David MeyerNov 6, 2022

I was just re-reading "Why Boltzmann Brains Are Bad", by Sean Carroll [1]. There Carroll notes (on page 13) that

"There is a possibility of confusion here, due to the sloppy way in which physicists use the word “quantum fluctuation.” It’s helpful to distinguish between three different notions [44]."

Indeed. 🙂 #physics #boltzmannbrain

References
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[1] "Why Boltzmann Brains Are Bad", https://arxiv.org/pdf/1702.00850.pdf

[2] "Is The Boltzmann Brain Theory Plausible?", https://medium.com/@amesett/is-the-boltzmann-brain-theory-plausible-85cf7b776c6d

Show threadretsilNov 6, 2022
@dmm I find it quite ironic that I skimmed the first page with growing suspicion that the paper was AI generated. However, I soon realised that my suspicion existed mainly because I hadn't heard of Boltmann universes.
Show threadDavid MeyerNov 7, 2022
@retsil Why did you think the paper was AI generated (just curious)?
Show threadretsilNov 7, 2022
@dmm. the mix of colloquial language in the title. Mixture of vastly different topics which may have just been pasted together.
Show threadDavid MeyerNov 7, 2022

The Bloch Sphere gives us an insanely beautiful way to think about (and visualize) the state of a qubit in 3-dimensional space. It is named after the Swiss-American physicist and Nobel physics laureate Felix Bloch [1].

My notes on the Bloch Sphere are here: https://davidmeyer.github.io/qc/bloch_sphere.pdf.

As always, questions/comments/corrections/* are greatly appreciated. #math #physics

References
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[1] ""Felix Bloch", https://en.wikipedia.org/wiki/Felix_Bloch

[2] "Bloch sphere", https://en.wikipedia.org/wiki/Bloch_sphere

Show threadazNov 7, 2022
@dmm Great job putting together these notes on the Bloch sphere! While I haven't gone through the notes fully, I noticed a few important mistakes. In the 2nd paragraph you mistake a superposition and mixed state as being the same and in the remark you call a superposition of states a coherent state. This is a common source of confusion so it is important to be precise.
Show threadDavid Meyer
@az Thank you for the comments! I removed the part about superposition just to make sure the confusion you note doesn't arise (see the new text in the attached image). I also added an acknowledgement. Thanks again, very much appreciated.
Nov 7, 2022 at 2:41pmWeb
Show threadazNov 7, 2022
@dmm No problem. To clarify, the linear combination of basis states would be a superposition. A mixed state appears when there is some lack of information about the state of the system. Mixed states cannot be written as a wavefunction and instead are described by a density matrix.