Quantum radar has been on… um… radar for a while now. Unfortunately, the theoretical and practical results of our explorations of the concept have been disappointing. But before I get to the disappointments, let me give all radar enthusiasts a reason to be hopeful. A new paper demonstrates that, under conditions of low signal-to-noise ratios (at the limit of the conventional radar range), the use of quantum technologies can provide a very significant improvement in accuracy.
Radar, in its simplest form, involves sending pulses of radiation that reflect off an object. The reflected signal is detected and the time of flight is measured. The time of flight is then translated into distance, while the direction in which the radar antenna was pointed when it picked up the reflection tells us the direction.
The horrible thing about radar is that the signal decreases very quickly, like the fourth power of the distance. This is because the power of the radiation we send out decreases as the square of the distance between the emitter and the object. And then it falls back like the square of the beach after being reflected and has to return to the receiver. You get run over twice by the reverse square rule.
Let me concretize this with a very rough estimate: a radar with a transmitter of 1 kW and an antenna with a gain of 10 should be able to detect a few nW (10-9 W) of power received to see a 1 m2 object at 5 km.
A quantum radar uses quantum entanglement to increase the sensitivity of the receiver. To make quantum radar work, we no longer send all of our photons in search of objects. Instead, we only send out half of a pair of entangled photons to reflect off objects; the other half is kept at the recipient. When the sent photon returns, it matches its partner more perfectly than any other photon that could be detected by the receiver. We can detect these matches, called correlations, with great sensitivity.
In microwave engineering terms, think of it as better than the best narrowband filter possible. In other words, a quantum radar does not increase the absolute level of the signal, but it does increase your certainty of distinguishing signal from noise.
Wake me up when it gets interesting
At first glance, it sounds exciting. Early calculations showed that entanglement should provide a 2- to 4-fold certainty improvement factor. Good, but probably not worth the added complications of working with entangled photons when it comes to practical applications. Worse yet, the early experiments with quantum radar all used optical frequencies rather than microwave frequencies, and they operated over distances so short that signal loss was minimal. Even on the brightest day, noise at optical frequencies is several orders of magnitude lower than that of microwaves.
Thus, practical applications, which would require the use of microwave frequencies, involve enormous losses. The hum of indifferent radar engineers was deafening.
To make quantum radar interesting again, theorists have delved into the theory and practice of radar. As it turns out, range accuracy (how good your average range estimate is) and range resolution (how confidently you can separate the range of two objects) aren’t quite the best of the best. bed mates. The range accuracy gets really bad when the ratio of the returned signal to the background noise is below a certain threshold. And it is at this point that quantum entanglement can apparently provide a big advantage.
Stretch that impulse
To improve accuracy, you need to stretch and chirp the pulse. Essentially, you sweep the radar frequency from high to low during the pulse (this type of pulse is also used in some conventional radars). This stretches each photon in time so that its frequency is much better defined. It also helps to better define its entangled partner so that they can be jointly detected with greater certainty.
At first glance, this reduces accuracy. An individual photon can be detected at any time over the duration of the pulse, which is now very long. But the microwave pulse is made up of billions of photons per frequency, so there are lots and lots of individual photons to be detected. The statistical variation in their detection time narrows with the number of photons, allowing you to generate an accurate time of flight.
It really shows its power when signal-to-noise levels drop below the typical threshold for accurate detection. When the signal is four times the noise, quantum radar is about 500 times more accurate than conventional radar (assuming the same transmit power). Even when the signal-to-noise ratio is one (roughly when I would give up), quantum radar is still three to four times more accurate than conventional radar.
How stretched are your impulses?
The advantage of quantum radar really depends on the elongation of the pulse. The researchers demonstrate this by calculating the quantum advantage of a W-band radar locating a small drone (a 1 cm radar section2). At 100 m, the drone is detected by a 10 ms pulse from a quantum radar with approximately 60 times more precision than with a conventional radar. But the utility window is limited; when the drone is one kilometer away, the same benefit is only obtained if the radar pulse lasts about two minutes, elapsed time from the drone.
The biggest problem is, unfortunately, the practicality. For this to work, high power sources of maximally entangled microwave photons are needed. Today, the best sources of entangled photons operate at optical frequencies and emit up to one million photons per second, which corresponds 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 need to be.
But, before you get too depressed, note that microwave sources are actually easier to build (and have a longer engineering history) than optical sources. And scientists have already demonstrated entangled microwave sources. So maybe there is a future here …
Physical examination letters, 2022, DOI: 10.1103 / PhysRevLett.128.010501 (About DOIs)