Scientists demonstrate quantum radar prototype

I'm a physicist working in a related field, but not involved with this research in any way. I'd like to provide a bit of context and point out an important technical detail that is not touched upon in the phys.org story.

The thing is, the claims made here, taken at face value, are rather extraordinary!

Under usual conditions, in order to observe quantum phenomena with microwaves, sub-Kelvin temperatures are required to prevent the quantum signal from being swamped by blackbody radiation. Theoretically, there could be a few ways around this limitation (e.g. N00N states), but all prior experiments (to my knowledge) have been performed firmly within the confines of cryogenic refigerators.

In this experiment, too, the non-classical state begins at a brisk temperature of 7 mK. To get a strong enough signal that can be brought out to room temperature, bounced off a target, and finally digitized by room-temperature electronics, the experiment includes a "classical" transistor amplifier in the signal path.

Now, another elementary result of quantum theory forbids amplifiers of this type from boosting quantum signals without significant added noise.

How did the experimentalists get around this limitation? It appears that they have chosen to present some of their results as a function of the number of signal photons, leaving out the added noise by the amplifier.

The problematic part for the purpoted quantum advantage is that the classical noise at the amplifier output is much larger than the quantum component. Hence, a target illuminated by this kind of "radar" will simply pick up the amplifier noise, and no "true" quantum advantage was demonstrated.

Note that the above conclusions can be found in the research article itself, starting at the end of second-to-last paragraph on page 3. Also note that, carefully reading the abstract and introduction, no claims of demonstrated beyond-classical performance are made.

Take-home lesson? At least when it comes to quantum physics, it's really really hard to accurately evaluate the significance of cutting-edge research. I'd be willing to assume the same holds for other branches of science, as well.

Looks like it operates on the same principle that this Chinese arms manufacturer announced a few years back.

https://www.popsci.com/china-quantum-radar-detects-stealth-p...

What is the resolution? Is it high enough to steer a missile or just good enough to tell you something is in the sky- a capability that already exists.

It does have some interesting traits. Is it resistant to jamming? Is it indistinguishable from background clutter to the target?

I wish articles like these define what "superior" means. Context in quote below..

> When benchmarked against classical low-power detectors in the same conditions, we see that at very low-signal photon numbers, quantum-enhanced detection can be superior.