Defense Intelligence Reference Document The Space Communication Implications Of Quantum Entanglement

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viewed from the slits will have reduced coherence. The path length difference at the
slits is, to a good approximation, inversely proportional to Lxs, Therefore, making Lxs
large-that is, placing the slits some distance from the source volume-can reduce the
path length differences to a value that increases coherence and allows observation of a
sharp two-slit interference pattern signal at detector Di. Alternatively, placing a thin,
double-concave diverging lens at point P of Figure 6 can have the same effect by
causing the shorter path lengths to pass through a greater thickness of lens glass. Such
a lens would also demagnify the source, producing the equivalent of a longer path
length and smaller source solid angle.
However, increasing source coherence has another consequence. The momentum
entanglement of photons from the source arises from momentum conservation.
Restricting the solid angle of the source, as viewed from the slits, means fewer photon
pairs can be entangled and still satisfy the geometrical constraints of the experimental
configuration. The Saleh group at Boston University has shown that there is a
complementary relation between source coherence and two-photon entanglement
(Reference 18). As the source-slit distance Lxs is increased, there are smooth transitions
from one-slit to two-slit interference patterns and from a highly entangled source to a
highly coherent source. We note, as mentioned above, that the parametric down-
conversion technology used in the Ghost Interference and Dopfer experiments was
fairly inefficient and noisy, with fairly improbable production of entangled pairs
competing with much more probable production of unentangled "noise" photons of the
same wavelength from pumped fluorescence in the crystal. This would tend to limit the
entanglement of the source.
Nonlocal communication using momentum entanglement requires source coherence.
Source coherence is needed in order to observe the "signal" of a two-slit interference
pattern and two-photon entanglement so that a measurement of one of the photons
"connects" with the interference pattern produced by the other photon. Where there is
coherence without entanglement or entanglement without coherence, nonlocal
communication with momentum-entangled photons is not possible. An unresolved issue
that requires further theoretical consideration and experimental testing is whether there
is a "sweet spot" in the experimental design that embraces both partial coherence and
partial entanglement and that permits the transmission of nonlocal signals.
VI. Nonlocal Communication Versus Signaling
The possibility of nonlocal communication is an unresolved issue. It is perhaps likely
that the coherence-versus-entanglement tradeoff is nature's way of preventing nonlocal
signaling, but that has not been demonstrated. In this section, we assume that nonlocal
signaling is possible and will examine its implications. As will be seen, they are so far
reaching that they could be taken as a strong indication that nature would not allow
such things and therefore nonlocal signaling must be impossible.
Figure 7 shows a variation of the Ghost Interference experiment (Reference 16) in
which the slit-imaging technique of the Dopfer experiment (Reference 17) is used to
ensure that entangled photon pairs passing through slits reach both detectors, and that
those intercepted by the opaque regions of the slits reach neither detector. In
particular, a lens of focal length f is placed in the path after the BBO crystal and before
the polarization splitter so that both entangled photons pass through this lens. A pair of
slits 51 is placed at a path distance f beyond the lens in the path of the "o" photons,
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