Fig. 1. (a) Schematic setup of classical Mach–Zehnder interferometer (MZI) used to test wave–particle duality of a photon. The MZI consists of the first beam splitter BS1, two mirrors (M), a phase shift (φ), and the second beam splitter BS2. BS2 can either be present in the path, or absent, or controlled by an external (classical or quantum) module. (b) Schematic setup of weak-measurement MZI (WM-MZI), where an interference screen (denoted by the blue thick lines) with high transmission and weak scattering replaces BS2.

Fig. 2. Schematics of the experimental setup of the WM-MZI. (a) Single-photon interference on a prism surface coated with a weakly scattering milk film as an interference screen. The interference pattern on the right is formed by the incidence of a laser beam into the WM-MZI setup. (b) A single-mode fiber (SMF) output single-photon source apparatus. Inset shows the sample structure where CdSe/CdS coreshell quantum dots (QDs) in PMMA serve as single-photon emitters. LPF, long-pass filter. (c) WM-MZI setup. The avalanche photon detectors APD1 and APD2 record two path way information, respectively. A wedge glass plate (WGP) is used to tune the optical length of path 1. APD3 with a position-tunable slit is for observing the interference pattern.

Fig. 3. Characterization of the photon paths of the setup. (a) Time traces of the photon detection rates of APD1 for the cases of reflection from a silver mirror (red) and refection from the prism surface (blue), respectively. Inset shows a schematic diagram of the measurement. (b) The same for APD2. (c) Coincidence measurement of the photon detection events of APD1 and APD2. Inset shows the spectrum of the photons in APD1 and APD2. (d) Dark count rate of APD3 as a function of time.

Fig. 4. Simultaneous measurements of path and interference. For longitudinal interference, (a) the photon detection rates of APD1 and APD2 change as a function of the optical path length tuned by the wedge glass plate (WGP); (b) the photon detection rate of APD3. The slit position is fixed in this series of measurements. For transverse interference, (c) the photon detection rates of APD1 and APD2 change as a function of the slit position; (d) the photon detection rates of APD3. The wedge glass plate is fixed in this series of measurements.