Quantum Radar is here

August 23, 2019 5:42 PM

And it uses entangled Microwave photons. Almost all the advances in quantum computing, cryptography, teleportation, and so on have involved visible or near-visible light. This team has used entangled microwaves to create the world’s first quantum radar.

Their device, which can detect objects at a distance using only a few photons, raises the prospect of stealthy radar systems that emit little detectable electromagnetic radiation.

A big advantage is the low levels of electromagnetic radiation required. “Our experiment shows the potential as a non-invasive scanning method for biomedical applications, e.g., for imaging of human tissues or non-destructive rotational spectroscopy of proteins,” say Barzanjeh and co.

orig: https://arxiv.org/abs/1908.03058

posted by aleph (20 comments total)

(Jim Gaffigan voice)

I have no idea what this even means...
posted by Windopaene at 5:47 PM on August 23, 2019 [6 favorites]

"Quantum" sounds weird the more you say it. Quantum quantum quantum.
posted by Mr.Encyclopedia at 5:50 PM on August 23, 2019 [1 favorite]

Entangled photons makes it quantum. This first time the photons are lower frequency microwave ones instead of the optical/visible ones used in quantum experiments before. Radar works at microwave frequencies.
posted by aleph at 5:53 PM on August 23, 2019

And entangling the photons means the one that goes out and bounces off the target is *much* easier to detect out of the noise floor by comparing it to its entangled buddy you kept back at home.
posted by aleph at 5:55 PM on August 23, 2019 [3 favorites]

Not to be confused with quantum gaydar, where whenever you observe someone's sexuality, it collapses from a superposition of gay and straight to some unit vector between the two.
posted by J.K. Seazer at 6:02 PM on August 23, 2019 [5 favorites]

Agreed
posted by aleph at 6:03 PM on August 23, 2019 [1 favorite]

Here in Tejas the bats have quantum radar . . .I swear to dawg!
posted by ahimsakid at 6:05 PM on August 23, 2019

Well, there's speculation about quantum effects in biological processes. Wouldn't surprise me if it turned up in sensory perception like bats.
posted by aleph at 6:07 PM on August 23, 2019 [4 favorites]

Almost all the advances in quantum computing, cryptography, teleportation, and so on

Wait, wut?
Ooookay.

Beam me up, Scotty.
posted by BlueHorse at 7:37 PM on August 23, 2019

WHERE IS TOM LEHRER WHEN YOU NEED HIM
posted by not_on_display at 10:13 PM on August 23, 2019 [1 favorite]

So does this mean we actually can have stealth in space
posted by Ray Walston, Luck Dragon at 10:15 PM on August 23, 2019

Cool. My undergrad QM was a long time ago, so I'm a bit fuzzy on this, but I was confused how you would maintain entanglement of photons after reflection. The MIT Technology Review article suggested the entanglement was in fact lost, which then made me confused why this was "quantum radar" and not just taking advantage of interference in coherent light. So I took a look at the paper on arXiv, which is mostly well beyond my ability to make sense of, but the key idea seems to be this:

Even though entanglement is lost in the round trip from the target, the surviving signal-idler correlations are strong enough to beat the performance achievable by any coherent-state transmitter using the same number of photons and bandwidth.

So if I understand correctly, it's basically that since the reflected microwave photons started out as entangled with the photons they're interfered with, their statistical correlations are better than you'd get with a conventional laser (or in this case since it's microwave, maser) source.

Physics is cool.
posted by biogeo at 10:27 PM on August 23, 2019 [5 favorites]

biogeo, I'm certainly no physicist - I'm just a f*&^ing gardener, but is that saying that entanglement keeps emitted photons in a pure\coherent state until reflection?
posted by unearthed at 11:40 PM on August 23, 2019 [1 favorite]

I think so. Entanglement in general is a pretty fragile phenomenon; as soon as either of the entangled particles interacts with another particle, the entanglement is lost. (Or maybe you could think of it as getting "distributed" across the rest of the environment.) So as long as the emitted photon doesn't interact with anything until it's reflected, entanglement should persist, and as soon as the emitted photon is reflected, the entangled state is lost. At least that's my understanding; if any actual practicing physicists happen to stop in and comment, I would certainly take their thoughts more seriously than mine.
posted by biogeo at 12:21 AM on August 24, 2019 [2 favorites]

You think quantum radar is strange, check out Quantum Darwinism.
Quantum Darwinism is a theory claiming to explain the emergence of the classical world from the quantum world as due to a process of Darwinian natural selection induced by the environment interacting with the quantum system; where the many possible quantum states are selected against in favor of a stable pointer state.
posted by Metacircular at 12:36 AM on August 24, 2019

Are all the kernels in my microwave popcorn popped? With quantum microwaves, they are and they aren't at the same time!
posted by SPrintF at 3:51 AM on August 24, 2019

Paging @physicsmatt
posted by sfts2 at 7:32 AM on August 24, 2019

physicsmatt's account is disabled... Hopefully just taking a break.
posted by biogeo at 9:09 AM on August 24, 2019

Makes perfect sense. I think you have it biogeo. Due to uncertainty you can't know what you sent precisely. By creating entangled pairs you can send one and measure the other for a reference. You now have a better interference filter for detection that allows you to crank down the probing sample volume while retaining the signal to noise because you do have that entangled reference sample that is woo-magic-at-a-distance the always matching to the signal you sent but can't measure.
posted by zengargoyle at 3:24 AM on August 25, 2019 [1 favorite]

"...because you do have that entangled reference sample"

I was surprised because I knew reflection would disrupt the entanglement but there was still enough left in the returned photon to make it easier to detect comparing it to the one of the entangled pair that you kept. Of course maybe it doesn't work the way they think it does but *something* seems to work being able to detect such a weak return signal.
posted by aleph at 9:02 AM on August 25, 2019