Based on the characteristics of superposition, entanglement, non-locality and non-clonality of quantum mechanics, quantum information science can break through the physical limits of classical information and open up a new information processing function different from classical electromagnetic application methods. Due to the advantages of high-energy single photon in practical applications, the research and application of optical quantum information technology dominates the development of current quantum information technology. However, the free-space transmission of light waves is greatly affected by weather conditions and atmospheric particles. Comparing with other wave bands, classical microwave signal shows good penetration ability when transmitting in free space. By introducing quantum mechanics, microwave signal also exhibits non-classical merits. As quantum microwave signal inherits both classical transmission performance and quantum non-classical features, it can be utilized as a significant signal source for diverse applications in microwave domain, such as quantum communication, quantum navigation and quantum radar, which are based on quantum technologies in large scale and dynamic free space transmission. There are three main experimental platforms on which quantum microwave is studied and produced. They are cavity quantum electrodynamics(C-QED) system, circuit quantum electrodynamics(c-QED) system, and cavity electro-opto-mechanical(EOM) system, involving with several nonlinear effects such as Kerr effect, Casimir effect, three-wave mixing, etc. In this paper, the setups of these platforms and the preparation principles are introduced. Meanwhile, the preparation principles and methods of microwave single photon, entangled microwave photons, squeezed microwave fields and entangled microwave fields are summarized and analyzed in detail from three aspects. The present status of experimental progress in the relevant fields are summarized and listed as well. Besides, key problems in the application of quantum navigation in free space utilizing quantum microwave are probed. Among them, the most pressing ones are preparation ability, decoherence in transmission and detection of entangled quantum microwave signals, which are also discussed and analyzed in this paper. Finally, we look forward to the future development of quantum microwave technology. It mainly consists of manufacturing microwave detectors with high efficiency, designing thermal photon filters, and developing suitable antennas. We hope that this study can provide useful reference for scholars who are engaged in or interested in research related to quantum microwave technologies.

As the core of quantum entanglement, two-mode squeezing is manifested in cross-correlations of incompatible observables between two subsystems, which makes the two-mode squeezed microwave an ideal resource for applications in quantum communication, quantum illumination, and quantum microwave navigation. Currently the preparation scheme of two-mode squeezed microwave, based on the Josephson parametric amplifier (JPA) and a superconducting 180° hybrid ring coupler, proves to be the most efficient and excellent in quantum properties. Nevertheless, the difficult phase locking processing restricts the further improvement of entanglement. There is no effective solution but the dual-path receiver with phase stabilization measures, and the phase error reaches as high as 0.3°, which still does not meet the requirements for phase locking precision and entanglement stability. To overcome the academic obstacle, we propose a phase locking scheme to achieve a stable two-mode squeezed microwave. There are two JPAs used to separately generate single-mode squeezed microwaves, between which the difference lies in the fact that the input of one JPA is phase-modulated but the other is not. A superconducting 180° hybrid ring coupler is used to distribute the two single-mode squeezed microwaves into two output paths, which are two-mode squeezing if the squeezing directions of the two single-mode squeezed microwave are orthogonal. That is to say, the relative phase satisfies the condition <inline-formula><tex-math id="M65">\begin{document}$\theta = {\text{π}}/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191348_M65.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20191348_M65.png"/></alternatives></inline-formula>. By mixing the unmodulated single-mode squeezed microwave and one output of the superconducting 180° hybrid ring coupler, a relative phase is obtained in subsequent process. Proportional integral derivative (PID) controller is used as the input of phase error, and the output is used to adjust the pump phase of JPA, which is the key to phase locking and stable two-mode squeezing. The present research not only provides an effective strategy to achieve stable two-mode squeezed microwave, but also may attract more attention to the precisive measurement of two-mode squeezed microwave.

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