Figure 2
Two-photon four-qubit Toffoli gate acting on the polarization and spatial DoFs.
Polarizing beam splitters in the circularly basis, CPBSi (i = 1, 2, 3, 4), transmits the R-polarized and reflects the L-polarized components, respectively. Single-qubit operations 
can be achieved by employing two half-wave plates oriented at 45° and 22.5°, respectively. The local single-qubit operations, heralded by the outcomes of the diamond NV center, via a feed-forward process are not shown in the sketch.
