Fig. 1. (a) Schematic diagram of the experimental configuration. ECDL, external-cavity diode laser; AP, attenuator plate; C, coupler; BS, 50:50 beam splitter; M, mirror; A, arm of the interferometer; PZT, piezoelectric transducer; SPP, first-order spiral phase plate; SPCD, single photon count detector; CCU, coincidence count unit. The ECDL is capable of generating coherent light, which serves as the light source within our experimental system. A Mach–Zehnder interferometer is constructed by M1 (with PZT), M2, BS1, and BS2. The PZT, integrated into the M1, enables precise control over the relative phase between the two arms of the interferometer, and the SPP1 modulates the phase structure of photons, thereby encoding their path information. Photons are detected and correlated through C3 and C4, along with their respective devices, at the two outputs of BS2. SPP2 and SPP3 are alternatively inserted before C3 and C4 to erase the OAM phase information. (b) Schematic diagram of OAM label erasure. The translation phase plate S^−1/2 operates on the input state, which is a superposition of |0⟩ and |+1⟩. As a result, some of the components |0⟩ remain unchanged, while some of the components |+1⟩ convert to |0⟩. The final output state is a coherent superposition of |0⟩ with a phase factor that depends on ϕ.

Fig. 2. The number of photons detected by SPCD1 and SPCD2, connected via SMF to C3 and C4, depends on the relative phase ϕ of the light field projected onto the state |0⟩. This figure shows the experimental data for the photon counts at C3 (blue dots) and C4 (orange dots) as a function of ϕ. The relative phase ϕ is controlled by a PZT that adjusts the optical path difference between the two arms of the MZI. The data demonstrate the correlation between the photon detection and the phase modulation.

Fig. 3. The number of photons detected by SPCD1 and SPCD2, connected via SMF to C3 and C4, depends on the relative phase ϕ of the light field projected onto the state |1⟩ (realized by SPP2 and SPP3). The blue and red dots in the figure represent the photon counts of C3 and C4, respectively. The figure clearly demonstrates the dependence of photon counts on the relative phase ϕ, which is controlled by a PZT adjusting the optical path difference between the two arms of the MZI.

Fig. 4. Projection of the state |0⟩ after erasing path information. The curve generated by fitting the data is consistent with the conclusion of the theoretical analysis, which is a cosine curve dependent on ϕ.