Bob qbit is 
> convenient to Alice.
> Selects a direction.

How do these qbits get shared, in the case of interstellar communication
without trusted couriers?  Wouldn't they have to get transmitted?

If Alice sent them to Bob, and he is sending back results that depend
on them, that is a round trip.

Or can Bob send all the necessary shared qbits to Alice at the same
time as his classical information, or slightly earlier?  In that case
you would avoid a round trip.

It seems like it's risky, though, to send a qbit (or two) in entangled
states, and then send classical information that tells exactly what basis
to measure with in order to get reliable information out.  Won't it be
harder to detect an eavesdropper if she knows what basis to use to avoid
perturbing the data?