The Short Weird Life -- and Potential Afterlife -- of Quantum Radar (sciencemag.org) 66
sciencehabit writes: A mini-arms race is unfolding in the supposed field of quantum radar, spurred by press reports in 2016 that China had built one -- potentially threatening the ability of stealthy military aircraft to hide from conventional radars. Governments around the world have tasked physicists to look into the idea. Whereas a conventional radar searches for objects by detecting pulse of microwaves reflected from them, quantum radar would utilize pulses of microwaves linked by a quantum connection called entanglement. The system would retain one pulse and measure it in concert with the one reflected from the object. Correlations between the two would make it easier to spot an object through the glare of the surroundings. Or so researchers hoped. Groups have demonstrated elements of a quantum radar, but only in limited experiments that a nonquantum system can still match. And fundamental physical limits suggest the scheme can't beat ordinary radar for long-range detection. Even one of the inventors of the basic concept thinks it won't work when applied to radar.
Aren't radars already quantum? (Score:2)
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Why are you saying that they're "coherent"? They're in phase with the same polarization but I can't see how you can call them coherent.
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They're in phase with the same polarization
Isn't that the definition of coherence?
Actually it is the definition of "coherence" (Score:3)
When I look at: https://en.wikipedia.org/wiki/... [wikipedia.org]
It seems we're talking about exactly the same thing. Interesting because when I did my course on radar in university, the term "coherence" wasn't used (I suspect that polarization of the radio signal is what messed up my understanding).
You learn something new every day.
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Coherence means not only polarized (actually, no idea if it does), but that the waves are synchronized, like marching soldiers. Hill and valley of the waves are at the same spot all the time.
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Re:Aren't radars already quantum? (Score:5, Funny)
Re: Aren't radars already quantum? (Score:2)
Since all the outgoing photons are coherent...
Not unless it's a maser.
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So my original question remains: a matter of degree, or of kind?
Obligatory Ant Man and the Wasp Quote (Score:5, Funny)
"Do you guys just put the word 'quantum' in front of everything?"
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Well, to be fair, those of us in the know see the word 'quantum' as a replacement for the words 'snake oil'.
Re: Obligatory Ant Man and the Wasp Quote (Score:2)
"Well, to be fair, those of us in the know see the word 'quantum' as a replacement for the words 'snake oil'."
My thoughts upon seeing "Quantum radar" were that these guys are kooks and they have been doing too many drugs.
Seriously and 100% honest, that was the very first thought that came to mind.
Re:Obligatory Ant Man and the Wasp Quote (Score:4, Insightful)
Well, "Blockchain Radar" sounded so unwieldy and they had to use something to sell their idea so they settled for the next best thing.
Does it stack? (Score:3)
Does it stack? Is "Quantum Blockchain Radar" even cooler?
Re: Does it stack? (Score:2)
No but if you can make a Machine Learning AI-powered Quantum Blockchain Radar and pay for with an open source cryptocurrency you will be a zillionaire. To make dead sure put an i before the name and X2000 after it. Or is becoming a zillionaire not cool anymore? I forget.
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Everything is better with 'quantum' in front of it !
Quantum doughnuts - are they really doughnuts or a meringue
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Yup, just like hot sauce, we put that shit on everything.
Does nothing against stealth, helps with jamming (Score:5, Informative)
Stealth works by reflecting incoming photons away the transmitter, entangled or not. If you do not detect any photons, you don't get measure entanglement. What helps against stealth is multi-static radars, where the transmitter is not colocated with the receiver.
What quantum entanglement helps against is jamming.
It does help detecting stealth aircraft (Score:5, Informative)
Background noise is as easy to filtered out as jam signal.
Defining feature of stealth aircraft is it reflects little radio wave towards the direction of originating radar. Whatever photon that get reflected back are greatly overwhelmed by background noise. Stealth aircraft thus escape detection.
With quantum radar, the radar *KNOWS* which photon comes from the station, by correlating with the reference photon. Bckgnd noise photon are probabilistically unlikely to correctly complement the reference photon. It is trivial to filter out the bckgnd noise in this scheme. With a noise floor so low, whatever low power return will stick out.
This super high S/N ratio allows another interesting feature of quantum radar: the source radar can be very dim, below bckgnd noise level. To the stealth aircraft, it would not know it has been scanned because the scanning beam is mask by bckgnd.
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Re:It does help detecting stealth aircraft (Score:5, Informative)
Most proposed implementation "trapped" by using delay element; i.e. kilometers long loop of fiber optics etc.
Incoming photon with correct expected arrival time are matched with the corresponding reference photon in the delay pipeline. If the quantum value are complementary, the photon is registered as a data point. Any decoherent return photon are treated the same as bckgnd noise(discarded).
You don't need a bright return of complimentary photons. With most noise eliminated, an image can be built up gradually with individual matching photons anyways.
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But wouldn’t the length of delay fibre correlate to a fixed distance requiring hundreds of delay loops of different lengths meaning you build up discrete concentric rings. Sounds unwieldy.
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Perhaps more importantly, how can you have an expected arrival time for a radar return? The whole point of radar is that you don't know if there's anything there. There can be no expected arrival time unless you already see the target. Moreover, it's not about filtering out noise, that can be done with standard computing. When an entangled photon interacts with a plane, it's compliment reflects that. That's what you
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The reflected photon, "reflects" its momentum changes to its entangled brother: that you can measure.
Seriously learn at least the most basic things about entanglement before posting such nonsense.
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Teensy problem - when do you measure it? Because once you have thats it, that photon is gone and too bad if it hasn't yet hit the target.
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Teensy problem - when do you measure it? Because once you have thats it, that photon is gone and too bad if it hasn't yet hit the target.
Photons are easy to generate; wasting the vast majority of them probably doesn't matter.
Assuming that the physical principle works, you're still left with a tricky problem in system design, including fitting it into your air defense systems. For example stealth aircraft can be detected using long wave radar, but those radars are bulky and don't give precise enough fixes for missile guidance. But long wave radar was reported to have played a part in the shootdown of a US F-117 Nighthawk; that took consid
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You see - not the plane it self - but its tail wind quite easy on weather radar.
Again: not enough to guide a missile, but enough to know where it is.
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The reflected photon, "reflects" its momentum changes to its entangled brother: that you can measure.
Seriously learn at least the most basic things about entanglement before posting such nonsense.
Wow, the irony, it's like physically palpable. No, that's most definitely not how entanglement works. There's no "reflection" of momentum: the change in momentum in the emitted photon doesn't change the state of the kept photon at all, because, again, that's just not how entanglement works. In fact you cannot use entanglement to carry information at all.
What you can do is use frequency correlation between returning and kept photons to help identify what photons are signal, and which are background noise, t
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Sorry, I wrote wrong work, it was not meant to be momentum but polarization.
In fact you cannot use entanglement to carry information at all.
Yes, you can. That is actually how that radar works, facepalm.
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Seems to me that in order to work, you'd need a conical tube as long as the range of the radar. The inner walls of the tube need to be detectors for your photons. Down this tube you send your local half of the entangled pairs (you'll need about a gazillion), and the gradually narrowing walls give continuous feedback so that you have a shot in hell of having a photon hit the target when it's compliment hits a detec
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They use glas fibres for "real photons", no idea what they use for radar/microwave.
But stuff like this is done since decades, just google it.
Re:Does nothing against stealth, helps with jammin (Score:4, Interesting)
A conventional radar works by detecting the handful of photons reflected back toward the source from a distant object. To the limited extent that I understand it, a quantum radar would work by detecting the change of state when an outgoing photon is reflected or absorbed at that selfsame object. That detection is done by observing some sort of state change in the photon's entangled partner which has somehow (how?) been kept around for observation. What, if anything, happens to the no longer entangled(?) photon pair after one of them strikes the target is of no interest?
Will we have quantum radars in the year 2021 or 2025 or 2030? Seem unlikely. In the year 2120? Maybe. Assuming they can actually work in theory, and can have a usable signal to noise ratio in real world situations, and can't somehow be jammed.
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That detection is done by observing some sort of state change in the photon's entangled partner which has somehow (how?) been kept around for observation.
This is a common misconception. Measuring the preserved photon alone tells you nothing about the remote photon. It's only when you measure both photons that you see that the correlation between the value pairs is different than if the photons were not entangled.
The article makes it clear you would need to measure the reflected photon.
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>Measuring the preserved photon alone tells you nothing about the remote photon.
If they are entangled then your understanding of the probability distribution from which the measurement of the remote particle draws is changed.
It doesn't transmit information, which may be what you mean. I guess if the attack plane kind chose to measure it for you and send back the result over a radio, it could work.
I see this the same as I see quantum computers - the decoherence will always get you. Time, distance and temp
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Stealth works by reflecting incoming photons away the transmitter
Reflecting or absorbing.
OK.. So they are being reflected away, or absorbed and radiated away as something unlike the energy absorbed.. The end result is the same, they don't go back to the source.
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Stealth works by reflecting incoming photons away the transmitter, entangled or not. If you do not detect any photons, you don't get measure entanglement. What helps against stealth is multi-static radars, where the transmitter is not colocated with the receiver.
What quantum entanglement helps against is jamming.
Stealth is, as you point out, not a universal property in all directions, but a characteristic of an object that changes it's observability from far away. This is an easy task when the illumination and observer are co-located, because you can imply the location of the observer by observing the source. This means, that stealth is defeated by separating the illumination source and the observer.
The easiest way to observer a stealth aircraft is to observe, not the return from the target, but the shadow it cas
How to parse this? (Score:3)
> potentially threatening the ability of stealthy military aircraft to hide from conventional radars
Why would it? It would still be hidden from conventional radars, even if it became visible to the newfangled quantum radars. Or somehow a dose of quantum radar exposure makes the hull of the plane turn permanently visible for the conventional radars?
Re:How to parse this? (Score:5, Funny)
It's just how quantum physics work. Old radars and new radars are entangled so if you measure a plane with new radar the old radar's measurement changes. It's weird but everything about quantum physics is.
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Stealth aircraft were defeated in the 1990s when British scientists realized that only the front profile was "stealthed" (and to a much lesser degree side profile) so detection from other angles would work fine. They analyzed signals from a metropolitan cellphone network and successfully tracked multiple stealth aircraft. It was suspected that this was how the US F-117 was shot down in Bosnia in 1999.
Of course you can't shoot the cash cow, so most of those researchers are probably working for Lockheed now
If it is quantum is has to be good (Score:2)
I can attest to that as I am an expert in Quantum Thermostatics.
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Looks like the sarcasm was too subtle. Anyway, Quantum Thermostatics is my favourite crackpot reserch subject.
By the way, have you heard of the "N-particle model" ?
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In other words, your sarcasm was not overly subtle, it was superpositioned because I was too lazy to collapse it by googling the term.
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I can attest to that as I am an expert in Quantum Thermostatics.
I agree, not because I know you, but because you stayed at a holiday inn express last night...
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I can't belive it ! How did you know I stayed at a Holiday Inn Express last night ?
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I can attest to that as I am an expert in Quantum Thermostatics.
So you build fridges?
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Quantum Radar (Score:4, Funny)
Quantum Radar, I can just imagine the feature list for this. The object is not where you observed it, but if it is, then it is no longer the same object it was, but a bowl of petunias.
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Oh, no, not again . . .
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Quantum Radar, I can just imagine the feature list for this. The object is not where you observed it, but if it is, then it is no longer the same object it was, but a bowl of petunias.
Damn, I thought the cat was in the box...
USA already has one then, right? (Score:2)
... else where did China copy theirs from? Americans reliably inform everyone that China can only copy and is unable to produce anything original.