Defense Intelligence Reference Document The Space Communication Implications Of Quantum Entanglement

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III. Nonlocality Versus Special Relativity?
If nonlocal communication is possible, would it be in conflict with special relativity, with
its well-known prohibition against faster-than-light signals? The answer is no.
The prohibition of signals with superluminal speeds by Einstein's theory of special
relativity is related to the fact that the definite simultaneity of two separated space-
time points is not Lorentz invariant. Since some hypothetical superluminal signal could
be used to establish a fixed simultaneity relation between two such points-for
example, by clock synchronization-this would imply a preferred inertial frame and
would be inconsistent with Lorentz invariance and special relativity. In other words, it
would be inconsistent with the even-handed treatment of all inertial reference frames in
special relativity.
However, if a nonlocal signal could be transmitted through measurements at separated
locations performed on two entangled photons, the signal would be "sent" at the time
of the arrival of the photon in one location and "received" at the time of arrival of the
other photon. By varying path lengths to the two locations, these events could be made
to occur in any order and time separation in any reference frame. Therefore, nonlocal
signals (even superluminal and retrocausal ones) could not be used to establish a fixed
simultaneity relation between two separated space-time points, because the sending
and receiving of such signals do not have fixed time relations. The transmission and
arrival instants of a nonlocal signal cannot be used for synchronization because the
transmission and reception instants are path- and delay-dependent variables.
To put it another way, the nonlocal connections of entangled photons lie along
segmented lightlike world lines that transform properly under Lorentz transformations.
Therefore, there is no conflict between nonlocal signaling and the Lorentz invariance of
special relativity. On the other hand, the principle of causality (cause must precede
effect in all reference frames) appears very likely to be violated (or at least violate-
able) if nonlocal signaling is possible.
Is it possible that the universe does have some preferred reference frame, perhaps that
laid down by the cosmic microwave background or implied by Mach's Principle?
Perhaps, but if such a preferred frame existed, its existence could not be established by
nonlocal communication.
IV. Momentum Domain Entanglement and EPR
Experiments
Einstein's original objection (Reference 4) that quantum mechanics appeared to be
nonlocal was made with arguments based on a gedankenexperiment in the momentum
domain. However, almost all of the modern EPR experiments testing the Bell Inequality
and demonstrating quantum nonlocality have been performed in the polarization (that
is, angular momentum) domain, usually with linearly polarized photons. Interestingly, it
appears that if nonlocal quantum communication is possible at all, it may be more
easily achieved in the momentum domain of Einstein's original focus.
The optical process of spontaneous parametric down-conversion (Reference 15) turns
out to be a very useful way of generating photon pairs entangled in either the
polarization or the momentum domains. In this process, a photon from a "pump laser"
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