Fast and precise controlling of the polarization of light has important applications in classical and quantum communication. However, in state-of-the-art single-photon polarization modulation schemes, linear polarization is only preserved in a set of orthogonal directions, e.g. vertical and horizontal states. As for other directions, the output polarization is either elliptical or circular. A common requirement raised for several optoelectronic applications, e.g., optical quantum computers, is in need of linearly polarized light for their operation. Moreover, fully linearly polarized single photons could also be used to realize multi-base vector quantum key distribution. Therefore, it is crucial to keep single photons fully linearly polarized while maintaining their quantum characteristics during the polarization control.

Using a periodically poled lithium niobate on insulator (PPLNOI) ridge waveguide, we demonstrate an on-chip Solc-type high-performance single-photon polarization-controlled electro-optic device. The ridge waveguide of the device is made by a high-precision diamond dicing method, which overcomes the shortcomings of the traditional titanium diffusion lithium niobate waveguide, such as low refractive index contrast and large mode field area. Similar to the Solc principle, linearly polarized incident light that satisfies the QPM condition can achieve TE/TM polarization coupling under the transverse electro-optic effect, thereby achieving the purpose of fully linearly polarizing the polarization state of single photons.

To demonstrate the device’s ability to control single-photon polarization states, we use the photon pairs generated from spontaneous parametric down conversion (SPDC) other than weak coherent pulses. After the polarization control of the single photons in the signal path, we perform coincidence counting with the single photons in the idler path. The measured interference shows a clear sinusoidal relationship with an average fitted visibility of V = 96.3 ± 2.65%, suggesting that our device can efficiently rotate the linear polarization state of single photons. We also prove the maintenance of quantum properties during the conversion by using the Franson-type interferometry. Thus, photon pairs after polarization control can still be used for quantum communication tasks.

Figure 1 | (a) Schematic of single-photon generation and polarization control. (b) Coincidence count as a function of the voltage on the ridge waveguide.

Figure 2 | Two-photon interference pattern (a) before and (b) after the polarization conversion.

Through the test experiment, the half-wave voltage of the device is 7V, which is approximately halved than previous versions. At the same time, the device's small signal modulation response rate reaches 100 MHz, and its operating wavelength can be temperature tuned by the built-in TEC chip. The results show that the device can realize the function of controlling the single-photon polarization state by controlling the applied voltage, and has the advantages of reduced driving voltage, fast-speed modulation and tunable operating wavelength. The scheme holds promise for realizing integrated quantum optics.

The paper was published on:
Jiafan Wu, Yiwen Huang, Chuanyi Lu, Tingting Ding, Yuanlin Zheng, and Xianfeng Chen, Tunable Linear Polarization-State Generator of Single Photons on a Lithium Niobate Chip, Physical Review Applied, 13, 064068 (2020).