[E-Lang] Quantum computing and capabilities

Ralph Hartley hartley@aic.nrl.navy.mil
Mon, 05 Feb 2001 14:32:52 -0500


hibbert@netcom.com wrote:

> hartley@aic.nrl.navy.mil said:
> 
>> QC requires something even stronger than a trusted courier: it
>> requires
>> an on-line connection between the parties.
> 
>> Does it? In what sense?
> 
> 
> As I understand the physics (I could be wrong) the two recipients receive 
> pairs of correlated photons generated at the same time.  There needs to be 
> a path connecting the generation to both of the recipients, and the 
> recipients must have detectors monitoring the transmission at the time the 
> photons arrive.  The connection could be line of sight, or it could be the 
> equivalent of a fiber.
> 
> That's not quite the same as on-line on a LAN, but the requirement is just 
> as tight.  You can leave a courier cooling his heels in your lobby until 
> you're free, but that won't work for the photons.

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.

> You also said:
> 
>> 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.
> 
> but my recollection of the physics is that you only get the QC guarantees 
> if there's no mediation between the source of the photons and the 
> receievers.  If you have a trusted third party source of bits that are only 
> guaranteed by conventional physics and the honesty of the broker, you 
> aren't doing better than one-time pads, you're doing worse.

There can be as much mediation as you like as long as there is no 
measurement (measurement has a special meaning in quantum mechanics). 
Qbits have to be treated gently, but they can be stored, relayed, and 
even error corrected, but never copied. Of course, if the broker 
converts the qbits to ordinary bits, by intent or neglect, all security 
is lost - except that Bob and Alice can tell that the qbits they were 
given are bad.

Ralph Hartley