Is quantum radar farther than traditional radar?

This paper discusses the basic principle, application and advantages, challenges to be overcome and future technologies of quantum radar.

The discussion of quantum radar technology begins with the description of "twin pairs" of photons, which are two photons. Each photon is one of two quantum states that can be in measurable physical properties (position, momentum, spin and polarization), but the state of each particle depends on the state of the other particle, even if they are separated from each other.

Twisted photon pairs are usually produced by a process called parametric down-conversion, in which a laser beam passes through a nonlinear crystal (usually barium β borate). This method is used to generate entangled photon pairs in the visible range. For quantum radar applications, these photons must be down-converted to microwave frequencies.

Chris Wilson of the Institute of Quantum Computing (IQC) of the University of Waterloo in Canada described in his paper "Quantum Enhanced Noise Radar" the work results of generating entangled photon pairs directly in the microwave range by using superconducting circuits. As an on-chip microwave circuit of superconducting aluminum, the nondegenerate Josephson parametric amplifier is used as a quantum microwave source. One of the challenges of this process is that it must be carried out under an extremely cold cryostat.

Theoretically, you can get the whole momentum vector of the target, not only its Doppler velocity, but also its whole momentum vector, all three-dimensional spaces and all three amplitudes of those dimensions in which the target moves.

The discussion of quantum radar has become more changeable, at least in the general media, there will often be completely different explanations or descriptions of the operating mechanism of quantum radar.

In one method, when paired entangled photons are separated, the process begins, one photon in each pair is directly transmitted along the storage path (idle photon), and the paired photon is converted into microwave frequency (microwave photon) and transmitted to the target as a traditional waveform.

The premise is that when microwave photons interact with the target, the quantum state will change in some way (such as phase or polarity). The return signal reflected from the target is received at the source, the photons are reversely converted to their original frequency state, and then can be compared with the frequencies of their unchanged idle winding pairs to provide information about what they have encountered.

However, there is another description of quantum radar theory, which describes a "long-distance strange interaction" link (a term coined by Albert Einstein), in which a photon of a split entangled pair is transmitted as a "photon beam".

However, in this case, regardless of the distance between them, the transmitted photons maintain communication with their winding pairs continuously and instantaneously in some way. The transmitted photon will not return to its source, while the untransmitted photon itself will see the environment according to the change of its winding pair, thus providing information about possible targets it may encounter without any known connection. So it is called a "weird" description.

Because of their work on the quantum radar project, the Lockheed team defined two types of quantum radars (called QuDAR).

However, as described by Dr. Ned Allen, chief scientific officer of Lockheed Martin (located in Bethesda, Maryland), as part of the DARPA Strategic Technology Office (STO) project in 2005, Lockheed Martin studied the concept of "long-range ghost action", which they called "no return to radar".

According to Allen, they think, "This is a disobedience to Einstein's special theory of relativity, which is far more accurate and credible than quantum physics. After studying for a period of time and bringing together a group of subject matter experts from universities and other top scientific entities, we didn't study this problem further because we thought it was not allowed according to the laws of physics.

Now, Allen also realized that "physics is in a turbulent period, and many of its problems are being reconsidered. He also pointed out: "Although it is not clear that we have learned enough physics and actually ruled it out completely, we are very confident in view of the physical performance available at that time, but it has not been recognized yet. "

Because of their work on the quantum radar project, the Lockheed team defined two types of quantum radar (called QUDAR)-Class1where all quantum effects remain on the radar transmitter/receiver, and Class2 is the transmission of "quantum resources" (photons) from point A to point B through a lossy medium (i.e. atmosphere).

Allen said that Class 1 quantum radar is being developed. "But it is not called quantum radar, but the' sensitivity improvement' of electronic equipment in the transmitting/receiving module, such as a better low-noise amplifier." Jonathan Baugh, an associate professor of IQC at the University of Waterloo, agrees: "This is one of the short-term benefits of developing a' quantum radar' system, in which more sensitive detectors and quantum-inspired signal processing methods may be used to improve the capabilities of classical radars."

Class 1 quantum radar technology may have an impact on stealth target detection. As Allen pointed out, "From a mathematical point of view, stealth is only the reduction of the radar cross section (RCS) of the target, because it is the signal-to-noise ratio (SNR) that determines whether the target can be detected. If the signal-to-noise ratio is improved by reducing the internal noise of the transceiver with good quantum function, smaller and smaller targets can be detected. Class 1 quantum radar may help to defeat some stealth methods.

Class2 quantum radar can further distinguish the degree of coherence between entangled pairs (idle photons and transmitted photons) in time and distance. In one case, the detected backscattered photons returning from the target will completely maintain their coherence. This will measure more aspects of the target than just its existence and Doppler effect. As Allen described, "Quantum interaction essentially measures the existence of a target along an infinite number of dimensions, not just the amplitude and phase, but the countless properties of quantum devices (photons).

In principle, using entangled beams, you can get the whole momentum vector of the target, not only its Doppler velocity, but also its whole momentum vector, all three dimensions and all three amplitudes of those dimensions in which the target moves.

In addition to decoherence, another challenge of quantum radar is photon flux, that is, the number of entangled photons generated and transmitted per unit time. As Baugh of IQC explained, "Suppose you send a photon every nanosecond (1-GHz rate), but if only11000 or110000 actually reflects back to you, then you will only detect a photon once every millisecond. In order to build a useful image, you need to emit photons very quickly to get enough information in a reasonable time. "

Baugh is carrying out a research project with the Canadian Defense Research and Development Department (DRDC) to develop an improved quantum light source, one of which is a quantum radar. The purpose of this project is to provide "very high rate" signals of entangled photons. Although the details of this method have not been announced, since IQC has not announced this technology, Baugh described it as "similar to a semiconductor, a nano-electronic device that works at the single-electron level, allowing electric signals to be converted into photons or a pair of entangled photons."

Since the light source works in an optical state rather than a microwave state (about 850 nm-near infrared, only at the edge of visible light), the direct application will be lidar, but Baugh said, "In the end, the idea is that other groups in the world are studying the coherent quantum wavelength conversion from visible light to microwave frequency, and their research results will become the way of our technology."

Today, when studying the development and possible practical realization of quantum radar technology, it is generally considered that the most promising method is the one that is most likely to be realized in the foreseeable future. It is called Quantum Illumination Radar (QIR).

In order to provide important information about the target encountered by the emitted photons, the returned emitted photons of the QIR entangled pair do not need to be consistent with other idle photons.

Baugh said that QIR can provide many advantages over traditional radar. "Under normal circumstances, using conventional radar, lidar or any type of remote sensing will send out an energy pulse, which contains billions or trillions of photons; This is a classic way to reflect electromagnetic radiation from an object. Back to the detector, it allows to measure the time of flight and calculate the distance to the object, and calculate its speed and direction as time goes by.

In contrast, QIR radar works at the level of a single photon, so starting with pairs of entangled photons, due to the principle of quantum mechanics, these photons themselves have stronger correlation than they did at first. If the returned photons are reflected back, the two photons can be jointly measured to show whether they are actually related at first, so that any unrelated photons can be separated, but it may just be background noise.

Due to the size reduction to very low power (single photon) level, quantum radar provides significant improvement in signal-to-noise ratio.

Fundamentally speaking, decorrelation is related to the second law of thermodynamics, which we have not really understood. If someone can find a way to overcome it, it will be very convenient to study.

Nevertheless, Baugh stressed that "QIR radar will not replace traditional radar. On the contrary, our idea is to enhance the ability of traditional radar to be challenged in specific systems, such as having a very strong background signal in a low SNR environment. Want to detect in the same frequency range, or try to detect invisible targets, or want to make the detection itself invisible. "

Baugh pointed out that another advantage of QIR is that it can provide detection because of the "tiny" power level of a single photon beam, but it is still undetected. "The target doesn't know that it is illuminated, because the number of photons per unit time used to detect it is too small to measure. QIR is 9- 10 orders of magnitude lower than traditional radar or lidar. "

In the paper of Bhashyam Bala Ji 20 18, the prospect of QIR is summarized as follows: "Quantum illumination radar can definitely be built, but the establishment of QIR requires Qi Xin's cooperation (that is, radar engineering indicators) and appropriate investment.

There are still many unknowns about the best quantum radar design or the best quantum signal processing. However, "the best should not be a better enemy." These efforts will require radar engineers to master microwave quantum optics, which is a very important application in the market and the benefits will be enormous.