Army researchers detect broadest frequencies ever with quantum receiver (2021) | Hacker News Reader

Army researchers detect broadest frequencies ever with quantum receiver (2021)

army.mil

56 points1970-01-0137 comments

0Hz is definitionally the lower limit of information-carrying frequencies (presumably this is not actually capable of detecting anything between zero and 1Hz - not that there’s much of interest in being broadcast in the micro-, nano-, or pico-hertz range anyway), and 20GHz is well into the realm of “millimeter-wave” broadcast frequencies. That’s ten orders of magnitude, and you only have about 1 order of magnitude more before you start seeing complete atmospheric attenuation on the scale of meters. So their claim that this is a genuinely general-purpose receiver is pretty much true, surprising as that may be.

For $20k you can have one, too, 100 kHZ - 20 GHz. [1]

[1] https://signalhound.com/products/sm200c-20-ghz-real-time-spe...

I'm curious: Aren't these kind of devices illegal in most jurisdictions? Who uses them anyway?

Of course not. Anybody doing serious RF engineering has a spectrum analyzer or access to one. For that kind of work a spectrum analyzer is more desirable than an oscilloscope.

Receivers, probably not; broadcasters may get the FCC breathing down your neck if you take the piss though. And there's probably specific laws about breaking encryption, e.g. law enforcement communications.

RF receivers were restricted in some jurisdictions. However, a 30€ SDR stick can receive pretty much any unencrypted voice transmission which makes these rules impossible to enforce.

Interesting they have a 10Gb interface with their own proprietary UDP SAR encapsulation. Pseudowires was a way to mux payloads like this onto the network but make a lot more sense for them to do it especially at that framerate. It could give you a pretty wide sample zone if you had a few of these and a lot of fiber.

Does 0Hz wave even fit inside our universe?

Technically it has infinite wavelength.

Universe is infinite so technically it fits.

0Hz is a constant value -- 0Hz means the electromagnetic field does not oscillate. The lowest frequency concievable would be something like ~2.2978 10E−18 Hz -- one period every 13.8 billion years.

[0]: https://en.wikipedia.org/wiki/Orders_of_magnitude_(frequency...

In a way the expansion of the universe is one half of a giant wave.

can a 0Hz wave move through space? or is it just a static field..

You can think of it as the "static" component of a variable field. Frequencies above 0 Hz oscillate, and the 0 Hz component keeps a static value.

I'd love to know the DC offset of the universe

What we've got here is a lot of marketing fluff around a Rydberg spectrum analyzer.[1][1a]. This is essentially an RF mixer[2] which, according to [1a]:

  rivals the performance of commercially-available spectrum analyzers with high sensitivity, 4 MHz instantaneous bandwidth, and over 80 dB of linear dynamic range.

While 80 dB of linear dynamic range is good, there are other designs, such as the Tayloe mixer[3][4] that are more sensitive and have a better dynamic range due to their inherently linear nature.

Careful reading of the linked paper shows that their best sensitivity measurements rely on conventional external low noise amplifiers. So, this is mostly grant writing driven marketing.

We've been getting signal detection this good with tube based WW2 and later ham gear my friend repairs. It's not uncommon to detect 0.1 μV signals, which is -127 dBm. These receivers have approximately 2khz of bandwidth

     -127 dBm - 10*Log(2000) --> -160dBm/Hz.

So, lots of interesting tangents, but nothing world changing as of now.

[1] https://arxiv.org/abs/2009.14383

[1a PDF] https://arxiv.org/pdf/2009.14383.pdf

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

[3 pdf] https://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf

[4] https://pa3fwm.nl/technotes/tn17c.html

One principal advantage is that the laser-cooled atoms are unaffected by Johnson noise which sets the ultimate limit in semiconductor (or vacuum tube) based systems. However, their stated sensitivity of -120 dBm/Hz is more than 50 dB above the Johnson noise at room temperature. They have a lot of work to do until this becomes relevant.

The Tayloe et.al. look a lot like Dr Sutton's work for NASA on his Synchronous Demodulator.

"Improved Analog Synchronous Demodulator: Output ripple is suppressed without an output filter" GSC-13179, NASA Tech Briefs, March 1992. Uses dual op-amp and 4053 CMOS switches.

"Digital Synchronous Demodulator: The digital version offers greater speed, precision, and reliability." GSC-13273.

Here Dr Sutton corrects me for mixing two of his unrelated NASA projects:

"Hi Bob,

The synchronous detectors were used in temperature monitors and temperature controllers designed to control temperatures on spacecraft at 60 milliKelvin +/- a few ucroKelvin. The preamplifier had to have a gain of 10E5 after which the demodulated signal had to be converted by a 16 bit ADC, with +/- 1LSB allowable error.... so of course, you can see that we were working with extremely small signals buried in the noise, and we had to go all out in an effort to beat down the noise. That's why we had to use a new improved synchronous demodulator. This project was as close to being impossible as you can get! I still have trouble believing that we actually made it work.

The active ("Black Hole") antenna was developed in another project, where we didn't want to transport a two meter long antenna that weighed 200 pounds.....so we miniaturized the hardware while simultaneously expanding the antenna field cross section. We wanted to receive the entire ELF-VLF bands all at once, so we had to have an extremely broadband antenna....like four decades of bandwidth or more. You wouldn't believe the arguments I had with the reviewer at Physics Essays. He just couldn't believe that one could do what we did....and if it was indeed true, then why hadn't someone done it years ago?.., "and what makes you so smart", .so, of course, "this must be nonsense, etc....." Progress in physics is so bloody difficult because most physicists think that everything worthwhile has already been discovered....so they expect nothing new. This is negative feedback which, of course, makes the system stable, I suppose." - John Sutton, Ph.D.

https://web.archive.org/web/20101201225337/http://unusualres...

There are some strong similarities, that's for sure. The phasing is wired completely different between the two, leading to different behavior.

It looks like Suttons work is a new variation of the chopped amplifier, a classic scheme for canceling out DC bias in instrumentation amplifiers.

"...marketing fluff around a Rydberg spectrum analyzer..."

How about the price point?

The paper speaks of DC to 1 THz. Any good spectrum analyzer I know of for anything close to that range, is going to have a lot of zeros in the price tag that puts it out of the budget of just about everyone.

Even covering all of 5G or newer automotive RADAR at 77 GHz is out of most budgets.

Marketing is not always about retail sales. Its a necessary evil ever since the Golden Fleece awards were used to neuter basic research.

Grant proposals are more about a marketing push than science these days. The article and associated paper are really just fluff so that they can keep doing research in spite of incompetent oversight.

We are slowly approaching state of affairs described by Lem in his Fiasco novel. I enjoy returning to this book to squeeze some new ideas from it every now and then. Highly recommended, even if you are not into Lem's style or sci-fi in general.

This looks interesting.

Will it fit in a backpack ?

Wonder if it could help with things like astronomy or planet to planet communication. If the receiver is quite good, that could help a great deal.

I hope this will open the possiblity of handheld Very-Low Frequency communication devices

Imagine being able to text someone directly, antenna to antenna, over distances of hundreds of KM

The bandwidth wouldn't be great, but it would be nice to send a simple text SMS to someone without any relays in between

VLF would also go through rock and water? Could be useful for spelunking (and submarine comms). Depends on if they can transmit without a huge antenna as well.

Could the rock not be the antenna? Distract the sandworm style

"...without huge antenna..."

That would be reinventing James Harris Rogers WW-I system.

T.E.Bearden claimed that this system has been rediscovered and "lost" several times.

http://www.rexresearch.com/rogers/1rogers.htm

> Impossible to read Arlington on the elevated antenna on account of static interference. United Fruit Co station of New Orleans interfered with signals from Arlington on main antenna, but offered no interference on underground antenna... [Missing Text ]

that's an interesting footnote...

There is some obscure evidence that submarine communications are being carried out by what can be considered Extended Electrodynamics.

Check out the list of Curl-Free Magnetic Vector Potential patents by Raymond C. Gelinas assigned to Honeywell.

That's already possible with a high sensitivity magnetometer.

It looks like a standard atomic physics setup with a vacuum system and a bunch of lasers. Definitely not built to be portable.

The performance is quite a bit worse than conventional semiconductor based RF spectrum analyzers which can be carried around. Maybe they have some ideas how to improve it in the future. Currently it is clearly just a proof of concept that shows that Rydberg atoms can be made sensitive to broadband RF at all.

Can someone eli5? How would this be used, as rx or tx or both? And when they talk about spread spectrum are they talking about detecting an enemies use of it, or better use by themselves, or both? What might be the advantages over conventional techniques?

Seems like a way to directly sense the prescence of electromagnetic fields without first confining them to a wire with an antenna.

Practically speaking, I guess antennas are shaped to work best in a specific range, so now you don’t need multiple antennas? Also there are some cases where just the presence of an antenna would be counterproductive, or simply infeasible, for instance measuring electric fields in a waveguide. Here are some examples from another article:

> Furthermore, the use of small vapor cells allows spatially-localized observations of electric field strengths such as, for example, at points within a microwave wave guide without perturbing the local field as would introduction of an antenna. Rydberg atom measurements can also be directly related to known physical constants and can therefore provide accurate absolute electric field values that can be used to calibrate more conventional antenna-receiver combinations.

https://www.openaccessgovernment.org/rydberg-atoms-quantum-e...

No, they use a conventional antenna, hence the title "Waveguide-Coupled Rydberg Spectrum Analyzer". The Rydberg atoms essentially serve as an active frequency mixer that produces the difference frequency between the RF input and a local oscillator. This is how conventional spectrum analyzers work, except that their mixers are based on semiconductors.

>4-MHz instantaneous bandwidth

Ah, I somehow thought it had the instantaneous bandwidth over the entire window. That would be insane.

That's exactly how I read the title as well.

Is there something that talks about this without the ad copy tone?

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