On our lab visit, we saw that a nonlinear crystal can convert one photon from a high energy pump laser to two photons of a lower energy. This technology is the most common way to generate single photons and entangled photon pairs. a. Why can't we make a single photon simply by attenuating a regular laser beam? b. Describe (in terms of photons) how energy is conserved in SPDC. c. How does the process of "heralding" let us use SPDC to make a single photon? d. Is SPDC an "on demand" photon source?