PMT2: Photon Bunching / Hanbury Brown & Twiss effect

This is the second video about photomultipliers and their use. In this video I set out to measure an effect called "Photon Bunching". Photon bunching is phenomenon characteristic for incoherent light It can for example be used to measure the angular diameter of stars and was discovered by Robert Hanbury Brown and Richard Quintin Twiss in 1954.

Video chapters:
0:00 Introduction
0:42 Brief description of coherence
4:01 Description of the experimental setup
10:17 Aim of the experiment
11:40 Main result
12:25 Explanation and discussion
13:10 What is a photon?
16:10 Relation field amplitude / intensity / probability
22:17 Second order correlation function described
25:23 The Hanbury Brown & Twiss effect
27:25 Trying to measure g(2); failure and succss

There is a typo in the second order correlation function formula presented at 22:25: The denominator should contain the square of the time averaged intensity instead of the time average of the intensity squared.

(I have no commercial affiliation, it's just great software)

Third party imagery and clips:
14:35 Image standard Model of elementary particles: Source WIkipedia
14:55 I got the "face slap" clip of a channel named @neilsandwichtv5186. Not sure if this channel indeed is the copyright owner. Contact me if you have more info on this.
13:36 I used a few very short clips from @ArvinAsh as illustrations of the particle presentation of light and photons. Arvin makes very high quality content on various scientific subject. But I guess his photon visualizations leave some room for improvement (;-).

Did I forget something? Let me know and I'll set it straight.

Music used : "Floating" (personal composition) performed by the Early Birds (1999).

[-vermin-]

There were at least a dozen points in this presentation where I wanted to hit the like button. Unfortunately I already had.

[SBTRIS]

Crazy that this caliber of content is available to us for free. Learned a lot - thanks.

[Richardincancale]

19:50 Thinking of photons as discrete transactions is so insightful - the most understandable explanation of photons and interference patterns I have encountered in my 67 years!

[gsuberland]

Fantastic video! I started watching this and as soon as I got to the astronomy part I rewound and re-watched it with my wife, who is preparing to perform this exact measurement at an observatory.

[FireStormOOO_]

You're the first person I've heard clearly articulate that the photon interactions are quantized, rather than the field everywhere. Would've been really helpful when I was first wrapping my head around this topic!

[joshlerner]

I am a new PhD student in biomedical imaging, and I’m currently taking a statistical optics course. Really enjoyed the video! We literally just derived the van Cittert–Zernike theorem in my class from the propagation of coherence.

[catbertsis]

I could not understand the basic concepts of quantum electrodynamics before, but seeing little people slap each other finally helped me.

Thanks Huygens Optics!

[sharplessguy]

I've studied the subject for years and watched countless lectures but never encountered it explained in this manner. It's intriguing

[brettito]

You really demystified the wave particle duality in this one. The fact that they are probable interactions that represent a wave was the a ha moment for me.

[QuantumFringing]

Great video! I as I saw some one point out sodium lamps and spectral filtering, there's another way that has been used to create what I'll define as pseudothermal light at a specific wavelength: namely a rotating diffuser. This technique was shown I think in the 60s, but you can read about it more thoroughly in goodmans statistical optics. The main idea is to consider a single frequency laser and look in the far field to see the speckle: it should be stationary. If we add several frequencies, each with their own speckle, we see that the speckles will sort of add together and when you detect youll average over the various speckle configurations leading to what looks like a speckle free beam. Now, if you take a laser and put a diffuser in front, youll end up with a random speckle pattern in front due to the random phases imparted to different spatial points of the beam on the diffuser. As the diffuser turns, the speckle pattern will change as the pattern of hills and valleys change on the diffuser. By rotating, and averaging over all realizations of the diffuser, youll end up averaging over all of the speckle patterns and thus something similar to the LED. I call this pseudothermal because the light is not coming from a blackbody source. This is easily implemented with a cheap RC car motor and some plastic (scotch tape works pretty well usually, but it depends on the wavelength of light and how big the small deviations in the surface). Ive used this (well using a random piece of plastic) in the lab to get g^2(0)=2 within experimental error.

Outside if this, I want to point out a couple of other things.

I think you glossed over the Hanbury-Brown-Twiss conteoversy too fast. There's a ton of interesting history and actually this discovery lead to what Ill call the second revolution in quantum optics. It took a painful derivation by Roy Glauber to show that indeed the g^2(x, t) is related to the first order coherence (the g^1(x, t)). The formulation to show this lead directly to the second revitalization of quantum optics.

Some other stuff that might be fun to try. First, you should try doing homodyne tomography of different quantum states of light. Effectively, this is taking your weak quantum state of light (thermal, coherent state, single photon) and interfering it at a beamsplitter with a strong laser. By subtracting the currents in either arm (called balancing), you can get access to reconstructing the quadrature state of the light. Most of these will be gaussian unless you can get a nonclassical light source.

On this note, you should also do the famous Hong-Ou-Mandel interference experiment that actually (and directly) shows the "particle" nature of light. In this case, take your beamsplitter and interfere two photons at the input ports (one at each), if the photons are truely identical (same polarization, spatial mode, and frequency distribution) then it turns out that the photons will always leave at the same port and never at different ports. Then as you change the states to become more distinguishable by, e.g., delaying one photon by some time, as the mode overlap between the states decreases, the coincidences will begin to increase. Specifically, if you have detectors at the outputs, then youll never see coincidences. Further, this cannot be due to a simple wavelike interference effect because it isnt phase dependent. You do not have an envelope with oscillations under it in this effect. If you did the same thing with coherent states (i.e. laser light), there is an oscillation effect that is phase dependent. Thermal light, also is phase independent, but the dip in coincidences will be smaller.

[powerdustlastname]

i love the explanation that photons are the interactions with the em field.
this is so much easier to think about compared to the way i am taught quantum physics in school
thank you!

[BigfootSF]

I showed several coworkers your video on the photo-multiplier. We work as construction inspectors. We sometimes use a Nuclear Density Gauge. These devices use a photo-multiplier. It was really cool to see the way a device works that is very difficult to understand.

[MiguelFMorales]

Very nice. As you so nicely explain the bunching time is related to the bandwidth, so another approach would be to use a narrow color filter on your white light source. You have plenty of light intensity to make this work. Essentially instead of going faster in the detection, go narrower in bandwidth. This is a trick used in HBT experiments.

Also, it is very fun that HBT works with any particle, and the HBT you explained is used with pions! Experiments where they collide gold nuclei at near the speed of light to make a quark-gluon plasma (e.g. RHIC), it ‘glows’ by emitting lots of pions. Since we can’t make mirrors for pions, they use the spatial bunching of HBT to make ‘images’ of the plasma as it expands and explodes.

[Lufteluke]

I'm not a physicist, and I've never "truly" understood the wave/particle duality, but I feel absolutely cheated that it took until now hearing about looking at the photons as interactions. That's way more intuitive, and my understanding about several concepts have changed significantly from that single insight

[dylanbiddle]

I was starting to believe that scientists were being deliberately dishonest about what a photon is, this video saved some of my faith in science.

[grantwilbur]

Amazing video as always! I've been doing g2 measurements in our lab for a few years now, albeit with single photon sources and ~$300k worth of detectors and timing electronics. It is unbelievable how much youve accomplished with just a scope and some PMTs!

I have a recommendations that might help you resolve photon bunching. You could replace the broadband LED with an atomic spectral line. If you have a sodium or neon discharge, you could use a diffraction grating to send a sinlge emission line to the input of your HBT setup. Most of those spectral emission lines are very narrow (sub-GHz), which corresponds to coherence times in nanoseconds. Ive used this technique to measure argon spectral lines in the near-IR to calibrate a spectrometer and it works very well.

The next logical step of course is to construct a spontaneous parameteic down-conversion based single photon source and characterize that 😋! You could even do Hong-Ou-Mandel interference, the wackiest measurement ever!

Cannot wait for your next upload!

[andax]

If it is the timing resolution that needs to increase i suggest using a TDC (time to digital converter) to measure the timing instances. You can get 10 ps resolution instead of 10 ns. If you have a fast oscilloscope with a fair amount of memory, you can do the TDC in software interpolating far beyond the sampling frequency. Very interesting topic. You are my favorite channel on YouTube.

[GangsterGod]

AT 1:47.... finally able to comprehend what wave function, oscilloscopes and wave diagrams, the wave itself; and, what they represent after watching hours and hours of videos discussing electrical engineering or electronics, photons and quantum effects, the science of sound, etc. etc. So, thank you for finally bringing a clear picture and understanding into my knowledge base!

[SaberTail]

The Hanbury Brown and Twiss effect seemed so wild to me when I first heard about it. But you're right that you can't argue with experimental results. I also appreciate that you were able to do all this stuff in software. So much more pleasant than doing this with a crate of analog electronics and a mess of coax cables.

[eternaldoorman]

This is definitely one of the best pieces of science I've ever seen on YouTube! It's awesome!