Ciro Santilli @cirosantilli 40 

Photonic quantum computer Updated 2025-07-16

Uses photons!

The key experiment/phenomena that sets the basis for photonic quantum computing is the two photon interference experiment.

The physical representation of the information encoding is very easy to understand:

input: we choose to put or not photons into certain wires or no
interaction: two wires pass very nearby at some point, and photons travelling on either of them can jump to the other one and interact with the other photons
output: the probabilities that photos photons will go out through one wire or another

Jeremy O'Brien: "Quantum Technologies" by GoogleTechTalks (2014)

Source. This is a good introduction to a photonic quantum computer. Highly recommended.

youtube.com/watch?v=7wCBkAQYBZA&t=1285 shows an experimental curve for a two photon interference experiment by Hong, Ou, Mandel (1987)
youtube.com/watch?v=7wCBkAQYBZA&t=1440 shows a KLM CNOT gate
youtube.com/watch?v=7wCBkAQYBZA&t=2831 discusses the quantum error correction scheme for photonic QC based on the idea of the "Raussendorf unit cell"

Photon polarization Updated 2025-07-16

The knowledge that light is polarized precedes the knowledge of the existence of the photon, see polarization of light for the classical point of view.

The polarization state and how it can be decomposed into different modes can be well visualized with the Poincaré sphere.

One key idea about photon polarization is that it carries angular momentum. Therefore, when an electron changes orbitals in the Schrödinger equation solution for the hydrogen atom, the angular momentum (as well as energy) change is carried out by the polarization of the photon!

Quantum Mechanics 9b - Photon Spin and Schrodinger's Cat II by ViaScience (2013)

Source.

clear animations showing how two circular polarizations can make a vertical polarization
a polarizer can be modelled bra operator.
light polarization experiments are extremely direct evidence of quantum superposition. Individual photons must be on both L and R states at the same time because a V filter passes half of either L or R single photons, but it passes all L + R photons

Physics bibliography Updated 2025-07-16

Physics course of the University of Oxford structure Updated 2025-07-16

Physics education needs more focus on understanding experiments and their history Updated 2025-07-16

This is the only way to truly understand and appreciate the subject.

Understanding the experiments gets intimately entangled with basically learning the history of physics, which is extremely beneficial as also highlighted by Ron Maimon, related: there is value in tutorials written by early pioneers of the field.

"How we know" is a basically more fundamental point than "what we know" in the natural sciences.

In the Surely You're Joking, Mr. Feynman chapter O Americano, Outra Vez! Richard Feynman describes his experience teaching in Brazil in the early 1950s, and how everything was memorized, without any explanation of the experiments or that the theory has some relationship to the real world!

Although things have improved considerably since in Brazil, Ciro still feels that some areas of physics are still taught without enough experiments described upfront. Notably, ironically, quantum field theory, which is where Feynman himself worked.

Feynman gave huge importance to understanding and explaining experiments, as can also be seen on Richard Feynman Quantum Electrodynamics Lecture at University of Auckland (1979).

'Making' - the best way of learning science and technology by Manish Jain (2018)

Source.

Physics journal Updated 2025-07-16

The strongest are:

early 20th century: Annalen der Physik: God OG physics journal of the early 20th century, before the Nazis fucked German science back to the Middle Ages
20s/30s: Nature started picking up strong
40s/50s: American journals started to come in strong after all the genius Jews escaped from Germany, notably Physical Review Letters

Physics Travel Guide Updated 2025-07-16

physicstravelguide.com/about

DokuWiki about physics, mostly/fully written by Jakob Schwichtenberg and therefore focusing on particle physics, although registration might be open to all.

Physics Videos by Eugene Khutoryansky Updated 2025-07-16

www.youtube.com/user/EugeneKhutoryansky

These videos can give some geometric insight and do have their value.

But they are sometimes too slow, there are never any mention of experiments, just "the truth".

And when things get "mathy", it sticks to a more qualitative view which may not be enough.

Very over the top with sexy demons and angels making appearances, and has some classic aesthetic artistic value :-)

Pion Updated 2025-07-16

Conceptually the simplest mesons. All of them have neutral color charge:

charged: down + anti-up or up + anti-down, therefore with net electrical charge $\pm 1$ electron charge
neutral: down + anti-down or up + anti-up, therefore with net electrical charge 0

Pirates (2005) Updated 2025-07-16

One of the first 1 million USD (zero artistic value) porn movie. And also a piece of shit! Hotter porn has been shot in kitchens around the world using iPhones.

IMDb entry: www.imdb.com/title/tt0477457/.

https://web.archive.org/web/20220330011213im_/https://cps-static.rovicorp.com/2/Open/MTI%20Home%20Video/Pirates%20(2005)/_derived_jpg_q90_310x470_m0/Pirates2005-PosterArt.jpg

Planck constant Updated 2025-07-16

Proportionality factor in the Planck-Einstein relation between light energy and frequency.

And analogously for matter, appears in the de Broglie relations relating momentum and frequency. Also appears in the Schrödinger equation, basically as a consequence/cause of the de Broglie relations most likely.

Intuitively, the Planck constant determines at what length scale do quantum effects start to show up for a given energy scale. It is because the Plank constant is very small that we don't perceive quantum effects on everyday energy/length/time scales. On the

lim_{h \to 0}

, quantum mechanics disappears entirely.

A very direct way of thinking about it is to think about what would happen in a double-slit experiment. TODO think more clearly what happens there.

Defined exactly in the 2019 redefinition of the SI base units to:

6.62607015 \times 1 0^{- 34} J \cdot s

Planck-Einstein relation Updated 2025-07-16

Photon energy is proportional to its frequency:

e n er g y = (pl an c k s co n s t an t) * (f re q u e n cy)

or with common weird variables:

E = h * ν

(2)

This only makes sense if the photon exists, there is no classical analogue, because the energy of classical waves depends only on their amplitude, not frequency.

Experiments that suggest this:

Planck's law Updated 2025-07-16

Used to explain the black-body radiation experiment.

Published as: On the Theory of the Energy Distribution Law of the Normal Spectrum by Max Planck (1900).

The Quantum Story by Jim Baggott (2011) page 9 mentions that Planck apparently immediately recognized that Planck constant was a new fundamental physical constant, and could have potential applications in the definition of the system of units (TODO where was that published):

Planck wrote that the constants offered: 'the possibility of establishing units of length, mass, time and temperature which are independent of speciﬁc bodies or materials and which necessarily maintain their meaning for all time and for all civilizations, even those which are extraterrestrial and nonhuman, constants which therefore can be called "fundamental physical units of measurement".'

This was a visionary insight, and was finally realized in the 2019 redefinition of the SI base units.

TODO how can it be derived from theoretical principles alone? There is one derivation at; en.wikipedia.org/wiki/Planck%27s_law#Derivation but it does not seem to mention the Schrödinger equation at all.

Quantum Mechanics 2 - Photons by ViaScience (2012)

Source. Contains a good explanation of how discretization + energy increases with frequency explains the black-body radiation experiment curve: you need more and more energy for small wavelengths, each time higher above the average energy available.

Plane wave function Updated 2025-07-16

In this solution of the Schrödinger equation, by the uncertainty principle, position is completely unknown (the particle could be anywhere in space), and momentum (and therefore, energy) is perfectly known.

The plane wave function appears for example in the solution of the Schrödinger equation for a free one dimensional particle. This makes sense, because when solving with the time-independent Schrödinger equation, we do separation of variable on fixed energy levels explicitly, and the plane wave solutions are exactly fixed energy level ones.

Pauli-X gate Updated 2025-07-16

The quantum NOT gate swaps the state of

∣ 0 ⟩

and

∣ 1 ⟩

, i.e. it maps:

x ∣ 0 ⟩ + y ∣ y ⟩ \to y ∣ 0 ⟩ + x ∣ y ⟩

As a result, this gate also inverts the probability of measuring 0 or 1, e.g.

if the old probability of 0 was 0, then it becomes 1
if the old probability of 0 was 0.2, then it becomes 0.8

[0110]

(2)

Equation 2.

Quantum NOT gate matrix

Figure 1.
Quantum NOT gate symbol
. Source.

Paul Allen Updated 2025-07-16

Particle creation and annihilation Updated 2025-07-16

en.wikipedia.org/wiki/Annihilation

Predicted by the Dirac equation.

We've likely known since forever that photons are created: just turn on a light and see gazillion of them come out!

Photon creation is easy because photons are massless, so there is not minimum energy to create them.

The creation of other particles is much rarer however, and took longer to be discovered, one notable milestone being the discovery of the positron.

In the case of the electron, we need to start with at least enough energy for the mass of the electron positron pair. This requires a photon with wavelength in the picometer range, which is not common in the thermal radiation of daily life.

Partial index partial derivative notation Updated 2025-07-16

This notation is not so common in basic mathematics, but it is so incredibly convenient, especially with Einstein notation as shown at Section "Einstein notation for partial derivatives":

\partial_{0} F (x, y, z) = \frac{\partial F ( x , y , z )}{\partial x} \partial_{1} F (x, y, z) = \frac{\partial F ( x , y , z )}{\partial y} \partial_{2} F (x, y, z) = \frac{\partial F ( x , y , z )}{\partial x}

This notation is similar to partial label partial derivative notation, but it uses indices instead of labels such as

x

y

, etc.