Figure 1. Structure of a [111]-oriented NV centre in diamond and experimental setup.

(a) Structure of a NV centre in diamond and electric dipoles E_x and E_y. (b) Schematic diagram depicting the unpolarized emission pattern (purple curve) caused by sum of emission from electric dipoles E_x (blue curve) and E_y (red curve) of a [111]-oriented NV centre. (c) The detection part of the experimental setup. For quantum state and process tomography, polarizer1 (purple rectangle) was inserted before a non-polarizing beamsplitter (NPBS). A λ/4 plate (green rectangle) was also inserted with its optic axis at 45° before the polarizer for measurement in the circular polarization basis. The setup for measurement of the second-order correlation function g⁽²⁾(τ) consisted of the NPBS, two avalanche photodiodes (APDs), a 100 m cable for electric delay and a time interval analyser (TIA). In other measurements, such as polarization angular dependences and tomography, pulses generated by APDs were counted by a data acquisition (DAQ) card. When we measured polarization correlations of the emitted single-photon stream, polarizer2, 3 (thick blue rectangles) were introduced between the NPBS and the APDs with HH, VV, HV and VH, where the first (second) character represents the polarization setting of polarizer2 (polarizer3).