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Quantum radar will expose stealth aircraft (phys.org)
64 points by jonbaer 9 months ago | hide | past | web | favorite | 62 comments

Always remember that stealth is not a boolean.

The enemy is constrained by size, cost, power, and heat dissipation. What can be done on the ground is not the same as what can be done in a fighter plane, and that isn't the same as what can be done in a small-diameter missile, and that isn't the same as what can be done in an anti-aircraft shell.

The enemy might be able to briefly detect at close range, reliably track at long range, or anything in between.

Because of this, stealth is not simply defeated, and it does not suddenly become 100% useless.

> and that isn't the same as what can be done in a small-diameter missile

Which was the genius idea behind Track-via-Missile in the Patriot SAM

Instead of having the missile work-out the best interception based on what it can detect, it relays its view of the World back to the ground station which returns its recommended geometry. Constantly and at high frequency.

And since the ground station is more powerful and can be upgraded more easily than the missile's electronics, Patriot has made huge improvements since the 1991 Scud incidents.

In an anti-stealth context having each missile 'see' from a different angle and being able to fuse that I to one picture is also useful in defeating shaped reflections. It's like a network of remote, lethal sensors.

> it relays its view of the World back to the ground station which returns its recommended geometry.

Can't the missile jam the signal?

Sure it could. And then the impactor could target the jamming transmitter. Therefore emitting a jamming signal would actually make the missile easier to intercept.

You could attach the jammer to the missile using a wire of some random length.

I see that you work in software, not hardware.

Aerodynamically this is quite a bad idea. Better to send two missiles, and have one use a jammer and the other not.

Or the jammer could be separate from the missile.

In this cycle, we see a war of complex resources (produced by logistical wealth) emerge. Those with the most of everything, to produce the greatest fighting flexibility, eventually win. Perhaps pyrrhicly.

That would be awfully nice if it were true. In reality, the opposition to this kind of tech just makes lots and lots of cheap missiles, while your interceptors cost millions each, and drive you bankrupt.

This is happening in Gaza. Hamas cuts down telephone poles and fills them with homemade solid rocket fuel and a 50 KG warhead, which are then called Katyushkas because they superficially resemble the 1940's Grad missiles. These are then intercepted by $100,000 Tamir interceptors.

The worst part is that misfiring telephone poles have killed about as many Gazans as Israelis.

Wouldn't the latency be a problem in highly dynamic chases. I presume its ok for ballistic missiles.

> Always remember that stealth is not a boolean.

And also it's not just about the technology in the design of the aircraft. It's a combined strategy such as knowing where the radar stations are and flying a path to minimise exposure.

I remember seeing a visualisation of what ISTR was a stealth flightpath into Iraq giving an idea of how they maintained their extremely low visibility. I've just has a search but can't find it at the moment.

The F-117 was retrofitted with the ability to fly a computer generated flight plan, which would not avoid radar sites but also adjust the plane's attitude in order to minimize the radar cross section. [1]

[1] Skunk Works: A Personal Memoir of My Years at Lockheed by Ben R. Rich and Leo Janos (page 101 on my copy)

The relevant Wikipedia article also mentions the system, but in less detail: https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Avion...

Great book, just finished the other day. Do you have any other similar suggestions?

I would recommend "Blind Man's Bluff: The Untold Story of American Submarine Espionage"

I would also HIGHLY recommend this book. I've described stories from it to interested colleagues several times.

"Stealth" is a principal, not a technology. There are a handful of radar techs that defeat traditional stealth technologies that seek to limit reflections, but the principal of hiding from radar is still going to be a thing.

Radars, even quantum radars, each have their issues. The most powerful stealth tech isn't defeating a radar but understanding it well enough to leverage its defects in coverage or operating range. The f-117's real trick was the preflight planning based on a deep understanding of the threat environment. A radar avoided need not be defeated.

> *Quantum radar...works by sending one of the [entangled] photons to a distant object, while retaining the other member of the pair. Photons in the return signal are checked for telltale signatures of entanglement, allowing photons from the noisy environmental background to be discarded. This can greatly improve the radar signal-to-noise in certain situations."

Why do they need to be entangled? Aren't there other ways of checking if the photons coming back are the ones you sent?

We do that now with spread-spectrum and chirps - you correlate the signal sent with the return signal and you can pick out reflections below the noise threshold. https://en.wikipedia.org/wiki/Pulse_compression

I'm guessing that this might be more effective at very small returns, where the reflection is so small that it suffers from shot noise - the fact that the energy comes in discrete packets makes it hard to correlate. This is another way to verify that the photons received are the same ones sent out (and not background noise)

Photons with identical properties are indistinguishable from each other, so not really! You can get really picky about sending out only certain kinds of photon and filtering them when they come back, but you might have some of that kind of photon in the environment anyway, or the target might be capable of generating them to swamp detectors.

What it can't do is generate photons which are entangled with another photon you already have, so it kind of cuts through all the incremental stuff you could otherwise do (and which has been done already).

At least, that's how I understand the physics involved at a fairly superficial level.

This sounds like it's a long way away from being useful though, that slight issue of not having an abundant source of entangled photons is a huge blocker.

They check for entangled photons, as that lets them easily filter out noise inherent in all the other particles.

It's really an idea not a complete system, needing a way to quickly entangle photons on-demand. Cool idea, definitely!, but not yet completed.

Back when I worked on skywave HF OTHR systems in the late 80s we knew that they could detect and pinpoint stealth aircraft easily.

Most stealth systems are designed to either absorb or reflect millimetric point source radar. HF long wavelength systems reflect nicely off the top surfaces of the aircraft, which aren't designed to dissipate and absorb those frequencies. To one of those systems a stealth aircraft shows up brighter than a 747...

So while this is neat science, it's not economical compared to the systems currently in use. The only real downside of OTHR systems is that you need a couple of miles space for the transmit antenna and a big empty space for the receiver; plus plenty of CPU to process the data.

I am quite offended by how the news provider phys.org reacts to me using an ad blocker: "It appears that you are currently using Ad Blocking software. What are the consequences? Click here to learn more." -> Will never ever click

Thanks, I remove those bookmarks too. (Being offended gives them more control over you than necessary, in my opinion.)

Not if you're offended enough to never visit the domain ever again.

I thought stealth aircraft is already visible on the latest Russian gear. They just use different frequencies to see it. The problem now seems to be not in seeing the plane per se, but homing the missile on a low observability target.


Latest? L-band early warning has been around for ages. The problem is the range and angle accuracy is not enough to guide an interceptor for fundamental physical reasons: the angular resolution is proportional to frequency and antenna size, and the range resolution is proportional to bandwidth.

> I thought stealth aircraft is already visible on the latest Russian gear

That's just a usual FUD stuff from Russians.

Sure. That’s why our government pisses itself every time Russians try to export S400.

An opinionated post about another quantum radar:

Chinese "quantum radar" is a thing that cannot exist https://motls.blogspot.com/2017/03/chinese-quantum-radar-is-...

I think that guy has gone too far in pushing against quantum woo. Also, I'm not aware of any actual science that forbids closed time like curves, either; AFAIK they're allowed by GR and anything that rules them out is strictly speculative.

Which reminds me of a paper I once submitted to HN, »Gravimetric Radar: Gravity-Based Detection of a Moving Point-Mass« [1]. But I don't remember much about it, especially how far this was from being practical and not even how credible the paper looked. But given that I submitted it, I must at least once have thought that it is not total nonsense.

[1] https://news.ycombinator.com/item?id=8369812

Going slight OT but related is the use of Gravity Anomaly Maps to aid submarine navigation. There are quite a few diverse papers about it but I think information about any real capabilities are largely under wraps.

Scientists who work on atomic clocks believe that they will be able one day to put up a network of quantum-entangled atomic clocks in orbit as a kind of ultra-GPS.

According to an article[1] I read, they should be able to gravimetrically image the volume of earth to within about a cubic centimeter resolution.

At that point it becomes very hard to hide high-density materials, like concentrations of U or Pu, anywhere.

[1] https://www.sciencenews.org/article/quantum-timekeeping

Lol, it's pretty ambitious to try tracking planes with gravitational effects. But now that gravitation wave detection works, I think there are plans (or at least ideas) to build such devices in earth or solar orbit for triangulation of distant objects like stars and black holes. Detection arms could be much longer and disruptions much lower than on earth, so results would me much more accurate.

You are probably thinking of the planned Laser Interferometer Space Antenna (LISA) [1]. Launch is scheduled for 2034 but the technical demonstration mission LISA Pathfinder [2] was already done between 2015 and 2017 and exceeded its targets with regard to sensitivity. Other proposed space-based gravitational wave detectors are TianQin [3] and DECIGO [4].

[1] https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...

[2] https://en.wikipedia.org/wiki/LISA_Pathfinder

[3] https://en.wikipedia.org/wiki/TianQin

[4] https://en.wikipedia.org/wiki/Deci-hertz_Interferometer_Grav...

I did some noodling a while back to see if LIGO could detect the mass energy conversion of a fission 'event' in North Korea. If memory serves it was definitely within the range in terms of impact to the gravitational field, but correlating a infinitessimally short pop of noise would be much more difficult than the death waltz of binary black holes.

However, a larger array of more sensitive detectors that are immune from the tremendous amount of terrestrial noise might make short work of it.

Why would an event in NK cause a change in gravity? Wouldn't the energy liberated in a nuclear event would cause the same amount of gravity to be felt elsewhere as the mass it came from?

Initially I thought the same for a couple of seconds because of mass energy equivalence, but then I realized:

assume the fission event to occur in empty space, assume the liberated energy is isotropically radiated, and assume all the energy is thus expanding spherically, then the mass/energy that was initially in the mother isotope is now after some time distributed on a spherical surface.

As long as the observer is outside of the shell of escaping energy, to the observer the gravitational field looks like all the mass is concentrated at the center of the expanding shell of energy, exactly where the mass/energy originally was, and hence gravitationally undetectable.

After a while the expanding surface has past the observer, and the observer is inside a spherically symmetric shell of mass/energy. Even if the observer is not at the center there will no gravitational field at all according to the observer. (consult any physicist or physics textbook, or do the exercise for the gravitational field in a 'hollow earth'). So to the observer this looks like a bit of disappearing mass at the location of fission event.

It seems extremely difficult to me to actually measure such an event gravitationally though..

Two theories off hand without research to double check.

1 - nuclear explosion involves the conversion of a tiny amount of mass into rapidly expanding cloud of energy conveyed by various particles. The significance being the drastic density difference before and after it’s blown to bits. This difference is equivalent to a sudden moment of regular mass since energy and mass are equivalent and energy has gravitational attraction. This is roughly the same as attempting to catch the gravitational waves theorised to be produced by a supernova. We can’t catch these yet due to insufficient sensitivity, but despite the nuclear explosion being ridiculously tiny compared to a supernova, we are also ridiculously close compared to the supernova, so the proximity to the event may “win” and make it strong enough for the detectors to see it.

2 - the equivalence of nuclear explosions to earthquakes in terms of energy displacement into the earths crust, and in particular underground tests of them, has been established. (This is obviously dependent on the size of the explosion and if it’s an underground test or not, since underground tests dump basically all their energy into the surrounding geology they make it a “more effective earthquake”). Earthquakes can speed up or slow down the earths rotation, which has tidal effects which involve the moon, so between the seismic and seismically induced tidal changes, we get to affecting the moons orbit and consequently we get a tiny gravitational wave which once again is detectable only because of how much closer we are to the source compared to the things we built LIGO to detect.

Of course I might be wrong on both of these. They were just fun to think about on my ride home from work. :-)

I ought to start writing scientific papers. I back-of-envelope calculated much the same conclusion years ago — only for the static case, but it was back-of-envelope work.

Are there interesting spinoff technologies that having an efficient entangled photon source would enable? My very spotty understanding makes me wonder if this could have an impact on quantum computing. It seems like an interesting question, since military funding is involved.

Yes, so much so that I would not call it a spin-off, rather one of the main tasks that people building quantum computing hardware are dealing with.

Entanglement is one of the main phenomena that permit quantum hardware to do things infeasible on classical hardware. In the context of communication it permits quantum key distribution, whose security depends only on quantum mechanics being a not-too-wrong description of nature (while classical schemes depend on less tested computational complexity conjectures). In the context of computing, it is an integral component of many of the "fault-tolerant error correcting" schemes, necessary in order to protect the extremely fragile qubits (similar to what ECC RAM does for classical computing).

In the context of sensing, it does permit some cool unbelievably sensitive detectors (the case discussed in the article), but it is only one of the avenues of intense research.

Does quantum computing require entanglement of more than two photons with each other? One of the reasons I could see this not actually advancing QC is if what's needed for quantum radar is easy generation of entangled pairs, while QC requires something more difficult.

TLDR; University of Waterloo is investing $2.7M to speed up the process of photon entanglement. One photon will be beamed to the object and the radar response will be checked to see if it was an entangled photon that came back.

What really is horrible here is the asymmetry of cost- the development of stealth technology costed billions and took from drawinboard to reliable implemenetation close to half a century.

And it seems that detection is always easier and cheaper. I wonder wether radar and quantum "noise"-bombs that basically spam the enemy detection methods would be a cheaper approach.

"quantum noise bombs"... this wont work as the photons that hit the receiver wont be entangled with the one you sent out... This makes it potentially un-jammable in that sense.

Of course, overloading the receiver may be an attack worthy of investigating.

> What really is horrible here is the asymmetry of cost

That's the fate of most military technologies throughout history, why would stealth be different?

Spam the enemy with thousands of drones.

Quantum radar will expose quantum stealth of quantum aircraft.

I'm getting quantum sick.

While it is true that quantum is used a lot as a buzzword, here it is used in its literal scientific sense. And it is an exciting technology that has taken a lot of effort to be created that does not deserve such knee-jerk derisiveness.

It’s not at all clear this isn’t a buzzword boondoggle.

RF photonics is barely even a thing, and no one is going to get much joy from more comfortably photonic frequencies operating in the Earth’s atmosphere.

Besides, entangled states are very delicate and disappear after any environmental interaction. So I’m not entirely clear how you’re supposed to maintain an entangled state while bouncing photons off a large object.

I'd say that a technology that attempts to exploit entanglement can reasonably be called 'Quantum'. Now, whether the technology is feasible is a separate question from whether it is accurately named.

"Quantum" is a science-y sounding word which sensationalist scientists use to push their otherwise inexplicable "exciting" ideas for fundraising. It's a joke in itself. A parody of what physics actually is.

Even the actual scientist who formulated Quantum Electrodynamics didn't quite understand what quantum is and what to make of it, nor did he find a way to properly explain it. This term and the theories behind it were used just to explain some connections in phenomena, not to invent new ones! Quantum laser, quantum liquid, quantum crystals, quantum nose, quantum ass!

There was a post here somewhere, discussing that science isn't exactly science anymore when it very closely resembles its own stereotype. A philosopher for example today isn't someone who actually thinks, but rather a person who writes non-sense papers just for the sake to be published in a major journal, because otherwise they can't find a job. So, it's not the scientists who publish science-y papers who deserve credit. I don't know who deserves any credit anymore. Academia makes me sick to my stomach.

You are referring to Feynman, but you are misunderstanding his quote quite a bit. He belongs to the "shut up and calculate" camp of science, and tried to say that it is not worth it to try to build "intuition" about a phenomenon so remote from the scale at which humans experience the world.

Instead of relying on hearsay to reinforce your beliefs, I implore you to challenge them and read an intro science textbook on quantum mechanics. It is that theory of nature that permitted us to build the day-to-day electronics that powers our lives and our civilization, and it is empowering to understand it. "Theoretical Minimum" is a good set of free courses with video lectures for instance.

Or if you prefer you can just continue with your self-aggrandizing pity and willful ignorance. It is true that there is a rat race in the sciences, but that does not devalue the amazing advances that science has nontheless given us.

Edit: for the more advanced I would suggest reading the first few chapters of Sakurai's or Griffiths' Quantum Mechanics textbook - if you know math they explain how quantum mechanics works quite well. Or Aaronson'a Quantum Computing since Democritus if you are more interested in computational complexity.

Stealth aircraft haven't been stealth for a while. The Serbians managed to destroy one F117a (https://en.wikipedia.org/wiki/1999_F-117A_shootdown) and damage another.

This was because the Radar battery were using a different frequency.

You don't need a direct hit with a missile, just be close enough when you explode it.

Also, all the "shared battle information" is done via radio, which with passive radar will give wonderfully accurate positioning with cheap off the shelf hardware.

Not really. The opsec was poor; the aircraft had been using the same ingress/egress routes, so the SAM batteries knew where to look. That helps tremendously. Also, the F-117 has to open it's bomb bay to release its weapon. When it does so, it increases it's radar return dramatically, if only for a very short period of time. This helped the Serbians a lot.

And passive radar isn't a thing. There's passive radio sensors, that can theoretically detect a transmitter, but frequency hopping etc make that tough.

Passive radar is a thing, because I use it.

it works by using known radio sources (ie tv transmitters) which has the advantage of being powerful , continuous and reasonably unique signal pattern (for timing)

radio sniffing is where you listen for the aircraft directly, and there are a number of offerings on that front too. Phased array antenna with SDR front ends, are smashing bits of kit.

now frequency hopping spread spectrum means you need more DSP to find the signal, but its still there. In fact it provides nice timing signals because they hop frequency so much. There is more, but thats for another time

In theory the frequency hopping can be random, but that requires a shared random source, which is hard to sync.

That's not really radar though. Radar systems have a dedicated transmitter that broadcasts. Phased array radars have transmitters as well. What you're describing is more akin to ESM. Conflating the two definitions diminishes both. It's like when submarine movies depict sonar using a hydrophone without a transmitter; that's passive compared to real sonar that creates a ping.

Passive and active are both valuable in their own regimes, but they're not the same.

I'd note that the Serbians kinda lucked out and got it when it was at its least stealthy:

> According to Dani in a 2007 interview, his troops spotted the aircraft on radar when its bomb-bay doors opened, raising its radar signature.

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