High-dimensional (HD) entanglement enables an encoding of more bits than in the two-dimensional case and can increase the channel capacity of quantum communications and improve robustness to noise. As a new intrinsic degree of freedom of photon, Orbital angular momentum (OAM) has infinite dimensions in theory, which provides an effective method to solve bottleneck problems of the capacity of optical communication system.
In 2001, professor Anton Zeilinger, a Noble Prize winner in Physics, first proposed the idea of realizing HD quantum entanglement by using photon’s OAM, which attracted wide attention as it not only substantially increased the information carrying volume of photon, but also benefit the safety of quantum key transmission.
However, one of the challenges for practical application of photon OAM was to develop efficient characterization method of HD OAM entanglement. Traditional full quantum state tomography is the standard technology to obtain all the information of the quantum state, however, it becomes impractical in HD system as the required measurements would increase exponentially with the dimension. Thus, an effective solution to this problem is highly expected to present HD entanglement with as few measurement as possible and without introducing unwarranted assumptions.
Recently, in collaboration with professor Wang Huitian from Nanjing University, the research team of the School of Physics from Nankai University led by professor Li Yongnan proposed the method of using two-dimensional detector to realize scan-free quantum tomography. The result was published in the journal Physical Review Letters under the title of “Two-Measurement Tomography of High-Dimensional Orbital Angular Momentum Entanglement”.
The idea of the research is to use two-dimensional array detector instead of the traditional single-pixel detector to obtain high-dimensional quantum information from the two-dimensional coincidence counting based on the principle of interference and Discrete Fourier Transform. This method is scan-free and independent of dimension, which only requires two measurement to reconstruct the density matrix with very high fidelity for two-photon HD OAM entanglement with any dimension.
The idea can be applied to other spatial mode entanglement, multi-photon entanglement and mixed state entanglement, laying a solid foundation for high-capacity quantum communication and quantum process tomography. In the future, it would provide more interesting and efficient measuring method for complex conditions with the combination of machine learning, such as the application of HD photon quantum information in the atmosphere and optical fiber.
Diagram of Efficient Characterization Method of Quantum OAM Entanglement
Nankai University is the first affiliation of the paper. The paper's co-first authors are graduate student Li Yi and PhD graduate Huang Shuangyin from Nankai University, while the co-correspondent authors are professor Li Yongnan from Nankai University and professor Wang Huitian from Nanjing University. The research is supported by the National Key Research and Development Program and the National Natural Science Foundation of China.