Many quantum radars currently studied in the literature use a phenomenon called entanglement to address the problem of distinguishing signal from noise, which is one of the most important problems faced by any radar. Until recently, entanglement-based quantum radars at radio frequencies existed only in theory; their practicality was very much in doubt. The situation has changed with a recent experimental implementation of all the necessary components of a quantum two-mode squeezing (QTMS) radar, which operates at microwave frequencies and can be described as a quantum range finder. In this article, we lay out the exact problem solved by this prototype, namely quantum noise, and explain how entanglement can overcome the existence of this noise. By analyzing the QTMS radar prototype, we point out a technological route to an entanglement-based quantum radar that can, in principle, perform all tasks that radars can and must do, such as array processing, clutter suppression, and image processing (including synthetic aperture radar and inverse synthetic aperture radar).

Entanglement-Based Quantum Radar: From Myth to Reality

We present a self-error-rejecting multipartite entanglement purification protocol (MEPP) for N-electron-spin entangled states, resorting to the single-side cavity-spin-coupling system. Our MEPP has a high efficiency containing two steps. One is to obtain high-fidelity N-electron-spin entangled systems with error-heralded parity-check devices (PCDs) in the same parity-mode outcome of three electron-spin pairs, as well as M-electron-spin entangled subsystems (2 ≤ M < N) in the different parity-mode outcomes of those. The other is to regain the N-electron-spin entangled systems from M-electron-spin entangled states utilizing entanglement link. Moreover, the quantum circuits of PCDs make our MEPP works faithfully, due to the practical photon-scattering deviations from the finite side leakage of the microcavity, and the limited coupling between a quantum dot and a cavity mode, converted into a failed detection in a heralded way.

Self-error-rejecting multipartite entanglement purification for electron systems assisted by quantum-dot spins in optical microcavities

The precision of optical gyroscope can be improved by enhancing Sagnac effect. Aiming at the accuracy of optical gyroscope restricted by the theoretical limit, shot noise limit, we firstly analyze the theoretical limit of Sagnac effect in optical regime with quantum theory, and point out the physical reasons for the precision limit. And then a quantum entanglement state, Yurke state, with photon number difference measurement is advised to break through shot noise limit for Sagnac effect enhancement. This proposed scheme can achieve the true quantum limit, Heisenberg limit, and offer a new idea for improving the performance of optical sensors based on Sagnac effect.

/pdf/the-theoretical-limit-of-sagnac-effect-521cgbbj0q.pdf

The Theoretical Limit of Sagnac Effect

The precision of gyroscope can be improved with Sagnac effect enhancement. We show that Sagnac effect can be enhanced with single-mode number squeezing technique by calculating the quantum Fisher information (QFI) of coherent state and Fock state and obtain the minimum phase uncertainty with parity measurement in ideal case. Considering that photon losses almost cannot be avoidable in the practical application, a lossy interferometer is modeled by inserting a beam splitter theoretically to calculate the QFI in the presence of photon losses. Through theoretical analysis, the effective transmission rate (or the maximum loss rate) which can enhance Sagnac effect is obtained. For instance, if the average photon number of coherent state $$|\alpha |^{2}$$
 is far more than the average photon number of Fock state $$N$$
 and $$N = 20$$
 , sub-shot-noise phase uncertainty can be achieved when loss rate is less than 85%, which indicates the prospect of practical application with the proposed scheme.

Sagnac effect enhancement with single-mode number squeezing in the presence of photon loss

Quantum entanglement is a unique phenomenon which can be described in quantum mechanics. There are two categories of quantum entanglement: one is based on a single-body system in various freedom degrees and another is found in the multi-body system. In recent years, the spin–orbit coupling effect has been widely concerned. Various electronic devices based on the spin–orbit coupling effect have been emerging in an endless stream and bringing great practical application value. At present, the relationship between single-particle commuting observables $$ l_{z} ,s_{z} $$ entangled states and the spin–orbit coupling is rarely reported. In this paper, based on the diverse freedom degrees of the quantum entanglement states in a single-body system, the correlation between the sole particle commuting observables $$ l_{z} ,s_{z} $$ entangled states and the spin–orbit coupling has been analyzed by MATLAB software and the degree of entanglement is measured according to von Neumann entropy. This work provides innovation to further understand quantum entanglement and demonstrates that there is a close relationship between the degree of entanglement and the spin–orbit coupling coefficient $$ {j \mathord{\left/ {\vphantom {j 2}} \right. \kern-0pt} 2} - {1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-0pt} 4} $$. As $$ j $$ increases, the degree of the maximum entanglement firstly decreases, then increases, afterward decreases and then increases repeatedly, with the maximum value of entanglement degree being 0.6828. When $$ j $$ is beyond 85.5, the degree of entanglement will not make sense any longer.

The relationship between single-particle commuting observables l z , s z entangled states and the spin–orbit coupling

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.

Preparation methods and progress of experiments of quantum microwave

A method of evaluating the quality of dual-path entangled quantum microwave signal generated based on von Neumann entropy

As a widely utilized information carrier, polarization microwave shows plenty of merits. Quantum microwave is booming gradually due to the development of superconducting technology, which makes it a promising potential to apply quantum entanglement to polarization microwave. In this paper, we introduce the concept of continuous variable polarization entanglement. Meanwhile, a scheme of polarization entanglement in microwave domain is proposed and simulated. The detail derivations are given and discussed. Polarization entangled microwaves are prepared by combining quadrature entangled signals and strong coherent signals on polarization beam splitters, and quadrature entangled signals are prepared by utilizing Josephson mixer. In order to probe the polarization entanglement between output signals, inseparability of Stokes vectors \begin{document}$I({\hat S_1},{\hat S_2})$\end{document} and \begin{document}$I({\hat S_2},{\hat S_3})$\end{document} , is analyzed in 100 MHz operation bandwidth of Josephson mixer. The relation between inseparability I and squeezing degree r and between inseparability I and amplitude ratio Q are analyzed respectively. The results show that \begin{document}$I({\hat S_1},{\hat S_2})$\end{document} is sensitive to the variation of Q, while \begin{document}$I({\hat S_2},{\hat S_3})$\end{document} is sensitive to the change of r. The physical reasons for these results are explored and discussed. Apart from these, \begin{document}$I({\hat S_1},{\hat S_2})$\end{document} remains its value above 1 under the condition in this paper, but on the contrary, \begin{document}$I({\hat S_2},{\hat S_3})$\end{document} keeps its value well below 1. It proves that \begin{document}${\hat S_2}$\end{document} and \begin{document}${\hat S_3}$\end{document} of Stokes vectors are inseparable from each other, thus output signals \begin{document}${\hat E_a}$\end{document} and \begin{document}${\hat E_b}$\end{document} of our scheme exhibit bipartite entanglement. The best entanglement appears nearly at about 70 MHz, at this point the minimum \begin{document}$I({\hat S_2},{\hat S_3})$\end{document} value is 0.25.

Continuous variable polarization entanglement in microwave domain

Adaptive optimal measurement for the squeezed vacuum and coherent state

Path-entanglement microwave signals detecting method based on entanglement witness

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