Quantum Radar has been on the…ahem…radar for some time. Unfortunately, the theoretical and practical consequences of our explorations of the concept have been overwhelming. But before we move on to the disappointments, let me give all you radar enthusiasts a reason to hope. a new paper demonstrates that, under conditions of low signal-to-noise ratio (at the edge of the classical range of radar), employing quantum technologies can offer a *Very* Significant increase in accuracy.

## Quantum radar?

Radar, in its simplest form, involves sending out pulses of radiation reflected off an object. The reflected signal is detected, and the flight time is measured. Time of flight is then translated into a range, while the direction in which the radar antennas picked up the reflectance tells us the direction.

The terrible thing about radar is that the signal drops very rapidly – to the fourth power of the distance. This is because the power of the radiation we send the droplets as the square of the distance between the transmitter and the object. And then it falls again as a square of the range after it is reflected and has to go back to the receiver. You get bogged down by the inverse square rule twice.

Let me make this solid with a very rough estimate: a radar with a 1 kW transmitter and an antenna with a gain of 10 would need to be able to detect a few nW (10)^{-9} W) 1 m of the received power. To see^{2} Object at 5 km.

A quantum radar uses quantum entanglement to increase the sensitivity of the receiver. For quantum radar to work, we no longer send all of our photons looking for objects. Instead, we send only half of the entangled pair of photons to reflect off objects outside; The other half is placed on the receiver. When the sent photon returns, it matches its companion more perfectly than any other photon that can be detected by the receiver. We can detect these matches, called correlations, with high sensitivity.

In terms of microwave engineering, think of it better than the best possible narrow-band filter. In other words, quantum radar doesn’t amplify the absolute level of the signal, but it does increase your certainty in separating the signal from the noise.

## wake me up when it gets interesting

It looks exciting when viewed from above. Preliminary calculations showed that entanglement should provide a factor of 2-4x improvement in certainty. Nice—but not worth the extra complications of working with entangled photons when it comes to practical applications. Worse, the first experiments with quantum radar all used optical frequencies instead of microwave frequencies, and they operated over such short distances that the signal loss was minimal. Even on the brightest day, the noise at optical frequencies is an order of magnitude lower than at microwaves.

Therefore practical applications, which would require using microwave frequencies, imply huge losses. Snoring was deafening from reluctant radar engineers.

To make quantum radar interesting again, theorists have delved deeper into radar theory and practice. It turns out that boundary accuracy (how well your average estimate of the range is) and boundary resolution (how confidently you can separate the boundary of two objects) are not entirely the best bedfellows. Range accuracy gets really bad when the ratio of the returned signal to background noise is below a certain range. And it is at this point that quantum entanglement can provide a huge advantage.

## pull that pulse out

To improve accuracy, you should have diffuse and chirping pulse. Essentially, you rotate the radar frequency from high to low during the pulse (this type of pulse is also used in some classical radars). This pulls each photon out over time so that its frequency can be better defined. It also better defines its entangled partner so that they can be jointly identified with greater certainty.

On the face of it, it degrades accuracy. An individual photon can be detected at any time in the entire pulse duration, which is now much longer. But the microwave pulse contains billions of photons per frequency, so there are lots and lots of individual photons to be detected. The statistical variation in their detection times decreases with the number of photons, allowing you to generate an accurate time of flight.

It really shows its power when the signal noise level drops below the classical limit for accurate detection. When the signal is four times the noise, quantum radar is about 500 times more accurate than classical radar (assuming the same transmitter power). Even when the signal-to-noise ratio is one (about when I’ll give up), quantum radar remains three to four times more accurate than classical radar.

## How wide are your pulses?

The gain of a quantum radar really depends on how spread out the pulse is. The researchers demonstrate this by computing the quantum gain of a W-band radar detecting a small drone (a radar cross section of 1 cm).^{2}) At a distance of 100 metres, the quantum radar detects the drone by a 10 ms pulse, with an accuracy of about 60 times higher than that of classical radar. But the window of utility is limited; When the drone is a kilometer away, the same advantage is available only if the radar pulse is about two minutes longer, by which time the drone has taken off and is gone.

The bigger problem, unfortunately, is practicality. To make this work, high-powered sources of maximum entangled microwave photons are needed. Currently, the best entangled photon sources operate at optical frequencies and emit up to one million photons per second, corresponding to a power of about one fW (10).^{-15} W). There are a few orders of magnitude between where we are now and where we should be.

But, before you get too depressed, note that microwave sources are actually easier to manufacture (and have a longer engineering history) than optical sources. and scientists already displayed Entangled microwave source. so maybe there *Is* Here’s a future…

*physical review paper*, 2022, DOI: 10.1103/PhysRevLet.128.010501 (About DOI)