[umvi]

I'm still so confused. I understand what a photon is (ish... I understand it can behave as a particle or wave, but in this case it is a "particle"). How do you "observe" a single photon? How do you "destroy" a single photon "in a particular way"? Heck, how to do keep something moving at the speed of light stuck inside a chip? Create a fiber optic loop somehow? How is the photon introduced into the loop?

[fsh]

Typically (for polarization qubits) a combination of waveplates, polarizing beam splitters and single-photon detectors (avalanche photodiodes, photomultipliers, or superconducting nano-wire detectors) are used to measure the qubit state. These detectors absorb the photons and turn them into detectable electrical signals. It is also possible to detect photons without destroying them (non-demolition measurement). However, this is much more difficult to do and leads to exactly the same results. After the measurement, the photon is no longer in a superposition state and is no longer entangled with the other one.

There is number of techniques for storing photon qubits. The easiest way is to send the light through a very long optical fiber spool. However, the achievable delay is limited due to absorption in the fiber. It is also possible to transfer the photon state onto a different system, such as a single ion or a superconducting qubit. Then the state of that system can later be measured.