LiNbO_3 waveguide phase modulators are components of single-photon interferometer in quantum cryptographic key distribution(QKD)system.The effects of the properties of phase modulation and polarization on QKD system are discussed.The parameters of both the linearly single-polarization LiNbO_3 modulator fabricated with annealing proton exchange(APE)technology and the LiNbO_3 modulator with low polarization dependent loss(PDL)property fabricated with Ti internal diffusion technology are presented.
LiNbO_3 waveguide phase modulators are components of single-photon interferometer in quantum cryptographic key distribution(QKD)system.The effects of the properties of phase modulation and polarization on QKD system are discussed.The parameters of both the linearly single-polarization LiNbO_3 modulator fabricated with annealing proton exchange(APE)technology and the LiNbO_3 modulator with low polarization dependent loss(PDL)property fabricated with Ti internal diffusion technology are presented.
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.
Polarization Mode dispersion (PMD) in single-mode fibers is a common source of problems in quantum key distribution (QKD) as well as all optical communication. Donald S. Bethune and William P. Risk developed a PMD-free phase modulator which is a single port component. We found that it is also necessary to construct a dual-port component. In this report, we demonstrate how PMD affects our single-photon interference experiment, and give a proposal on developing a dual-port PMD-free integrated phase modulator. This new component can be used in QKD as well as in other optical fiber communication schemes. And it will make long-distance information exchange more stable and effective.
A new differential phase shift quantum key distribution system based on the intensity modulator with Faraday-Michelson interferometer is demonstrated experimentally. Some technologies are employed to improve the performance of the quantum key distribution system. These technologies include the clock generation technology of arbitrary frequency by a small scale frequency-split and phase locked loop scheme, the new method to acquire the half-wave voltage of the phase modulator with a high accuracy in the condition of the single photon count, and the auto-compensated polarization technology by the Faraday-Michelson interferometer. Finally, the quantum key distribution is achieved with a quantum bit error rate of 3.9% when the transmission distance reaches 50 km.
Quantum key distribution (QKD) allows distant parties to exchange cryptographic keys with unconditional security by encoding information on the degrees of freedom of photons. Polarization encoding has been extensively used for QKD along free-space, optical fiber, and satellite links. However, the polarization encoders used in such implementations are unstable, expensive, and complex and can even exhibit side channels that undermine the security of the protocol. Here we propose a self-compensating polarization encoder based on a lithium niobate phase modulator inside a Sagnac interferometer and implement it using only commercial off-the-shelf (COTS) components. Our polarization encoder combines a simple design and high stability reaching an intrinsic quantum bit error rate as low as 0.2%. Since realization is possible from the 800 to the 1550 nm band using COTS devices, our polarization modulator is a promising solution for free-space, fiber, and satellite-based QKD.
Lithium niobate (LiNbO3) serves as one of the most competitive materials in nonlinear integrated photonics and becomes a key platform for the realization of integrated optical quantum technology, because it is physically and chemically very stable and it has a larger refractive index, a wider transparent wavelength range and better nonlinear optical characteristics than most of the traditional materials. Using the second-order nonlinear effect of periodically polarized lithium niobate (PPLN) crystal, single-photon frequency conversion was successfully realized to satisfy the quantum measurement requirements. LiNbO3and PPLN Waveguides were widely developed to solve the problems such as easy beam divergence in crystals, so that the improved nonlinear conversion effect promotes the successful fabrication of many quantum devices such as single photon sources, entangled photon sources and single photon detectors etc. By combining LiNbO3waveguides with other waveguide technologies, one can fabricate various LiNbO3integrated optical components serving as the core devices of quantum interfaces, quantum memories and etc. Based on these progresses, we see the attractive potential of LiNbO3material to play a deterministic role in quantum information technology.
We present a new optical scheme for BB84 protocol quantum key distribution (QKD). The proposed setup consists of a compact all-fiber polarization encoding optical scheme based on LiNbO3 phase modulators, single laser source and two single-photon detectors. Optical scheme consists of standard telecommunication components and is suitable for both fiber and free-space quantum communication channels. Low losses (~2dB) in Bob's device increase both the key generation rate and distance limit. A new technique for solving polarization mode dispersion (PMD) issue in LiNbO3 is implemented, allowing two crystals to neutralize the effect of each other. Several proof-of-concept experiments have been conducted at 10 MHz repetition frequency over 50 km of standard optical fiber under laboratory conditions and over 30 km of urban fiber with high losses (13dB), which is a link within a QKD network. To achieve this, calibration algorithms have been developed, allowing the system to work autonomously and making it promising for practical applications.