Figure 5: Quantum beam splitter in the MZI.
From: Equivalence of wave–particle duality to entropic uncertainty
In this scenario, the second beam splitter is in a superposition of ‘absent’ and ‘present’, as determined by the polarization state 
at time t2. The quantum beam splitter (QBS) can be modelled as a controlled unitary, UPQ=|H›‹H|P⊗Q+|V›‹V|P⊗U(R), where U(R) is the unitary on Q associated with an asymmetric beam splitter with reflection probability R. Polarization-resolving detectors (PBS=polarizing beam splitter) on the output modes help to reveal the ‘quantumness’ of the QBS.
