Defense Intelligence Reference Document The Space Communication Implications Of Quantum Entanglement

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interacts with a nonlinear crystal and is transformed into two photons with energies and
vector momenta that add up to those of the original pump photon. Depending on the
type of down-conversion process, there are well-defined polarization correlations
between the entangled photons. The down-converted photons may also be easily
prepared in momentum-entangled states, because within the nonlinear medium, the
vector momenta of the down-converted pair of photons must add to give that of the
pump photon.
The first measurement using
momentum-entangled down-conversion
photons that might be related to
nonlocal communication is the Ghost
Interference experiment reported in
1995 by the Shih group (Reference 16),
shown schematically in Figure 2. The
experimenters used degenerate
collinear Type-II down-conversion of
351-nm UV pump radiation from an
argon-ion laser passed through a 3-
mm-long BBO (~-BaB2O4) crystal that
had been cut with the optic axis at a
phase-matching angle of 42. 2° to the
pump beam to produce a pair of
collinear momentum-entangled 702-nm
photons with opposite polarizations. The
entangled photons emerge from the
crystal very nearly parallel with the
pump beam. The pump beam is then
split off from the pair using refraction in
a quartz prism (UV Prism), and the
entangled photons are separated with a
polarization-selecting beam splitter (BS)
that reflects the "extraordinary"
vertically polarized photon (e) and
transmits the "ordinary" horizontally
polarized photon (o). Both photons are
UV Prism
1 DBBO
7 □ 2nm
1 e
BS I
I f
0 Slits 1
X ;
2 ... fiber
062
351 nm <J
Ar Laser
Gating
Gated
Figure 2. Schematic of the 1995 Ghost Interference
Experiment (Reference 16) of the Shih group
passed through 702 ± 10-nm wavelength-selective filters (f1,2) and then detected
(D,,2).
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