Quantum teleportation can transfer arbitrary unknown quantum states between two distant users with the help of quantum entanglement, thereby facilitating the construction of quantum networks, implementation of quantum logic operations, and advancement in quantum computing. Continuous variable (CV) polarized light field is an important quantum resource, with advantages such as high efficiency in preparation, transmission, and measurement. It is suitable for long-distance quantum state transmission and can directly interact with atomic nodes. Therefore, it is desired to implement a quantum teleportation network of the CV polarized light field. However, the precise control and transformation of the polarization state of the light field are key to the quantum teleportation of an arbitrary CV polarization state. Quantum teleportation of the CV polarization state requires reversible control, enabling transformation from any arbitrary polarization state to a predetermined state for quantum teleportation, followed by restoration to the initial polarization state. Nevertheless, existing studies on polarization control primarily focus on unidirectional control, which fails to meet these requirements. This scheme intends to realize reversible precise regulation of any polarization state of the light field and achieve effective conversion between the initial polarization state and the predetermined polarization state.
The amplitude-division polarization measurement method and the wave plate polarization controller are employed in this paper to achieve polarization measurement and reversible control. The field-programmable gate array (FPGA) and host computer are utilized for control optimization and information display. First, a combination of half wave plate (HWP) and quarter wave plate (QWP) is used to generate incident field polarization states uniformly distributed on the Poincare sphere. Second, an amplitude-division polarization measurement system consisting of the partial polarization beam splitter, polarization beam splitter, HWP, and QWP is employed to spatially modulate the incident polarization state of the light field. Full Stokes parameters are measured in real time through inversion, which are then converted into azimuth and elliptic frequency. Additionally, based on the obtained polarization measurement information, a wave plate polarization controller comprising of HWP and QWP is used to convert arbitrary polarizations into preset polarizations and restore the initial polarizations, thereby enabling reversible control over the polarization state of the light field. Finally, communication between the systems for both polarization measurement and control is realized by combining FPGA with the host computer, while an optimization algorithm designed specifically for controlling errors caused by optical systems enhances control accuracy.
The reversible control system for the polarization state of the light field exhibits precise measurement and effective manipulation of polarization. The azimuth is measured by rotating HWP to generate linearly polarized light. The average error between the measured result and the theoretical value is 0.543°. The ellipticity is measured by rotating the QWP to produce different degrees of ellipticity polarization. The average error between the measured results and the theoretical value is 0.432°. The aforementioned results of the polarization azimuth and elliptic ratio measurements demonstrate the precise effectiveness of the polarization measurement system, thereby providing valuable test outcomes for the polarization control component. Through the forward polarization control of the incident polarized light field by QWP 1 and HWP 1, the azimuth of the converted linearly polarized light is close to 90°, and the average error is 0.474°. The forward polarization control successfully achieves precise and efficient conversion from the arbitrary polarization state to the predetermined target polarization state, with the measured value closely approximating the set value. The average error of the azimuth is 0.636°, and the average error of the ellipticity is 0.479° for the inverse polarization control of the incident polarized state by HWP 2 and QWP 2. The reverse polarization control successfully achieves accurate and efficient conversion from the preset polarization state to the initial incident polarization state, resulting in a restored polarization state that closely approximates the initial polarization state.
The measurement and reversible control of the polarization state of the optical field are realized by using the amplitude split polarization measurement method and the wave plate polarization control method. Algorithm optimization and semi-open-loop structure design have been employed to achieve an average measurement error of less than 0.543° for any polarization state. Furthermore, the average preset conversion error is less than 0.474° and the average reduced conversion error is less than 0.636° for any polarization control conversion. The system can realize the effective conversion between the initial and the preset polarization states, which provides key technical support for the efficient quantum teleportation of the CV polarization state. Using an optical fiber system and free space is the main way to realize the long-distance transmission of quantum states. A quantum key distribution of 200 km can be achieved by maintaining a polarization offset in the optical fiber. Free space is not sensitive to polarization, so the polarization state can be easily and directly realized for long-distance quantum communication. This scheme can realize the efficient conversion between the initial and preset polarization state of the light field, so it is of great research significance for the long-distance state transmission in free space or long-distance optical fiber quantum state transmission combined with the bias-preserving controller.
