Quantum Radar Hides In Plain Sight

Radar was a great invention that made air travel much safer and weather prediction more accurate, indeed it is even credited with winning the Battle of Britain. However, it carries a little problem with it during times of war. Painting a target with radar (or even sonar) is equivalent to standing up and wildly waving a red flag in front of your enemy, which is why for example submarines often run silent and only listen, or why fighter aircraft often rely on guidance from another aircraft. However, researchers in Italy, the UK, the US, and Austria have built a proof-of-concept radar that is very difficult to detect which relies upon quantum entanglement.

Despite quantum physics being hard to follow, the concept for the radar is pretty easy to understand. First, they generate an entangled pair of microwave photons, a task they perform with a Josephson phase converter. Then they store an “idle” photon while sending the “signal” photon out into the world. Detecting a single photon coming back is prone to noise, but in this case detecting the signal photon disturbs the idle photon and is reasonably easy to detect. It is likely that the entanglement will no longer be intact by the time of the return, but the correlation between the two photons remains detectable.

Of course, in reality they don’t use just a single photon. The process — known as quantum illumination — uses the weakened correlation and sampling to beat the performance of other techniques using the same number of photons and bandwidth.

Other than the quantum part, the rest of the setup is pretty simple to understand. The signal frequency is a little more than 10 GHz, while the idler is at almost 7 GHz. The receiver down converts to an intermediate frequency of 20 MHz and then uses a 100 MHz 8-bit A/D to grab the signal and do a Fourier transform.

Most of what we see involving quantum physics involves computing. We are still hoping [Sean] will work out his quantum coffee pot.

38 thoughts on “Quantum Radar Hides In Plain Sight

  1. That’s a pretty great idea. So it basically uses way less photons than traditional radar, but can still achieve an image through the noise because of quantum entanglement effects? Whereas the enemy wouldn’t have the entangled particle as a reference and thus the radar ping would be below SNR to reliably locate the source? This is what I’m picturing, wondering if that’s wrong.

    Does anyone know if there’s an up-to-date list anywhere of practical devices which use quantum mechanics and superconductivity? I feel like that would be a great wikipedia trap or something to read.

  2. Big news. There were claims that the Chinese had built ond of these and that it could detect underground structures. Seems more plausible now.

    If I understood it correctly, this lets a higher frequency ‘interrogation’ signal get carried by a lower frequency, deeper penetrating illumination signal. It’s a breakthrough technology.

    I speculate, it could perhaps be used to detect the chemical composition of asteroids…

    1. Sorry, but whatever Quantum buzz word you’re adding, you’re still limited by Shannon’s theorem. If you want the bandwidth of the signal to be recovered, you still need to send (obviously) and sample at, at least, twice the frequency.
      So, any detector will see the HF coming in.

      That being said, the scheme is called AM, as far as I understand (just reversed).

      1. Leaving aside any quantum tricks, there are methods to communicate using signals lower than the noise floor. Synchronous detection as the “lock-in amplifier” is an example. Such techniques does not contradict Nyquist sampling rate (sample at least at double the bandwidth) and does not contradict the Shannon limit regarding the maximum bandwidth of a channel in the presence of noise.

        Same methods of detecting own signals buried under the noise floor can be used for radar applications. Indeed, the target will receive some signal coming from the radar, but the signal will be so faint that the target has no way to distinguish it from the background noise.

        At the bottom line, the target will hear some usual RF noise, but there is no way to tell if that’s only noise or noise+radar.

        For the radar operator side, the problem with this method is it can not use too much power, so the detection range is small.

        If entanglement is added on top of that radar signal, the detection range is increased even more (without the target being able to tell if that photon is from the radar or it’s just background noise).

        The technique of using entanglement to increase the noise ratio of own signals is called Quantum Illumination. It really works, and it was tested experimentally many times over the years.

        1. I was thinking at one time an array of lock-in amplifiers to cover a wider bandwidth more pertinent to the situations in case the signal frequency shifts due to propagation would be useful.

          I have to read into Quantum Entanglement and Quantum Illumination more as I’m not able to visualize yet.

  3. “Whereas the enemy wouldn’t have the entangled particle as a reference and thus the radar ping would be below SNR to reliably locate the source?”

    It seems to work how you picture it, but not as well as you describe.

    Because the signal you send still has to be much stronger than the reflection you get back, things you can barely detect with your increased SNR may still get a signal that’s above their SNR. What changes is things twice that far away might not be able to detect you now.

    1. Or you could get the same detection range with less power = less chance of being detected by the target.
      Keep in mind that nowadays almost all* military radars use some form of phased array antenna, which can wave the beam around stupidly fast . This drastically cuts the dwell time, lowering the amount of energy you send towards the target. Reduce the energy even further and it becomes even harder to detect.

      It makes a big difference in a BVR dogfight if you have even just an extra 20-30km detection range before the target even notices you.

      *almost…I’m looking at you, Eurofighter…

    2. Thanks. I guess it depends on just how much effective gain you get from the quantum entanglement effects; I suppose the info available so far doesn’t go into such specific figures, but if it’s strong enough then maybe both the outgoing and incoming signals would be too faint to detect without access to the entangled particles. That’s just speculation on my part, of course.

  4. “The quantum stuff is pretty comlpicated, but the radar is, like, radar. Then: profit” — The path length of the signal seems to be on the order of the stored signal, so this will be interesting to put in a 100km device. Also, quantum.

        1. The experiment was just a proof of concept device. For a commercial or military application, optical photons can be used instead of microwave photons, so the delay line can be easily implemented with a spool of optical fiber.

          Another method could be to entangle the microwave photon with an optical photon to be put in the optical delay line. Not sure how hard would be to do that in practice, but in theory it should work.

          1. Hm. Not sure if that’s how it works. Can you entangle particles with such vastly different energies and characteristics?

            And I don’t think optical photons can be used in either commercial or military radar—certainly not at 100km ranges. The atmosphere tends to absorb those rather quickly. Such a thing would be called lidar or similar, and it’s used for very different ranges.

        1. Because you’re measuring it.
          When the signal photon returns it interacts w/ it’s paired ‘idle’ photon in a predictable way. Counterintuitively, the signal of the signal photon
          off of your radar target isn’t what you’re measuring; it’s interaction with the idle photon is.

        2. Because the return signal is very weak. You receive something, but you don’t know if that’s an echo from the target, or it is just noise from outer space.

          However, if you keep listen for long enough, you will start notice there is some correlation between what you sent and “the noise” you receive. If it were to be only random noise you won’t see those statistical correlations you are seeing when the photons are entangled, therefore you can figure out if what you are receiving is just noise, or is it an echo of your own signal.

          If you throw away the other signal, you’ll have nothing to compare with, so you can not tell if what you receive is noise or is it the echo from a target.

    1. Nah, still no information may be transmitted faster than the speed of light through vacuum. Doing so would always always defeat causality—even with quantum magick—which as far as we know isn’t on the menu.

  5. I too figured it was ‘simple’; stick to observing the remaining particle.
    .oO(On the other hand, observing it will affect both it and the signal particle – doesn’t that give the game away?)
    *Arghh!*

  6. This would be great for getting fast response times in space. You don’t have to wait for the reflection to come back. It’ll see objects previously invisible to radar if you use the time of flight by varying the phase angle rather than just did it respond.

    1. You could trap them and sort them by phase, relying on shear weight of numbers to overcome loss.
      Or you could build a nanostructured material like a forest of nonotubes and balls on an etched substrate capacitor with a grating over it. Then you sit that on top of something akin to a ccd.
      It’ll act like a 2D tesla coil array that responds to phase. The varying charge read off by the ccd as an image. Depending on the mask you could potentially determine, speed distance and size all at the same time.

    2. Getting the reflected photons back is a key element, and can not be skipped no matter what. Without measuring the reflected photons, no conclusion can be drawn.

      Other said, there is no way to transmit (back) any information faster than light, here the information we want being if the target was hit or not by our photons.

      1. Nope. They are entangled. So you just measure the reference photon and let the other keep going until it hits something, no matter how far away it goes. [ especially in space where there’s less stuff to hit ] Then you do an average duration of variability on the state to determine the object passively.

        1. Also, the effect is due to the fact we have lag in our perception. The combination of standing waves that form the photon have done multiple passes of those point in time. When you “observe” a photon it is absorbed and if remitted, is actually a copy formed from a different set of standing waves, therefore doesn’t then continue to effect the figure eight style loop that formed the origin pair, both events are solidified by printing on secondary standing waves. The lag in our perception means we never see the proverbial wave front. It’s concurrent superimposed iterations. Think parallel load shift register with a separate output enable.

        2. Once again—where are y’all getting this idea that it’s a one-way trip for the radar signal? The article very clearly states there’s a reflected signal that must be collected. Surely that isn’t done just for funsies.

          And citation needed on quantum entanglement transmitting usable information faster than the speed of light in a vacuum. Pretty sure the no-communication theorem eliminates that and preserves causality. I think this is one of the larger areas of misconception in QM, and I admittedly might be the mistaken one here so I’d like to see a more credible source on that somewhere.

  7. isnt everyone still having issues with entangled pairs simply disentangling for no reason we understand yet? part of why we can’t get enough functional qubits to make a true quantum computer thats faster at a specific purpose than its traditional silicon counterpart? for the radar does each “transmission’ sent out need to have new quantum pairs generated or is the assumption you’ll get the “sent” one back? does that even apply? quantum stuff is still too spooky for me :(

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