📚 Node [[quantum]]

# [2017-06-03] pilot wave theory

The other thing you should know is that Copenhagen is not any longer widely accepted amongst people who actually work on interpretations of QM or in foundational physics in general, it's just accepted outside the community as something that most people have at least some knowledge of.
Now, what's the problem with Copenhagen. The biggest one is the so-called measurement problem, that there is an ill-defined delineation between a system and its observer, or between "quantum" and "classical". There's may others though, e.g. that randomness is postulated rather than a result of the theory.


Pilot wave theory: the pilot wave, particle follows it. Bohm trajectory.
If we could determine position and velocity, we would be able to predict trajectory exactly.

Incompatible with relativity, non-local.

global hidden variables pilot wave?
pilot wave theory is both physical and deterministic

# [2015-08-26] .

a |00> + b |01> + c |10> + d |11>
Measure of entanglement: ad - bc

For 1 < p < inf, Lp is reflexive

Riesz theorem: p ∈ (1, inf), phi ∈ Lp*.
There exists a unique g ∈ Lq, s.t. phi(f) = \ip{g}{f}. Norms are equal.
T: Lq -> (Lp)*
T(u) f = \ip{Tu}{f}

# [2015-08-26] Lower bound for quantum search

Theorem: any quantum algorithm must take at least O(sqrt(N)) queries.
Lemma: 1 query > no algorithm can guarantee success prob > const / N.
t steps: P[success] = O(t2)/N

# [2015-08-26] continuous and discrete symmetries [[symmetry]]

commutator with Hamiltonian => time derivative (todo what?)
V = I - i eps G (generator)
[H, V] = 0
[H, I - i eps G] = 0
[H, G] = 0

degenerate level. Degeneracy is a consequence of symmetry

# [2015-08-26] bosons and fermions

P(ra, rb) = Pa(ra) Pb(rb)

P+-(ra, rb) = A (Pa(ra) Pb(rb) +/- Pb(ra) Pa(rb))

• for bosons
• for fermions

spin-statistics?

X : symmetry operator

X P(ra, rb) = P(rb, ra)
[X, H] = 0
complete set of simultaneous eigenvalues

P(ra, rb) = +- P(rb, ra): fundamental law; symmetrization requirement

two identical particles can’t occupy same single-particle states. Pauli exclusion principle.

# [2013-09-21] Prover, verifier (Из лекции Шеня в ПОМИ 30 сентября 2012)

1. Sequential repetition
2. Parallel repetition
3. Sequential repetition (multi-prover)
4. Parallel repetition (multi-prover)

Левый игрок: получает (a), вводит (c). Второй игрок: получает (b), вводит (d). Победа – если (c \otimes d = a \lor b).

Оптимальная стратегия: один игрок вводит (0), другой – (1). В (\frac34) случаев (a \lor b) равен (1), соответственно, они будут выигрывать с вероятностью в (p_C = \frac34) процентов.

Как доказать, что нельзя больше:

• Перебрать все стратегии (рассмотреть 16 случаев, ни одна не дает больше (\frac34), значит, и смешанная не даст больше)
• Заметим, что (c)((d)) можно выразить как линейную функцию от (a)((b)): (c = \lambda a \otimes \mu), (d = \lambda’ b \otimes \mu’), их xor тоже линеен, а функция «или» в правой части нелинейна, поэтому будет фейл хотя бы в одном из четырех случаев.

# [2019-05-06] quantum mechanics - What equation describes the wavefunction of a single photon? - Physics Stack Exchange

In quantum field theory, the Dirac equation and the Schrödinger equation have very different roles. The Dirac equation is an equation for the field, which is not a particle. The time evolution of a particle, ie, a quantum state, is always given by the Schrödinger equation. The hamiltonian for this time evolution is written in terms of fields which obey a certain equation themselves. So, the proper answer is: Schrödinger equation with a hamiltonian given in terms of a massless vector field whose equation is nothing else but Maxwell's equation.


# [2019-05-06] wavefunction - Can particle quantum spin be described with a wave function? - Physics Stack Exchange [[spin]]

Within the context of first quantization (the Schrodinger equation), spin itself is not characterized by the spatial or momentum space wave function (the function that defines a complex quantity over all space). It is instead treated with a separate Hilbert space that quantifies the spin states 'tacked on' to this wavefunction by an outer product. It is a matter of semantics whether you consider 'wavefunction' to refer to the complex field alone, or the composite field/spin hilbert spaces.

More advanced field equations incorporate spin more naturally. The Dirac equation, for example, no longer describes the evolution of a complex scalar field, but the evolution of a four-vector, that naturally describes spin states. For this system, spin isis characterized in the wavefunction itself.


# [2018-12-24] quantum mechanics - Infinite square well in momentum space - Physics Stack Exchange

space is a bounded region of the real axis, so no translation group can be defined


huh, that’s interesting…

# [2019-01-28]Why The Schrodinger Equation Fails at Relativity /r/Physics

Physics students always say that the Schrodinger equation doesn't hold for SR, but it does. It's not the Schrodinger equation that fails though, it's the non-relativistic Hamiltonian that fails. The Dirac equation *is* the Schrodinger equation, but with the Dirac Hamiltonian. The Schrodinger equation is valid.


## [2019-01-28]Why The Schrodinger Equation Fails at Relativity /r/Physics

Yeah the schroedinger equation just defines the hamiltonian as the generator of time translations, it can't be *wrong* any more than the number 4 can be wrong.


# [2020-07-21]quantum mechanics - Is Schrodinger&#8217;s Cat itself an observer? - Physics Stack Exchange

The point is, it has made you think about the issue. Whereas we all might agree a hydrogen atom is not an observer and a human is an observer, the case of a cat is not so clear. The point of the thought experiment is to expose problems with the Copenhagen interpretation - which it does very successfully.


# [2020-07-21]quantum mechanics - Is Schrodinger&#8217;s Cat itself an observer? - Physics Stack Exchange

As you open the box, this quantum state again evolves unitarily (i.e. without any kind of "collapse") into


# [2020-07-21]Copenhagen interpretation - Wikipedia

Wigner's friend

Wigner puts his friend in with the cat. The external observer believes the system is in state ( | dead ⟩ + | alive ⟩ ) / 2 {\displaystyle (|{\text{dead}}\rangle +|{\text{alive}}\rangle )/{\sqrt {2}}} (|{\text{dead}}\rangle +|{\text{alive}}\rangle )/{\sqrt {2}}. However, his friend is convinced that the cat is alive, i.e. for him, the cat is in the state | alive ⟩ {\displaystyle |{\text{alive}}\rangle } |{\text{alive}}\rangle . How can Wigner and his friend see different wave functions?

The Copenhagen interpretation: The answer depends on the positioning of Heisenberg cut, which can be placed arbitrarily. If Wigner's friend is positioned on the same side of the cut as the external observer, his measurements collapse the wave function for both observers. If he is positioned on the cat's side, his interaction with the cat is not considered a measurement.


# [2020-08-03](2) Quantum Mechanics 12b - Dirac Equation II - YouTube

studying spinors – try to translate dirac equation solution from moving reference frame (only px component) to still

# [2020-08-03](2) Quantum Mechanics 12b - Dirac Equation II - YouTube

make some flashcards from this video..

# [2019-01-07] quantum mechanics - How is the Schroedinger equation a wave equation? - Physics Stack Exchange [[pde]]

Actually, a wave equation is any equation that admits wave-like solutions, which take the form f(x⃗ ±v⃗ t)f(x→±v→t). The equation ∂2f∂t2=c2∇2f∂2f∂t2=c2∇2f, despite being called "the wave equation," is not the only equation that does this.


hmm, so if you do variable transform and substitute in pde, the substitution behaves like a wave… interesting

# [2020-08-11]Majorana equation - Wikipedia[[sm]]

right, so it’s unclear whether neutrinos get their mass from Higgs or something else

So far most particles we discovered seem to be described by Dirac equation? We haven’t seen evidence of Majorana particles so far?

# [2020-08-12]Circular reasoning?[[spinor]]

We can even deﬁne a ‘metric’ that is antisymmetric: a two-dimensional space called spinor space has such a metric, and it turns out to be of fundamental importance in physics.


hmm, interesting

# Tweet from @hamishtodd[[towatch]]

@hamish_todd: The Schrödinger equation describes the wave function or state function of a quantum-mechanical system. It states that, over time, the humps will move in a direction perpendicular to the colored stripes


# [2014-07-18] Quantum entanglement is not nonlocality, it’s correlation! [[quantum]]

Vector potential is important because it is impossible to state action principle in a Lorentz invariant way directly in terms of E and B fields.

# [2016-09-03] infinite dimentional Hilbert space is crucial for quantum mechanics? [[quantum]]

The most famous such equation is the position/momentum commutation relation mentioned earlier, which for our purposes is just the following matrix equation:
AB – BA = I.
This equation can’t be satisfied by any finite-dimensional matrices, since AB and BA have the same trace, so Tr(AB-BA)=0, but Tr(I) is nonzero.


# [2019-07-17]Quantum Zeno effect

## [2020-12-20] quantum zeno effect: measure the system often enough, it appears as if it’s not changing. [[drill]]

evolution is linear, so after time dt the amplitude of changing state is proportional to dt, and probability to dt2. So by making dt low enough, probability of ever changing state is arbitrary close to 0

# Uncertaincy principle nice video [[quantum]][[viz]]

https://youtu.be/VwGyqJMPmvE?t=469 – if you limit the time domain of a pure note, counterintuitively, you’ll have to use more sine waves to approximate it, so widen the frequency domain

# [2019-12-11] Māris Ozols on Twitter: "Helping my students to see beyond equations. https://t.co/4TioiOP5Sk" / Twitter [[quantum]][[notation]]

Helping my students to see beyond equations.


# [2019-08-02] 〈 Berger | Dillon 〉 on Twitter: "A changing Magnetic flux induces an Electric field. This can create a current in a wire to power a💡! No battery needed! https://t.co/ggeayJedFD" / Twitter [[viz]][[inspiration]]

A changing Magnetic flux induces an Electric field. This can create a current in a wire to power a
No battery needed!


very nice visualisation!

# [2020-03-24] New essay: "How quantum teleportation works"

I'm excited to announce the launch of a new essay, "How quantum teleportation works" (joint with Andy Matuschak). Teleportation is fun and surprising, and a fundamental primitive in quantum computation. This essay explains in detail how it works. It's available here:


# [2021-01-16](2) Quantum Waves visualized in 3D - YouTube[[quantum]][[viz]]

basic idea: draw the wavefunction in the orthogonal plane, then it’s easy to see where the wavepacket is moving

# [2021-01-17](2) QCD: Visualizing the Strongest Force in the Universe: Quantum Chromodynamics - YouTube[[quantum]]

nice derivation of strong force energy via E=mc2, the uncertaincy principle, and radius of the nucleus

# Tweet from @nonlocalpodcast [[quantum]]

@nonlocalpodcast: Seasons greetings! We - @captainhamptons , William, @henryquantum - are excited (and nervous!) to announce a project we've been working on: a quantum computing podcast! We have two episodes posted, and a bunch more to come. Tune in @ https://t.co/Nwz2cbHD8W, Spotify, or iTunes!


# [2019-02-18] Lecture by Douglas Hofstadter: Albert Einstein on Light; Light on Albert Einstein - YouTube [[biography]]

Einstein was given Nobel Prize for photoelectic effect; no one in the commitee believed in light quanta

# [2019-02-18] Lecture by Douglas Hofstadter: Albert Einstein on Light; Light on Albert Einstein - YouTube [[physics]]

apparently, Planck still didn’t believe in light quanta in 1913

# [2020-08-21]Chirality (physics) - Wikipedia

For massless particles – photons, gluons, and (hypothetical) gravitons – chirality is the same as helicity; a given massless particle appears to spin in the same direction along its axis of motion regardless of point of view of the observer.


# [2020-08-01]Klein paradox - Wikipedia

However, Klein’s result showed that if the potential is of the order of the electron mass, V ∼ m c 2 {\displaystyle V\sim mc^{2}} V\sim mc^{2}, the barrier is nearly transparent. Moreover, as the potential approaches infinity, the reflection diminishes and the electron is always transmitted.


wow, pretty insane

# [2014-11-01]What stops the Hydrogen atom from being destroyed? /r/askscience

That is a weak interaction effect and not something mediated by the electromagnetic interaction, which was specifically what the OP was asking about.
To discuss that we'd really need to move to QFT and beyond just the Schrodinger equation.
Though I presume the next logical question is why the weak interaction doesn't induce electron capture with hydrogen...
In this case, it is because a free neutron is a higher energy, less stable system. Electron capture is only relevant in systems where the resulting neutron is stabilized by the rest of the nucleus.
In cases where the neutron is *not* stabilized, you get positron emission. It is no accident that positron emission is a *common* decay mode among neutron rich nuclei.


# [2019-04-20] Aharonov-Bohm Experiment https://physicstravelguide.com/experiments/aharonov-bohm#tab__concrete

The best explanation can be found here: http://gregnaber.com/wp-content/uploads/GAUGE-FIELDS-AND-GEOMETRY-A-PICTURE-BOOK.pdf at page 18ff


# [2019-06-17] Quantum computing | Microsoft https://www.microsoft.com/en-us/quantum/

The only scalable quantum solution
We've taken a scalable topological approach, harnessing superior qubits that perform more complex computations, at lower cost, with higher accuracy.