Defense Intelligence Reference Document The Space Communication Implications Of Quantum Entanglement

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Assume then that the fiber coils of Figure 10 remain rolled up and stored in a corner of
the laboratory and that the source can be made strong enough so that the average rate
at which the entangled photon pairs are detected is 10 MHz (which would correspond to
the efficient detection of about 3 nW of 702-nm photons). This would require a source
of entangled photons considerably stronger than is achievable with the "bulk" BBO or
LilQ3 crystals used in the Ghost Interference and Dopfer experiments, which produce a
usable entangled photon pair from only about 1 in 10 10 pump photons.
Fortunately, a recently emerging technology makes the production of entangled photon
pairs much more efficient through the use of "periodically poled" nonlinear crystals. In
"bulk" nonlinear crystal there is a "walk-out" phenomenon that limits the distance
within the crystal over which phase matching holds, permitting entangled pairs to be
produced efficiently by down-conversion. However, nonlinear crystals like potassium
titanyl phosphate (KTiOPQ4 or KTP) have very large nonlinear coefficients and are also
ferroelectric, with a large electric dipole moment. By using a large pulsed electric field
during crystal production, one can "write" on KTP crystals to change the orientation of
their local dipole moment over small distances (a few tens of wavelengths) periodically
along the pump direction through the crystal, so that the phase drift regularly reverses
and cancels out as the pump radiation progresses through the crystal. This is called
"periodic poling." With this kind of crystal, the walk-out is suppressed, and one can use
very long crystals that efficiently produce entangled pairs of photons over their entire
length. The IQOQI group of Anton Zeilinger in Vienna, using a periodically polled KTP
crystal 25-mm long, has measured the entangled pair production rate at 2.73 x 10 5
pairs per second per mW of pump radiation per nm of wavelength 20 . Such a source
should be easily able to produce the 10-MHz detection rate assumed above.
Now consider that the "sending" detector system and the "receiving" camera are in the
same room and separated by a distance of 1 meter or less. If the switch is set on the 0
or 1 position, the "message" that it is in that position begins to arrive at the camera SO
μs before the switch position is moved. If 100 photon counts constitute a signal, then,
even allowing for the latency in signal reception, the message could be received 40 μs
before it was sent. This would be a direct demonstration of retrocausal signaling using
nonlocal communication and would constitute a direct violation of the principle of
causality.
IX. Paradoxes and Nonlocal Communication
The setup described above, with its retrocausal communication link, raises some time-
communication paradoxes. First, let us consider the issue of "bilking." Suppose we
construct a million linked systems of the type shown in Figure 10 (or use 10 7 km of
nonattenuating fiber). Then the transmitted message would be received 40 seconds
before it was sent. Now suppose a tricky observer receives a message from himself 40
seconds in the future and then decides not to send it. This produces an inconsistent
timelike loop, which has come to be known as a "bilking paradox." Could this happen?
If not, what would prevent it?
There are discussions of such bilking paradoxes in the physics literature by Wheeler and
Feynman (Reference 22), who were considering the retrocausal aspects of the advanced
waves of absorber theory, and by Kip Thorne and colleagues (Reference 23), who were
considering the paradoxes that might arise from timelike wormholes. The general
consensus of both groups is that nature will forbid it and will require a consistent set of
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