AAWSAP DIRD, The Space Communication Implications of Quantum Entanglement and Nonlocality, March 2010

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The experimenters demonstrated that
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passing the vertically polarized photon
(e) through a double- or single-slit
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system before detection at D1 produced
a "comb" interference distribution or a
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"bump" diffraction distribution,
respectively, in the position X2 of the
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horizontally polarized photon (o)
detected at D2 when the pair of photons
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is examined in coincidence. In other
words, the position distribution of the
straight-through photon shows patterns
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Detector 2 position (mm)
characteristic of the single- or double­
slit system through which its twin
entangled photon passed. Figure 3
shows the observed position
distributions for the two cases.
From the viewpoint of nonlocal
communication, we note that modifying
the slit system before D1 through which
the reflected photon passes, which can
be thought of as the action of a
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"sending" observer, nonlocally causes
Detector 2 position (mm)
an observable change in the Xrposition
distribution of the undeflected photon,
Figure 3. Ghost Interference Position Distributions
as detected by a "receiving" observer at
at x,
D2. This is a nonclassical effect that demonstrates the nonlocal connection between the
entangled pair and that might form the basis for transmission of a nonlocal signal
between the two observers. However, the Ghost Interference experiment does not, in
the form reported, demonstrate nonlocal communication, because of its use of a
classical communication link in imposing the coincidence requirement between the
detected photons.
In their paper, the authors comment that with the two-slit system in place, in the
absence of coincidences there is no observable two-slit interference pattern
distributions at either D1 or D2. They attribute this lack of an interference "signal" to the
horizontal variation in the creation position of the down-converted photons. The
variation is enough to cause the "e" photons to arrive at the two slits with relative path
lengths that may differ by more than a wavelength, thereby randomly shifting and
washing out any interference pattern. Furthermore, their source of entangled photons
was very inefficient and noisy. Only about 1 in 1010 pump photons produced an
entangled pair, while many unentangled "noise" photons of the same wavelength were
created by fluorescence in the crystal. Therefore, even if the coincidence requirement
had been in principle removable (see below), it is not surprising that coincidences were
required to observe the reported effects.
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(a) Two slits
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