Fig. 1: Essential schematic based on the Mach-Zehnder interferometer.
From: Observing the quantum Cheshire cat effect with noninvasive weak measurement
A polarized beam is split into two paths with a non-polarizing beam splitter (NPBS). With half-wave plates (HWP), the polarizations in the upper and lower paths, respectively, are set at \(\left|\,\text{D}\,\right\rangle\) and \(\left|\,\text{A}\,\right\rangle\). The polarizing beam splitter (PBS) ensures that only the \(\left|\,\text{D}\,\right\rangle\) polarization reaches the detector. To probe the locations of the photon itself and its polarization state, an absorber (ABS) or an HWP is inserted in paths a or b.
