In May 2020, MIT Technology Review stated that a multinational research team created the world’s first quantum radar system using entangled microwave photons.
However, the central enterprise China Electronics Technology announced as early as August 2016 that the first quantum radar system based on single-photon detection was successfully developed at the 14th Institute, and the real object was displayed at the Zhuhai Air Show in 2018.
So, quantum radar, who is the world’s number one?
What is quantum radar?
After the U.S. Naval Research Laboratory developed the world’s first pulse radar in 1934, countries all over the world have competed to develop radar technology, and have been constantly improving, improving and exploring.
To improve the accuracy of radar and obtain high-quality images, radar needs to emit higher frequency electromagnetic waves. However, classical radar is limited by the theory of classical electromagnetic waves, so it is necessary to develop a new radar system-quantum radar.
Conceptually speaking, quantum radar is a quantum sensor that modulates quantum information into radar signals and transmits/receives quantum signals to achieve target detection. It can be used to detect, identify and distinguish radio frequency stealth platforms and weapon systems, etc., and theoretically detect distance Extremely far away, useful for planetary defense and space exploration.
To put it simply, classic radar perceives information such as the target’s position and speed by emitting electromagnetic signals and receiving the echo signals reflected by the target. However, quantum radar only uses a few or even a photon as an information carrier to detect the target, using the particle characteristics of light.
Take stealth aircraft, for example, which rely on special paint and airframe designs to absorb and deflect radio waves, making them invisible to conventional radars. But when it intercepts the photons emitted by the quantum radar, the original quantum characteristics of the photons will be destroyed, and due to the non-reproducibility of photons, the false signals sent by the stealth aircraft cannot re-simulate the physical characteristics of the previous photons. As long as quantum radar recognizes the single photon state reflected back by the target, it can see through its interference behavior.
In addition, quantum radar can emit electromagnetic waves in an entangled state, and use one of the entangled photon pairs as an imaging photon and the other as a detection photon. When detection is performed, the imaging photons remain in the quantum memory, and the detection photons are emitted, reflected by the target and re-accepted by the quantum radar. In this way, according to the principle of quantum entanglement, by comparing the respective quantum states of the two photons in the entangled photon pair, the detection performance of the radar can be significantly improved.
Compared with classical radar, in addition to being less susceptible to interference, quantum radar also has the advantages of higher sensitivity and stronger concealment.
Classical radar makes measurements by emitting an electromagnetic signal towards a target and receiving a modulated echo signal from the target. The accuracy of the measurement (such as distance, angle and speed, etc.) has a signal-to-noise ratio limit of N, where N is the average number of photons detected in the signal. Therefore, the measurement accuracy limit of classical radar is 1/√N, which is caused by shot noise, and is called the standard quantum limit.
The limit of quantum measurement is limited by the basic principle of the quantum world – the uncertainty principle, which is called the Heisenberg limit.
If some strategies are adopted in the measurement process, the measurement sensitivity may break through the standard quantum limit and approach the Heisenberg limit, ie 1/N. When the sensitivity of a measurement is better than the standard quantum limit and approaches the Heisenberg limit, it is called an ultrasensitive measurement.
Another benefit of quantum radars: Since they emit very little energy (photons), they are difficult to detect.
All modern radars emit electromagnetic radiation to detect objects. This radiation also enables the radar itself to be detected. It’s a lot like being in a dark room with lots of people with flashlights, turning on the flashlight allows you to find other people, but the beam of the flashlight is directed back at you, revealing your presence and location.
The advantages of quantum radar, which are less susceptible to interference, higher sensitivity, and stronger concealment, have strong practical significance.
In 2012, under the funding of the US DARPA single-light quantum information project (Information in a Photon), the Institute of Optics at the University of Rochester successfully developed an anti-interference quantum radar, which uses the quantum properties of polarized photons to detect and image targets.
Since any object that receives a photon signal changes its quantum properties, the radar could easily detect stealth aircraft and be virtually unjammable, the research team claims.
Classification of quantum radar
As early as 1966, P. A． Bakut first proposed the feasibility demonstration of using quantum signals in radar systems, and in the 1980s, research progresses that broke the standard quantum limit continued to emerge. In 1991, the U.S. Navy proposed a patent for using quantum detectors to improve the sensitivity of conventional radars.
Then it gave birth to emerging research fields such as quantum ranging, quantum synchronization, quantum sensing and quantum imaging, and attracted the attention of the US Defense Advanced Research Projects Agency (DARPA). DARPA launched the Quantum Sensor Program (Quantum Sensor Program) in 2007. , QSP) and the Quantum Lidar project (Quantum Lidar), marking the formal formation of the field of quantum radar research.
At the beginning of the 21st century, researchers represented by research teams such as MIT, Louisiana State University, Northwestern University, University of Texas, Raytheon BBN, Harris Corporation, and ITT Corporation proposed a variety of different systems. Quantum radar solutions mainly include interferometric quantum radar, quantum enhanced lidar at the receiving end and quantum illumination.
The earliest proposed interferometric quantum radar uses a non-classical light source (entangled or compressed) to illuminate the target area, and performs classical coherent detection at the receiving end. Utilizing the quantum properties of the light source, the distance resolution and angle resolution of the radar system can break through the classical performance limit.
We can perform phase measurements with a Mach-Zender interferometer. In this interferometer, light from a light source at the input is split into two beams, which are reflected by two different mirrors and reach an output detector (screen). By measuring the phase difference between the two beams at the output, the distance to the target can be determined.
However, it is found through research that the performance of interferometric quantum radar is easily affected by loss and atmosphere. Therefore, in the later stages of the quantum sensor project, DARPA shifted its research focus to the quantum-enhanced lidar at the receiving end.
This kind of quantum radar is different from the interferometric quantum radar. It uses a classical light source to scan the target area. In the receiving process, it uses the high-latitude coherent characteristics of the microscopic quantum to improve the angular resolution of the radar and increase the detection distance of the radar.
Quantum-enhanced lidar at the receiving end is the fastest-growing solution among the three quantum radar solutions, but the solution with the best performance in theory is quantum illumination radar.
Quantum irradiation radar uses non-classical light sources to scan the target area in the transmission signal, and uses quantum high-latitude coherent characteristics in the receiving process to perform quantum optimal joint detection, thereby achieving highly sensitive detection of targets.
Since Seth Lloyd of the Massachusetts Institute of Technology proposed the target detection scheme based on quantum irradiation in 2008, the research team of MIT has studied the quantum irradiation based on Gaussian state, the receiver design of the quantum irradiation target detection system, and the angle of the quantum irradiation target detection system. issues of resolution.
The current theoretical and experimental studies show that even under the conditions of increased signal loss due to the real environment and the presence of background noise, the target detection system based on quantum irradiation still has excellent characteristics of high sensitivity.
Simply put, interferometric quantum radar only uses non-classical state (quantum state) information processing technology at the transmitting end; quantum enhanced radar at the receiving end only uses non-classical state (quantum state) information processing technology at the receiving end; Both the receiving end and the receiving end adopt non-classical state (quantum state) information processing technology, and the machine adopts quantum technology.
In contrast, the implementation of quantum irradiation technology is much more complicated, but it can greatly break through the limit of existing radar performance, and has attracted more attention. In recent years, most of the research at home and abroad has focused on the latter.
In addition, according to the different detection signal forms, quantum radar can be divided into single-photon detection quantum radar and multi-photon detection quantum radar. The former is an ideal detection scheme. Its advantage is that it is almost free from interference, but its disadvantage is that it is difficult to implement. Although the latter will be interfered to a certain extent, it is relatively easy to implement and has greater practical significance.
Who is number one in the world?
Now it is time to answer the opening question. Generally speaking, the world’s first quantum radar system was developed by the University of Rochester in 2012, but the technology is at a very early stage. The quantum radar announced in May this year is the world’s first microwave quantum irradiation radar.
The quantum irradiation radar scheme was proposed in 2008. In 2013, Lopaeva et al. in Italy realized the quantum irradiation radar for the first time with an experimental method. Has target detection capability.
In 2015, Shabir Barzanjeh, who was still at RWTH Aachen University in Germany, conducted in-depth research on microwave quantum radiation detection. Barzanjeh is now an associate professor at the University of Calgary in Canada and the lead author of the microwave quantum irradiation radar article published in May of this year.
The research was initiated by Prof. Johannes Fink’s research group at the Austrian Institute of Science and Technology (IST), of which Barzanjeh is also a member. In addition, there are collaborators from the Massachusetts Institute of Technology in the United States, the University of York in the United Kingdom, and the University of Camerino in Italy. The researchers demonstrated a new detection technique called microwave quantum illumination, which uses entangled microwave photons as the detection method.
The system starts with a Josephson parametric converter (JPC) inside a dilution refrigerator that generates entangled microwaves at frigid temperatures just one-thousandth of a degree above absolute zero (-273.14 degrees Celsius) photon.
Instead of using conventional microwaves, the researchers entangled each other with two groups of photons, called signal photons and idler photons. Signal photons are emitted to the detection target, while idler photons are measured in relative isolation without interference and noise.
When the signal photon is reflected back, the quantum entanglement between the signal photon and the idler photon is lost, but a small amount of correlation remains to unambiguously distinguish the reflected signal photon from background noise, resulting in a signature or pattern that describes the target object exists or does not exist.
Quantum radars are less affected by background noise, consume less power, and do not reveal themselves when detecting distant targets. The researchers believe the technique has potential applications in ultra-low-power biomedical imaging and security scanners.
Let’s go back and talk about China’s quantum radar. As early as August 2016, CETC announced that the first quantum radar system based on single-photon detection was successfully developed in 14th Institute, reaching the international advanced level.
CETC stated that “the quantum radar system was developed by CETC’s 14 key laboratories of intelligent perception technology. With the joint efforts of China University of Science and Technology, CETC’s 27 institutes and Nanjing University, and through unremitting efforts, it has completed The mechanism of quantum detection, the study of target scattering characteristics and the experimental verification of the principle of quantum detection.”
The researchers completed the target detection test in the real atmospheric environment in the field, obtained the detection power of 100 kilometers, the detection sensitivity was greatly improved, the indicators all reached the expected effect, and achieved major research progress and results.
At the 12th Zhuhai Air Show held in November 2018, CETC publicly demonstrated the first single-photon detection quantum radar prototype for the first time. According to the staff, it is a quantum-enhanced lidar.
According to the analysis of “Chinese and foreign ship news”, the radar is a laser radar based on single-photon detection and quantum enhancement technology working in the near-infrared spectrum. It belongs to the product between the second type of quantum enhanced radar and the third type of quantum irradiation radar. Based on the wave-particle duality of light, in order to improve the particle characteristics of the detection medium, the laser radar in the near-infrared frequency band is used, and the quantum enhancement technology is used at the receiving end. But because the technology of quantum entanglement is not used, it has not yet reached the stage of the real third quantum irradiation radar.
The practicality is also open to question. The detection capability of 100 kilometers is not enough to provide the air defense requirements of stealth aircraft or missiles. A larger detection range is needed to achieve the best military use.
Objectively speaking, the microwave quantum irradiation radar system proposed by the Austrian Institute of Science and Technology in May this year is a more advanced quantum radar. However, quantum radar is still in the early stages of research, and some problems must be overcome. At present, there is no practical quantum radar.
A major obstacle to quantum radar is the problem of quantum decoherence, where a quantum system loses its quantum behavior when exposed to the external environment for a long time. This imposes a range limit on existing quantum radar systems, since the longer the distance, the longer the exposure to the surrounding environment.
Professor Wang Qun, a military expert at the National Security and Military Strategy Research Center of the National University of Defense Technology, said: “The research on quantum radar is still very preliminary in general, basically in the stage of experimental verification, far from meeting the level and requirements of technology promotion.”
For example, U.S. defense contractor Lockheed Martin has been trying to build its own quantum radar for long-range detection since 2007, but has had no public reports of deployment.
In recent years, in some media reports, the word “the world’s first quantum radar system” has been mentioned many times. In fact, the current research on quantum radar in various countries is almost on the same starting line, so “the world’s first” is not yet of practical significance.
