How to Build Your Own Quantum Entanglement Experiment (2013) | Hacker News Reader

How to Build Your Own Quantum Entanglement Experiment (2013)

blogs.scientificamerican.com

153 pointsabrax314137 comments

I just played this online Qubit game that I thought I would put down after a few minutes. Ended up playing for 50 min to finish and actually got a much better sense of how quantum computers work than before.

https://quantumai.google/education/thequbitgame

Relatedly, I recently discovered https://algassert.com/quirk — it does require a bit of quantum computing knowledge to use, but the UI is excellent, and I found it incredibly useful for ‘getting a feel’ of how various quantum algorithms work.

This website is incredibly helpful for quick simulation. My favorite design detail is that the circuits are stored in the url, allowing easy bookmarking or sharing of some specific circuit.

Thanks for suggesting this game. I can definitely see Townsend's influence, though I prefer his A Dark Room .

How did you make headway with the research project subsystem ?

I've tried submitting just algorithm responses with lots of valid samples, but that doesn't seem to correlate with success. The svelte js for the game is minified, so I couldn't tell what it tested to choose success or failure. Thanks

Much happier game than the time I destroyed the universe with paperclip manufacturing.

This is really fascinating. It would be cool if we had an online group or forum that attempted to brainstorm the simplest/cheapest/(safest?) ways of performing interesting experiments like this that you wouldn’t think you could do at home. I’ve read about a few experiments for determine the speed of light using a microwave that I thought were pretty cool.

There's a neat book called "Exploring Quantum Physics through Hands-On Projects" that is well worth a look, for those so inclined.

https://www.amazon.com/Exploring-Quantum-Physics-through-Pro...

https://www.youtube.com/watch?v=ejKCiBZwB34

determine the speed of light using a microwave and marshmallows :)

I'd like to know whether the frequency printed on the microwave was calculated using the speed of light, in which case you'd just be reversing someone else's calculation rather than making an independent measurement of the speed of light.

In an optics class I took in college one day the professor picked up a ruler and said "Let's measure the speed of light". The class laughed, and then he proceeded to measure the speed of light with his ruler.

It was a metal ruler with the distance markers raised. He put the ruler on a table, reflected a laser off the ruler onto the blackboard and got a diffraction pattern, marked the positions of the lines of the diffraction pattern with some chalk, marked the position of the ruler on the table, and then turned off the laser.

Then using the ruler he measured the distance from the mark on the table to the blackboard, and measured the spacing of the lines in the diffraction pattern. It was then a short bit of math later and he had the speed of light.

Of course he had to use the frequency of the laser in that calculation, and the frequency of the laser was almost certainly calculated using the speed of light so in all likelihood all his demonstration actually did was verify that the frequency of the laser was really what the manufacturer said it was.

That then raises the question of whether there is a way we can determine the frequency of visible light without knowing the speed of light.

We could do it for radio probably fairly easily by a method I'll describe below but I don't know if that could be pushed all the way to visible light. On the other radio might be good enough. A 2.4 GHz radio wave has a wavelength of around 12.5 cm. I'd guess that it would be practical to build something that could make a diffraction pattern with that and let you then calculate the speed of the wave just like my professor did, albeit you'd need a lot more room than we had in the lecture hall.

Here's how I'd go about trying to get a radio wave around 2.4 GHz without knowing the speed of light or using anything that depends on knowing the speed of light.

First, I'd build two variable electronic oscillators and adjust them to match the frequency of the D right above middle C on my piano. That's 293.665 HZ.

Then I'd tune one of the oscillators up until it is twice the frequency of the other. You can build circuits that can detect when one oscillator is twice the frequency of another. You can even use those to build feedback circuits to adjust the tuning of the second oscillator to keep it exactly twice the first if either drifts a little due to things such as temperature changed.

Then tune the first to be twice the second. Keep doing this. If you can do this enough to get 23 doublings you'll end up at 2.46 GHz.

You probably can't actually do that with the original two oscillators, at least if you start off with simple EE 101 circuits. You might have to build more sophisticated oscillators along the way.

Actually, it might be best to build 23 oscillators instead of swapping, so you can have the complete chain of frequency doublings available, with each oscillator locked to twice the frequency of a neighbor by a feedback circuit.

To extend this all the way to visible light you'd need 37 to 44 doublings, depending on which piano note you use as your reference.

37 halvings brings visible light down to 2900-5800 Hz. The notes F#7-C8, 2959.96-4186.01 Hz, (82-88 on the standard 88 key piano) are in that range. You can halve visible light up to 44 times and remain in piano range. 44 halvings bring you to 23-45 Hz. The notes A0-F1, 27.5-43, 6535 HZ, (keys 1-9) are in that.

I don't think we have nearly as much flexibility with light as we do with radio when it comes to making variable oscillators and with making things that can detect when one light source is a frequency that is an integer multiple of another, so I don't know if it this method could be extended far enough.

What you are describing is essentially a "frequency chain" which was historically used for measuring the frequency of laser light. The idea is to start with a cesium atomic clock which (by definition) has a frequency of 9 192 631 770 Hz. The frequency of the signal is then multiplied up using some non-linearity, and a phase locked loop is used to stabilize the output of a higher-frequency oscillator to the up-multiplied signal. This is repeated in multiple steps, until optical frequencies are reached. The laser frequency is then an exact multiple of the frequency of the atomic clock. This technique was for example used for the "last measurement" of the speed of light [1]. Later, the speed of light was defined to be exactly 299 792 458 m/s, so it is no longer possible to measure it (all lengths are now defined with respect to the speed of light and the frequency of a cesium clock).

Frequency chains were monstrously complex experiments, since they contained all kinds of different oscillators in a wide range of frequencies that were linked with phase locked loops. Only a few were ever build, mainly by national metrology labs. The technique was made completely obsolete in 1999 when the optical frequency comb was invented. Frequency combs are lasers that directly link optical frequencies to radio frequencies without requiring intermediate steps. In 2005, Hall and Hänsch shared half of the Nobel Prize in Physics for the invention [2].

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.29... [2] https://www.nobelprize.org/prizes/physics/2005/prize-announc...

Many years ago, only the wavelength of a laser could be measured and the frequency was indeed computed using the speed of light.

Nevertheless, there is almost a half of century since it began to be possible to measure directly the frequency of lasers.

Nowadays, the frequency of the lasers is what is known, by measurement, and their wavelength is computed using the speed of light.

Before the speed of light became a conventional constant, there was a wavelength that determined the unit of length and a frequency that determined the unit of time.

At that time, measuring the speed of light was done by measuring both the wavelength and the frequency of a laser, as ratios to the standard of wavelength and to the standard of frequency.

Now, when the speed of light is a conventional constant, what the experiment described by you did, is measuring the length of a ruler using a laser of known frequency, and verifying that the measured length is the same as the length marked on the ruler.

The measurement of the frequency of lasers is done using a special kind of laser, which produces pulses of light, not continuous light, and which can be tuned so that there is a precise ratio between the frequency of the light and the repetition frequency of the pulses. (This kind of lasers are called by the not very explicit name "optical frequency combs")

The repetition frequency of the pulses is in the MHz to GHz range, so it can be compared with the frequency of other oscillators using electronic counters.

The frequency of the light can be compared with the frequency of other light sources using various optical interference methods (taking advantage of the fact that periodic pulses of light have a specter with a very large number of equidistant spectral lines, some of which will be close in frequency to the frequency that must be measured). So in the end, the ratio between the frequency of any low-frequency oscillator and the frequency of any light source can be determined.

A student just now told me about an open source rpi muon detector that can be built at home. That should be added to such a list. I work at Andøya Space.

Here's part 2:

https://blogs.scientificamerican.com/critical-opalescence/ho...

Simples. So now we can check for ourselves...

Yes, part 2 is key because it's only when he starts measuring polarization that he demonstrates entanglement rather than just the fact that photons are being emitted in pairs.

The article mentions a photo of the setup ("see photo below"), but I don't see it.

Side note, with the invasion scare it's been almost impossible to buy any sort of consumer radiation monitors. Everything is sold out or on backorder.

Sorry, what "invasion scare"?

The parent is referring to the Russian invasion of the Ukraine, stoking fears of a bigger and possibly nuclear war.

Are you suggesting there has been no invasion? Anyway they made explicit threats.

https://www.reuters.com/world/europe/russia-warns-baltic-nuc...

> Are you suggesting there has been no invasion?

If there has been an invasion (of where, exactly? Presumably Ukraine?) then it is not a "scare".

I assume you are talking about Finnland, Sweden and Russia?

I can assure you there are some very scared people in whole Europe and elsewhere because of Ukraine invasion and Russian inflammatory rhetoric.

In English, the use of "the X scare" means something X that people were afraid would happen but didn't, often because it was never really going to happen. It can also mean being scared of something that people think has happened, but hasn't, such as Orson Welles' War of the World scare. It doesn't mean something that has happened and that scares people - nobody would call the invasion of Ukraine by Russia "a scare".

Not sure why, but the article doesn't have the original photos that went along with the post(s):

https://web.archive.org/web/20160919103031/http://blogs.scie...

https://web.archive.org/web/20150926005801/https://blogs.sci...

One thing I don't get: he is measuring gamma rays, which will go through the wall of the Geiger tube as easily as they'll go through the front window. Why does it matter how the tube is oriented with respect to the source? Seems like the only factor that should affect the count rate is distance.

TIL that there readily available radioactive material/disks sold online…

I have taught a chemistry class where we use those disks. They have a stamp on them that say something like "NRC exempt quantity."

So, apparently, the disks have enough radioactive material to detect, but not enough to cause anyone to worry.

Smoke detectors have radioactive material

Not all though. Photoelectric detectors also exist and seem to be more common in homes these days.

TIL that you can buy antimatter on the internet.

Has anyone got the book 'Exploring Quantum Physics through Hands-on Projects', it sounds pretty cool

It is a fantastic book IMO. Some of it is fairly advanced, although there's never more math than is really necessary to explain what to expect from the experiments. The chapters on recreating the early CRT experiments of people like Hertz, Crookes, and Thomson are especially nifty.

If you wanted to perform most or all of the experiments it will probably cost you at least a few thousand dollars (and a few months' time), but the authors have done a great job with the presentation and content and clearly know their stuff.

Should not have (2013) in the title?

(2013)

so last year

You can create superpositions with guitar strings.

Not a physicist, so maybe I have a completely wrong idea. But is not superposition of waves different than (in any event less surprising than) superposition in discrete particles?

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