
Scientists demonstrate quantum radar prototype - dnetesn
https://phys.org/news/2020-05-scientists-quantum-radar-prototype.html
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_8091149529
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.

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magicsmoke
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...](https://www.popsci.com/china-quantum-radar-detects-stealth-
planes-missiles/)

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2OEH8eoCRo0
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?

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mkchoi212
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.

