Science
In reply to the discussion: Quantum Entanglement, Dark Counts, Coincidence Detection [View all]caraher
(6,278 posts)I guess I'm thinking about things like hyperentanglement experiments, where people created photons that are entangled in many degrees of freedom. One newly-popular one is spatial mode, using Laguerre-Gaussian beams of different orders (which are mutually orthogonal) to supplement other resources like polarization. Frequency and momentum are, of course, other degrees of freedom that are available, but there's some appeal to using discrete variables like polarization relative to a fixed basis or LG mode rather than a continuous variable because those things are readily interpreted as quibits. In the delayed choice experiment I linked, we're looking at the measurement of a single photon; but to do anything interesting that measurement on one photon must affect a photon with which it is entangled. It's a matter of how you extend the experiment. Cramer's setup looks like 2 MZ interferometers, one for each photon, while Aspect's experiment is a single MZ since there's just one photon.
With the BBO pair one thing you can do is set up your pump laser so it only drives downconversion in one crystal or the other (set the pump to 0 or 90 degrees relative to one of the BBO optical axes). You actually need the phase shifter to do the polarization entanglement for Type I; in other words, if you're just making one "cone" of one polarization it's unnecessary, but if you want to make 2 overlapped cones by pumping with a 45 degree polarization, you need the phase shifter to make the two cones indistinguishable in principle with the correct relative phase. You basically need to make the phase of the horizontal component of the blue pump light a bit different (I forget which way) from the vertical component to compensate for things like the birefringence of the BBO (one polarization propagates faster than the orthogonal polarization).