Polarization controller (PC) serves as a critical component for manipulating the polarization state of light and holds great significance in both classical and quantum communication systems. Anisotropic electro-optic (EO) modulation in thin-film lithium niobate provides a new avenue for realizing high-speed PCs. This paper presents a high-performance on-chip PC with a simple architecture, assembled from linear tapers, Y-branch splitters, and EO phase shifters. The proposed PC has an ultra-broad working bandwidth that covers the entire optical communication spectrum. It enables arbitrary polarization manipulation. Endless polarization generation is realized at typical wavelengths of 1550 and 1310 nm, while exhibiting a record-high dynamic polarization extinction ratio >43 dB. High-speed random four-polarization encoding for quantum key distribution at a switching speed of 20 MHz is demonstrated. Enabling polarization manipulation with high speed, high PER and broad bandwidth, the PC offers huge potential for performance enhancement in optical systems.
Polarization controller (PC) serves as a critical component for manipulating the polarization state of light and holds great significance in both classical and quantum communication systems. Anisotropic electro-optic (EO) modulation in thin-film lithium niobate provides a new avenue for realizing high-speed PCs. This paper presents a high-performance on-chip PC with a simple architecture, assembled from linear tapers, Y-branch splitters, and EO phase shifters. The proposed PC has an ultra-broad working bandwidth that covers the entire optical communication spectrum. It enables arbitrary polarization manipulation. Endless polarization generation is realized at typical wavelengths of 1550 and 1310 nm, while exhibiting a record-high dynamic polarization extinction ratio >43 dB. High-speed random four-polarization encoding for quantum key distribution at a switching speed of 20 MHz is demonstrated. Enabling polarization manipulation with high speed, high PER and broad bandwidth, the PC offers huge potential for performance enhancement in optical systems.
Quantum key distribution (QKD) relies on quantum state manipulation to achieve unconditionally secure key transmission. Polarization encoding, a mainstream QKD scheme, faces practical limitations due to the bulkiness and instability of discrete optical components. This work presents a polarization-encoding chip based on hybrid thin-film lithium niobate (TFLN) and silicon-on-insulator (SOI) integration, enabling dynamic control of four polarization states | R 〉, | A 〉, | L 〉, and | D 〉 with extinction ratios over 16.78 dB, using only a single thermal and electro-optic phase shifter in a compact architecture. Experimental results demonstrate that the chip achieves a secure key rate (SKR) of 328 kbps and a quantum bit error rate (QBER) of 1.96% at 100 MHz repetition rate in a 10 km fiber-based QKD system. Compared to conventional silicon-based carrier-depletion modulators (CDMs), the TFLN-SOI hybrid integration design avoids phase saturation and dynamic loss induced by carrier depletion, significantly enhancing the stability and scalability of polarization encoding.
In recent years, quantum communication has received extensive attention due to its high security of transmitted information. Quantum key distribution (QKD), an important branch of quantum information, is developing rapidly and has been gradually moving toward practicality and networking. The use of phase coding through fiber optic channels is the basis for the implementation of QKD systems. In QKD systems, electro-optical modulation techniques are mainly used to change the photon phase through phase modulators to realize the phase coding scheme. Among them, lithium niobate is a common material for making phase modulators in QKD systems. Lithium niobate (LN) crystals are an optical material with excellent acousto-optical and electro-optical properties. It has good physical and chemical stability, a wide optical low-loss window, a large electro-optical coefficient and an excellent second-order nonlinear effect. It has a wide range of applications in high-speed electro-optical tuning, holographic storage, nonlinear frequency conversion, etc. Thin-film lithium niobate (LNOI), as a new integrated optical material, can well combine the excellent electro-optical, acousto-optical and nonlinear properties of the material with a compact optical waveguide. It also has the advantages of a small waveguide cross-section size, high electric field density, strong nonlinear effect, low half-wave voltage length product, and small size. It has significant advantages in the integration of optoelectronic devices. In the phase-coded QKD system, the coding object of the information is the phase of the optical signal. The polarization state of the optical signal can have a serious impact on the system. The phase-encoded QKD system based on the Faraday-Michaelson interference loop is able to self-compensate for the polarization variations in the system to remove the relevant effects of polarization variations on the QKD system. The application of a phase modulator based on thin film lithium niobate preparation in quantum key distribution can effectively enhance the rate of the quantum key distribution system. However, there is still a need to study the transmission and modulation characteristics of LNOI waveguides on polarized optical signals. In this paper, we develop a phase modulator based on thin-film lithium niobate for high-speed QKD systems. Simulation and analysis of the polarization mode of the optical signal transmitted in the optical waveguide. Test and study the transmission loss and modulation efficiency difference of a thin-film lithium niobate optical waveguide for TE and TM polarization state optical signals. To build a test system for application to the measurement and modulation test of the polarization state of optical signals in a high-speed phase-encoded QKD system.
Time-bin encoded quantum states of light are crucial for quantum technology applications. The integration of manipulation functionalities into chip-scale devices is essential for deploying scalable, high-performance, and cost-effective quantum networks. Here we develop a fully integrated, high-throughput quantum receiver based on the thin-film lithium niobate (TFLN) platform, capable of high-speed electro-optic manipulation of time-bin encoded quantum states. The device's novel architecture enables active switching of time-bin quantum states with an electro-optic bandwidth exceeding 30 Ghz, while supporting real-time arbitrary projective measurements with a bandwidth of over 1 GHz. We showcase its versatility and performance through several applications, including the certification of entanglement with Bell's inequality violation by 38 standard deviations and with >95% visibility. We then apply it to a fiber-based quantum communication scenario, where we experimentally demonstrate an entanglement-based quantum key distribution (QKD) protocol, achieving stable finite-size secure key rates exceeding 25 kbit/s over 12 hours of continuous operation. By leveraging a high-speed active switching scheme, the system overcomes the need for temporal post-selection, eliminating a fundamental loophole that compromises the security of time-bin entanglement-based QKD protocols and relaxes the temporal resolution requirements of single-photon detectors. Moreover, it enables active selection of the projection basis, increasing the flexibility for communication parties. This approach establishes a versatile and scalable architecture for time-bin encoded quantum communication, enabling practical protocols on industry-grade photonic technology.
We report on a unique photonic quantum source chip highly integrating four-stage photonic elements in a lithium niobate (LN) waveguide circuit platform, where an aperiodically poled LN (APPLN) electro-optic (EO) polarization mode converter (PMC) is sandwiched between two identical type-0 PPLN spontaneous parametric down-converters (SPDCs), followed by an EO phase controller (PC). These core nonlinear optic and EO building blocks on the chip are systematically characterized stage by stage to show its high performance as an integrated quantum source. The APPLN EO PMC, optimally constructed by a genetic algorithm, is characterized to have a broad bandwidth (>13 nm), benefiting an efficient control of broadband type-0 SPDC photon pairs featuring a short correlation time. We demonstrate an efficient conversion of the |VV› photon-pair state generated from the first PPLN SPDC stage to the |HH› state through the APPLN EO PMC stage over its operating bandwidth, a broadband or broadly tunable polarization-entangled state can thus be possibly produced via the superposition of the |VV› state generated from the other PPLN SPDC on the third stage of the chip. Such a state can be further manipulated into two of the Bell states if the relative phases between the two polarization states can be properly modulated through the EO PC on the fourth stage of the chip. Such a multifunction integrated quantum photonic source chip can be of high value to developing a compact, efficient, and high-speed quantum information processor.
Controlling the polarization state of light at a fast speed is important in both classical and quantum regimes. Here, we present an easily integrated compact tunable linear polarization-state generator of single photons in the telecom wavelength band on a periodically poled lithium niobate on insulator (PPLNOI) chip. The linear polarization of single photons can be continuously rotated by applying a transverse electric field utilizing the electro-optic (EO) effect. With strong light confinement and the large EO coefficient in the PPLNOI ridge waveguide, dramatically reduced driving voltage and fast-speed modulation is realized. Moreover, quantum entanglement is well maintained during the switching of polarization states, as proved in the time-energy entanglement measurement. The scheme holds promise for realizing integrated quantum optics.
The rapid development of photonic quantum information processing necessitates precise and programmable control over optical frequency, a capability critical not only for achieving photon indistinguishability but also for exploiting a virtually unbounded frequency dimension. However, efficient and scalable processing of frequency-encoded photon states remains challenging, primarily due to the limited nonlinear optical interaction in most photonic materials. Here, by harnessing the high-performance thin-film lithium niobate electro-optic (EO) platform, we demonstrate an integrated quantum photonic frequency processor that enables coherent and programmable control of photon frequency with high precision. We establish a scalable architecture for frequency-encoded quantum information processing. Using a fully integrated photonic chip, we realize a universal set of frequency-encoded quantum logic gates, including arbitrary single-qubit rotation gates and the two-qubit controlled-phase gate. Furthermore, we demonstrate its application in high fidelity characterization of frequency-bin entangled states. Our work reveals the unprecedented potential of utilizing the frequency degree of freedom in integrated quantum photonic systems.