Lecture 1: Introduction to Superposition
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it as a clear, intuitive account of superposition and sequential Stern-Gerlach style measurements, both to explain the ideas and as the source people quote when asking follow-up questions.
Recommended by 4 people across 4 different months, 2015 to 2023.
What it is
The first lecture of MIT 8.04 Quantum Physics I (Spring 2013), taught by Allan Adams. After about ten minutes of course logistics, it uses thought experiments with hypothetical 'hardness' and 'color' box apparatuses to introduce uncertainty and superposition.
Cited in 1 Wikipedia article, including Allan Adams.
What people said
Recommends isometry · 2016-10 · Physics Stack Exchange · on “How do we know a quantum state isn't just an unknown classical state?”
Perhaps try this Intro To Superposition ( [this video] ). Color and Hardness are code for z-spin and x-spin. The box is a Stern-Gerlach device.
Recommends Timón · 2021-03 · Physics Stack Exchange · on “How deep our experiments on non-determinism are?”
Disclaimer: Sorry in advance, because I've probably misunderstood something from this introductory quantum physics lesson, or something else related to quantum physics or math. But, hey, I need to know what it is that I'm missing. MIT 8.04 Quantum Physics I, Spring 2013 (2013) [this video] There's no need to watch the video unless I summarize very badly, hopefully the examples and induction will do the deal. Letting aside how the quantum universe we live in is more concretely, we can conceive an abstract simpler universe in which we just have sources of particles, and two types of filters, namely Up/Down and Left/Right and then detectors were the particles end up so we can "see them". We can comb
Recommends Shailesh Pincha · 2021-09 · Physics Stack Exchange · on “Can particles be in a superposition of times as well as positions?”
@mebaker But the superposition isn't with a different particle, it's with a different eigenstate. And since all the particles in a double slit experiment start with being in a superposition of going through slit 1 as well 2, we will see interference even if particles are shot after a gap of even days. Experiments are done keeping in mind that they eliminate the possibility that multiple particles interact with each other before reaching the screen. Allan Adams has a beautiful take on the subject here: [this video]
Recommends Terminality · 2023-08 · Physics Stack Exchange · on “What is the actual version (using electrons) of this quantum thought-experiment from an MIT lecture?”
In the first lecture of the MIT quantum-mechanics 2013 course, the professor develops basic abstract concepts, by inventing hypothetical binary measurable properties of electrons (hardness (H)= hard or soft (H/S), color (C)= black or white (B/W)) and apparatuses that measure them. Based on these, he explains a series of experiments with using apparatuses one after the other in various orders to establish an idea of the uncertainty principles (e.g. if you measure hardness twice, you get consistent results, if measure hardness, color and then hardness again, you get 50%/50%). The link to the lecture is here [this video] and he establishes these basics from the 11 minute mark to the 30 minutes mark
Cites Yukterez · 2015-02 · Physics Stack Exchange · on “Is simulating the entire universe possible?”
Is it concievable that we may one day simulate the entire universe with every single particle Who should type the properties of every single particle into the computer? Even if the calculation power were available (which it aint) there is nobody who would live the time to make the input. But more seriously, Stephen Wolfram has some good recitals on Youtube about the universe possibly beeing a cellular automaton [LINK] , which means that it would take the whole universe to simulate the whole universe (because no simplifications can be made if you want to trace every particle). The next problem would be that the quantum world is rather probabilistic [this video] than deterministic.
Cites knzhou · 2016-09 · Physics Stack Exchange · on “Why can interaction with a macroscopic apparatus, such as a Stern-Gerlach machine, sometimes not cause a measurement?”
Consider a Stern-Gerlach machine that measures the $z$ -component of the spin of an electron. Suppose our electron's initial state is an equal superposition of $$|\text{spin up}, \text{going right} \rangle, \quad |\text{spin down}, \text{going right} \rangle.$$ After going through the machine, the electron is deflected according to its spin, so we get $$|\text{spin up}, \text{going up-right} \rangle, \quad |\text{spin down}, \text{going down-right} \rangle.$$ In a first quantum mechanics course, we say the spin has been measured. After all, if you trace out the momentum degree of freedom, we no longer have a spin superposition. In simpler words, you can figure out the spin by which way the e
Cites suncup224 · 2016-12 · Physics Stack Exchange · on “When does Quantum Mechanics Measurement lead to Mixed State”
Consider the context of the Stern-Gerlach experiment. As is stated on numerous sources (e.g. Feynman Lectures [LINK] , MIT Lecture [this video] ), a silver atom in the state of $\vert+z\rangle$ that is put through a Stern-Gerlach apparatus in the $x$ direction followed another apparatus oriented in the opposite direction will recombine the two beams and restore the state as $\vert+z\rangle$. My question is why shouldn't the new state be the mixed state given by the following density operator? $$\rho = \frac{1}{2}|+x\rangle\langle+x|+\frac{1}{2}|-x\rangle\langle-x|$$ Now, consider the scenario where I have a friend standing in between the two $x$-oriented Stern-Gerlach apparatus to observe that in
Cites YoTengoUnLCD · 2016-12 · Physics Stack Exchange · on “Regarding Energy and Momentum in QM”
I've been trying to learn Quantum Mechanics for a few months now, and there's a pretty fundamental thing I never quite understand: What is energy and momentum in quantum mechanics? I've been following MIT's QM course [this video] , and the instructor talked about the energy and momentum of light/wave-like objects, and said that De Broglie proposed that $E=\ \hslash \omega$ and " it turned to be true ". My question is, what is/was the QM definition of energy before de Broglie, and how did $E=\hslash \omega$ "turn out to be true"? Same question goes for momentum (the only definition of momentum I know is $\mathbf p = m\mathbf{\dot x}$).
Cites Zulire · 2019-08 · Physics Stack Exchange · on “Sequential Stern-Gerlach experiment”
Consider the following diagram: [LINK] (Sakurai) In the first lecture of MIT OCW Quantum Physics 1, 2013 ( [this video] ), Allan Adams implies that if we remove the barrier on the Sx- beam, thus allowing the Sx+ beam and Sx- beam to merge together and pass through the next SG measurement setup, the output of that final measurement will be only Sz+ atoms, rather than both Sz+ and Sz-. Why does this happen? Why do the atoms not care about being collapsed into Sx eigenstates in this scenario?