# How is the creation of entangled two-photon states such as $|00\rangle\pm|11\rangle$ compatible with the conservation of angular momentum?

Quoting, "Spin is the simplest example — two photons would either be (+1 and -1) or (-1 and +1), two electrons would either be (+½ and -½) or (-½ and +½) — and you don’t know which is which until you measure it" Source: https://medium.com/starts-with-a-bang/ask-ethan-whats-so-spooky-about-quantum-entanglement-e62c35ba339d

Question: The above states of photon polarization: (+1 and -1) relates to entangled state of 01+10 and is understandable for conservation of ang. momentum.

How is the state 00+11 realized in terms of polarization. Does not having two photons with (+1 and +1) violate conservation of ang. momentum?

• are you asking how a two-photon entangled polarization state can be generated in practice?
– glS
Dec 6, 2021 at 13:23
• yes ,and especially in the state of 00+11. Dec 6, 2021 at 13:28
• – glS
Dec 6, 2021 at 13:37

Given that you're already happy with producing $$|01\rangle+|10\rangle$$, it's enough to see how another state such as $$|00\rangle+|11\rangle$$ can be made out of the first one. This just requires the introduction of a bit of kit known as a half wave plate. It pretty much just swaps 0 and 1 and vice-versa, so you apply it on just one of the photons.