Entangled microwave photons can amplify radar 500 times
Extended, interlocking microwave pulses are key.

Quantum radar has been on... my radar for a while. Unfortunately, the theoretical and practical results of our explorations of this concept have been very poor. But before we get into frustration, let me give all of you radar fans a reason to hope. New research suggests that by using lower signal-to-noise ratios (at the edge of classical radar), the use of quantum techniques may dramatically increase accuracy.

Quantum radar?

In its simplest form, radar involves sending radioactive pulses that are reflected from an object. The reflected signal is detected and the flight time is measured. The flight time then becomes a range, while the direction shown by the radar antenna when the reflection is received tells us the direction.

The horrific thing about radar is that the signal goes down so much. Fast - as the fourth power of distance. This is because the radiant power we emit decreases with the square of the distance between the transmitter and the object. And then, after the reflection, it falls again as the square of the range and should go back to the future. You are doubly confused with the inverse square rule.

Let me make this concrete with a very rough estimate: a radar with a 1 kW transmitter and a 10 gain antenna should be able to detect it. A few nW (9-10 W) of energy were received to see an object of 1 square meter at 5 km.

Quantum radar uses quantum entanglement to increase the sensitivity of the receiver. For quantum radar to work, we no longer emit all of our photons to look for things. Instead, we emit half a pair of entangled photons to reflect things off. The other half is kept in the receiver. When the transmitted photon returns, it is so perfectly matched to its partner that any other photons are detected by the receiver. We can detect these matches, called correlations, with high sensitivity. Advertising

Regarding microwave engineering, consider it better than the best possible narrowband filter. In other words, quantum radar does not increase the absolute signal level, but rather increases your confidence in detecting the signal from the noise.

Wake me up when it's fun

In appearance, it looks sexy. Preliminary calculations showed that entanglement should produce a factor of 2-4 times the certainty. This is fine, but probably not worth the trouble of working with entangled photons. Worse, the first experiments with quantum radar all used optical rather than microwave frequencies, and they worked over short distances with very little signal loss. Even on the brightest days, the noise at light frequencies is much lower than at microwaves.

Thus, practical applications that require the use of microwave frequencies involve huge losses. The snoring of the radar engineers was indifferent.

To make quantum radar interesting again, theorists have delved deeper into radar theory and its application. Scope accuracy (how well you estimate your scope) and scope resolution (how confident you can separate the boundaries of two objects) don't seem to be quite the best. The resolution of the range becomes very poor when the ratio of the return signal to the background noise is below a certain threshold. At this point, quantum entanglement appears to have a significant advantage. Basically, it scans the radar frequency from high to low during the pulse (this type of pulse is also used in some classic radars). This pulls each photon in time to better determine its frequency. It also helps to better identify the partner in question so that they can be recognized together with greater confidence.


In appearance this reduces accuracy. A single photon can be detected at any time during the entire pulse duration, which is now very long. But a microwave pulse is made up of billions of photons at each frequency, so there are a lot of separate photons to detect. Statistical changes in detection time are reduced by the number of photons, which allows you to establish the exact flight time.

This really shows its power when the signal-to-noise level is below the cut-off threshold for accurate diagnosis. . When the signal is four times greater than the noise, quantum radar is about 500 times more accurate than conventional radar (assuming the same transmitter power). Even when the signal-to-noise ratio is the same (roughly when I give up), quantum radar remains three to four times more accurate than classical radar.

How tall are your hills? < p The benefits of quantum radar really depend on how much of the pulse is drawn. The researchers demonstrated this by calculating the quantum advantage of a W-band radar identifying a small UAV (radar cross-section of 1 cm 2). At an altitude of 100 meters, the drone is detected by a 10-ms quantum radar pulse with an accuracy of 60 times that of classical radar. But the tool window is limited. When the drone is one kilometer away, only if the radar pulse lasts about two minutes, it has the advantage that the drone is gone by that time.

The biggest problem, unfortunately, is practicality. To do this, high-energy sources of microwave photons are required. Currently, the best entangled photon sources operate at optical frequencies, emitting up to about one million photons per second, which corresponds to a power of about fW (10-15 W). There are many times between where we are now and where we should be.

But before you get too depressed, note that microwave sources are actually easier (and have a longer engineering history). from light sources. Scientists have already demonstrated entangled sources of microwaves. So maybe there's a future here...

Physical Review Letters, 2022, DOI: 10.1103/ PhysRevLett.128.010501 (About DOIs) Xiaomi will launch the Redmi Note 11S in the European market on February 11th

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