A laser pointer at 2B FPS [video]

Tl:dw for how this works:

He scans one line at a time with a mirror into a photomultiplier tube which can detect single photon events. This is captured continually at 2MSample/s (2 billion times per second: 2B FPS) with an oscilloscope and a clever hack.

The laser is actually pulsing at 30KHz, and the oscilloscope capture is synchronized to the laser pulse.

So we consider each 30KHz pulse a single event in a single pixel (even though the mirror is rotating continuously). So he runs the experiment 30,000 times per second, each one recording a single pixel at 2B FPS for a few microseconds. Each pixel-sized video is then tiled into a cohesive image

Thanks for the explanation. Honestly, your explanation is better than the entire video. - I watched it in full and got really confused. I completely missed the part where he said the light is pulsing at 30kHZ and was really puzzled at how he is able to move the mirror so fast to cover the entire scene.

FWIW he explains it better in his earlier video about the original setup. He might be assuming people have seen that.

Good explanation. One detail though: it is one pixel at a time, not one line at a time. Basically does the whole sequence for one pixel, adjusts mirror to next one, and does it again. The explanation is around the 8 minutes mark.

Just want to make it clear that in any one instant, only one pixel is being recorded. The mirror moves continuously across a horizontal sweep and a certain arc of the mirror's sweep is localized to a pixel in the video encoding sequence. A new laser pulse is triggered when one pixel of arc has been swept, recording a whole new complete mirror bounce sequence for each pixel sequentially. He has an additional video explaining the timing / triggering / synchronization circuit in more depth: https://youtu.be/WLJuC0q84IQ

Yup, this technique also allows oscilloscope capture signal with frequency higher than their Nyquyst bandwidth.

The downside is it only works with repeative signal.

One piece I'd like to see more clarification on is, is he doing multiple samples per pixel (like with ray tracing?). For his 1280x720 resolution video, that's around 900k pixels, so at 30Khz, it would take around 30s to record one of these videos if he were to doing one sample per pixel. But in theory he could run this for much longer and get a less noisy image.

I find it interesting that a project like this would easily be a PhD paper, but nowadays Youtubers do it just for the fun of it.

And the reason it matters that this is a single pixel at two billion times per second is that we can hypothetically stack many of these assemblies on top of each other and get video of a single event that is not repeatable.

I believe this technique is known as "equivalent-time sampling".

The author uses "real time sampling" to acquire evolution of light intensity for one pixel at 2 GSps rate. The signal is collected for approximately one microsecond at each firing of the laser, and corresponding digital data is sent from the oscilloscope to the computer.

"Equivalent time sampling" is a different technique which involves sliding the sampling point across the signal to rebuild the complete picture over multiple repetitions of the signal.

https://www.tek.com/en/documents/application-note/real-time-...

The author explained that he originally attempted to pulse the laser at 30 KHz, but for the actual experiment used a slower rate of 3 KHz. The rate at which the digital data can be read out from the oscilloscope to the computer seems to be the main bottleneck limiting the throughput of the system.

Overall, recording one frame took approximately an hour.

You should check the other channel by the same person, where he goes into more details about the system: https://www.youtube.com/@BetaPhoenixChannel

From what I remember, recording one frame took about an hour.

What you've invented there is a camera sensor :) Silicon photomultipliers do exist and are used in some LIDAR applications. The bigger problem would be creating the 921600-channel oscilloscope to capture all this raw data.

It's humbling how well-rounded Brian (and other Youtubers such as Applied Science and StuffMadeHere, HuygensOptics) is on top of clearly being a skillful physicist: electronics, coding, manufacturing, ... and the guy is _young_ compared to the seasoned professionals I mentioned in the parentheses.

Yea, he’s recording several thousand samples per pixel. That’s how it becomes a video instead of a snapshot.

Check out his previous video <https://www.youtube.com/watch?v=IaXdSGkh8Ww> for more details about that part.

I think parent meant that the image construction technique is analogous to equivalent time sampling. You’re correct in the mode of the oscilloscope’s use. However, the mode of the larger system is using a repetitive signal and sliding sampling points across it.

Can we not find why the strange behavior of a double split experiment occurs using this setup?

No, all the light seen as in "beams" is scattered off fog. That scattering is a measurement from the perspective of QM.

Huh. I watched a lot, but not all, of the video, and I thought he made it clear early on that he was stitching together 1px videos & repeating the event for each pixel (about a million times for that 720p result)

Rare enough though that the Fs/2 is higher than the analogue bandwidth on an oscilloscope.

In one of the appendix videos he mentions that would improve the noise, the issue is the data rate exporting from the scope is a bottleneck and it would slow things down even more.

Yes and no. The mirror is continually scanning horizontally because the mechanics simply can't position that accurately. This is fine, really because the amount the mirror moves in 33uS is essentially zero. He's still capturing single pixels, just while the mirror is moving. Any artifacts from the motion are probably entirely drowned out by the noise in the rest of the system

We already know why and the double slit experiment has been by thousands if not millions of students and researchers.

But as sibling said, this is still a measurement and will collapse the quantum system. You can't use this to peek under the hood and look at the quantum mechanics.

Mainstream oscilloscopes typically have sampling frequency at least five times greater than the bandwidth of the analog front-end. For example 1 GHz bandwidth oscilloscope will have sample rate of 5 GSps.

https://www.tek.com/en/products/oscilloscopes https://www.keysight.com/us/en/catalog/key-34771/infiniivisi...

"Sampling" oscilloscopes are a much less common product -- they are useful for analyzing signals that are too fast to digitize in the ordinary way. They typically sample at a very slow repetition rate -- some hundreds of kilohertz, but each sampling aperture can be exceptionally short, allowing to record signals to 100 GHz frequency.

The BW of analog front end is where the amplitude drops in 3db. So if all you care is whether the signal is present and what is the frequency of it, you may get away with it.

Something is off here, 2MSamples/sec wouldn't be 2 billion times per second, that would be 2GSamples/sec

If he randomized the position with blue noise or something he could use compressed sensing or ai denoising for many less samples. The raw image wouldn't be as good but by the time it is compressed it should much better for the same sample count. It might not be as easy to move the mirror in both axes between each sample though.

edit: saw below he is using a continuous scan so randomizing it probably wouldn't be workable