On-Chip Generation and Manipulation of Entangled Photons Based on Reconfigurable Lithium-Niobate Waveguide Circuits

A consequent tendency toward high-performance quantum information processing is to develop the fully integrated photonic chip. Here, we report the on-chip generation and manipulation of entangled photons based on reconfigurable lithium-niobate waveguide circuits. By introducing a periodically poled structure into the waveguide circuits, two individual photon-pair sources with a controllable electro-optic phase shift are produced within a Hong-Ou-Mandel interferometer, resulting in a deterministically separated identical photon pair. The state is characterized by $92.9\pm 0.9\%$ visibility Hong-Ou-Mandel interference. The photon flux reaches $\sim 1.4\times {10}^7\text{pairs}{\mathrm{nm}}^{-1}{\mathrm{mW}}^{-1}$. The whole chip is designed to contain nine similar units to produce identical photon pairs spanning the telecom $C$ and $L$ band by the flexible engineering of nonlinearity. Our work presents a scenario for on-chip engineering of different photon sources and paves the way to fully integrated quantum technologies.

Figure 1

(a) The photonic chip with dimensions of $50\mathrm{mm}\times 5\mathrm{mm}\times 0.5\mathrm{mm}$. The widths of single mode waveguides are $2\mu \mathrm{m}$ for the pump and $6\mu \mathrm{m}$ for entangled photons. The periodically poled section is 10 mm long with a period of $15.32\mu \mathrm{m}$. The interaction length of $C_1/C_2$ is $650\mu \mathrm{m}/1300\mu \mathrm{m}$ at a gap of $4\mu \mathrm{m}$. The upper inset is the structure of the electro-optic modulator. The electrode pairs are 8.35 mm long with a separation of $6\mu \mathrm{m}$. The buffer layer (${\mathrm{SiO}}_2$) is etched along the gap between the electrodes to suppress dc drift. The lower three insets are the micrographs of the $Y$ branch, the PPLN waveguide, and the directional coupler, respectively. (b) The whole structure of the chip. (c) Photograph of the chip with fixed fiber pigtails.

Figure 2

(a) Experimental setup of on-chip two-photon interference. The fiber laser is polarization controlled and connected with the input fiber $L_0$. Before single-photon detectors, 14 nm bandwidth interference filters centered at 1560 nm are used to further eliminate the pump. The coincidence counting measurements are made between $R_1$ and $R_4$, $R_1$ and $R_1$, and $R_4$ and $R_4$, respectively. The results are recorded in (b)–(d). The solid lines are sinusoidal fitting, and error bars show $\pm \sqrt{\text{counts}}$.

Figure 3

(a) Experimental setup of HOM interference. The photonic chip is controlled to emit separated photon pairs. In each arm there are two interference filters of 14 nm bandwidth. (b) Coincidence counts (cc) versus the displacement of the free-space delay line. Error bars show $\pm \sqrt{\text{counts}}$.

Figure 4

(a) The layout of nine channels with the same waveguide circuits but different poling periods on a LN wafer. (b) Efficient SHG wavelengths in different channels at $25.5°\mathrm{C}$. The red line is a linear fitting.