[extropy-chat] Fortune: the future of computing

[scerir]

Eugene Leitl:
I don't see the point of communicating by entangled
photon pairs, I must admit. In terms of cryptography,
that's snake oil. Throughput is awful (single-photon
sources) even over short distances, and so is 
susceptibility to disturbances.
So what is the point of quantum communication over
long distances? (I would see the point for short
distances, for QC).

### Essentially the point is the possibility
of establishing a global quantum communication
network, via a space-based distribution
of entangled pairs. One potential advantage
could be that quantum communication provides
means to establish a (more) secure communication.
So a quantum global network would be (more) secure.

But there are different scenarios, depending on
whether satellites are used to carry sources
of entangled photons, or receivers of entangled
photons, or relay devices capable of manipulating
qubits, or even a third party device (on earth, 
on space) performing Bell-state measurements, 
or any possible combination among these different 
scenarios. 

You can also think of a scenario like
two receivers orbiting around the earth, or moving
fast through the space, and one transmitter of
entangled photons. In this case it would be
possible to study the *interesting* question
of relativistic effects (ie Lorentzian boosts) 
on specific correlation between entangled states, 
or relativistic effects on quantum 'collapse',
and the effects of gravity on quantum entanglements
that is to say the Lens-Thirring dragging of frames.

In principle, a space-based (less decoherent) quantum 
network would make possible to perform a space-based
quantum computation even *before* its quantum imput
is defined (ie on Earth). The basic idea is to perform 
the space-based quantum computation (QC) at some *earlier* 
time with qubits which are part of an entangled state. 
Only at a *later* time a Bell-state measurement is performed 
and this projects (it is a sort of teleportation) the QC 
output state onto the correct one.

But I do not know what of the above is, at present,
pure speculation, and what is reasonable.