Generating, losing and reviving entanglement
In their experiment, the researchers generated entangled photons by sending light from a high-power “pump” laser into a nonlinear crystal. Under conditions where the photons’ energies and momenta are conserved, one pump photon will produce two entangled photons in a process called spontaneous parametric down conversion (SPDC). The two photons are entangled in all their properties. If a photon is detected at one location, for example, the position of the other entangled photon is automatically determined. The correlation exists for other quantities as well, such as momentum, angular position and orbital angular momentum.
As seen through the witness without any corrective measures, the researchers observed that position entanglement between photons disappears after about 4 cm of propagation. On the other hand, something interesting happens for angular-position entanglement. It disappears after about 5 cm of propagation, but after the photons have travelled another 20 cm, entanglement appears again (see figure). The researchers corroborated their experimental results qualitatively with a numerical model.
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