Quantum crypto broken?

[Matt Crawford]

> The Oxford announcement doesn't present quite
> the risk implied.  Cloning in their case results
> in an energy loss of 1/2 which is easily detected
> through various means including error rate.
> You have to conserve of energy ...

Excuse me.  If you duplicate the input photon, you duplicate its
wavelength as well as its polarization state.  Therefore you have two
output photons each of the same energy as the original.  The energy
is supplied by the excitation of the atoms in the crystal.  Think of
it as a toned-down laser.

Every now and then, your duplicator must absorb other otherwise
scatter an input photon, but I'm sure you needn't lose 1/2 of them.

But I agree that the use of this device can be detected by the
communicating parties.
				Matt Crawford
			   (former quantum mechanic)

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