Questions tagged [quantum-mechanics]
Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy, and the uncertainty principle and is generally used in single-body systems. Use the quantum-field-theory tag for the theory of many-body quantum-mechanical systems.
32,497 questions
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Quantum Computing Bloch Sphere Convention [closed]
Could somebody please explain why the convention for the Bloch sphere of a qubit has the $|0>$ state pointing up and the $|1>$ pointing down? I understand that the Bloch sphere representation ...
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Conceptually, what's the effect of an exponential gravitational tilt in non-unformal gravity on a Quantum Harmonic oscillator? [closed]
Diving into this topic, I'd like the know whether the results of diving into this question, and/or could be explain very simply using textbook knowledge or if it's a feasibly novel to a degree.
Thank ...
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Rotating-wave approximation (RWA) for two-tone spectroscopy
I am having a hard time figuring out what the correct rotating-wave approximation (RWA) should be in a two-tone spectroscopy simulation. Apologies in advance for this being an easy question, I work ...
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Quick question about self-orthogonality of non-Hermitian Hamiltonians
Let's say we have some non-Hermitian Hamiltonian $H(\vec{\xi})$ depending on a set of parameters $\vec{\xi}$. Whenever $H(\vec{\xi})$ is not diagonalizable (two or more eigenstates coalesce), let's ...
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A question regarding the applicability of Lieb's theorem to Kitaev's honeycomb model
In the fourth chapter of "Anyons in an exactly solved model and beyond", it is claimed that following Lieb's theorem in "Flux phase of the half-filled band", the ground state ...
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What would Spin Operator and Spin State Vectors be in higher dimensions?
If we represent spin quantum state of a particle in $\pm z$ direction with $\vert\pm\rangle$ then we know that the state vectors in remaining $x$,$y$ directions would be such that:
$$\vert\langle S_x;...
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How can one see the disappearance of entanglement as one transitions from a quantum to a classical system?
I am trying to combine two concepts in this question: quantum entanglement and the deformation quantization.
From the definition of entanglement on Wikipedia:
Consider two arbitrary quantum systems $A$...
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Alpha particles and the very unwavelike behavior [duplicate]
We're still trying to figure out why alpha particles move in straight lines instead of a wave-like probability moving in all directions. I am wondering if anyone has put any thought into maybe they ...
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Twistorr Agilent 74fs Vent Valve + Controller (not venting fully)
I am using a Twistorr Agilent 74fs Turbo with a vent valve kit attached to it (https://www.agilent.com/store/en_US/Prod-9699844/9699844) as a part of a system that cools a coldhead to ~1K. This should ...
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Zeeman splitting of Mercury green (546 nm) line: why no spin-orbit?
I'm teaching the Zeeman Effect and have gone through books and other lecture notes. I understand why, for the sodium D-line, we need to include spin-orbit coupling and then apply the Zeeman shift to ...
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Dirac-Hartree-Fock [closed]
I am trying to work my way through Dirac-Hartree-Fock. I have reached a point where I am baffled. I will present the problem here, but will have to define a few things since nomenclature is so varied ...
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How is the separation between core and valence electrons justified?
The full crystal Hamiltonian, strictly speaking, depends on all electrons, protons, and neutrons. Even before applying the Born–Oppenheimer approximation, one typically assumes — implicitly — that the ...
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Meaning of band gap structure [closed]
In a hydrogen atom, the electron can take only certain energies (quantized energies) only. When two hydrogen atoms come close, the electrons can take up more energy states but still the states are ...
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Does Wigner's theorem only imply left inverse?
From wikipedia
https://en.wikipedia.org/wiki/Wigner%27s_theorem
For unitary case
$$\langle U \Psi, U \Phi \rangle = \langle \Psi, \Phi \rangle .\tag{1} $$
If I apply the definition of adjoint
https://...
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Can entanglement correspond to a state where the thermodynamic arrow of time becomes locally undefined?
While studying quantum entanglement and thermodynamics, I started wondering about a possible conceptual link between the two.
In a bipartite pure entangled state we have:
Total entropy: S_total ≈ 0
...
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Greens function in Many Body Physics [duplicate]
What is the physical intuition for retarded many body greens function
$$G^{R}(rt,rt')=-i\theta(t-t')\langle[\hat{\psi}(rt),\hat{\psi}^{\dagger}(r't')]\rangle$$
where the square bracket stands for ...
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Computation of scattering cross section using Dirac equation
The differential cross section formula of the scattering of an electron on a nucleus are known to coincide when computed using either classical mechanics approach (Rutherford) or the Schroedinger ...
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Stopping criteria for Self-consistent calculations
I am currently doing self-consistent calculations for various parameters $\Delta_{i}$, but i run into the problem that I have a U(1)-symmetry break, so when I reach the minima my simulations start ...
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Collapse of wave function of large objects [duplicate]
When we measure the position of a particle,it collapses to a new wavefunction which is a dirac delta function.It's probability density spikes at a particular position value.So when we measure its ...
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Can counterfactual measurements be used to measure incompatible observables?
Imagine we have a qubit system prepared by choosing one of two states as measured in one of two possible, incompatible bases, i.e. there are four total possible states, such as e.g. polarizations of 0,...
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How earlier physicists like Maxwell specifically accounted for delay and propagation of changes in fields rather than their instant settlement? [duplicate]
We can see directly from Maxwell`s equations that his equations are for local changes not global which translates to delay and propagation of field changes rather than instant settlements
Question :
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Subsystem in ground state?
If I have a quantum system and trace out some degrees of freedom (i.e. in the Green function formalism), I get a subsystem governed by an effective Hamiltonian. Now, if we assume that the full system ...
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Physical meaning of a complete set of compatible observables
I am a Math student, new at Quantum Mechanics, and I am having some troubles understanding the physical meaning of the notion of “complete set of compatible observables". I know its mathematical ...
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When is the post-jump state in the Lindblad equation pure, and when is it mixed?
In the Lindblad master equation,
\begin{equation}
\frac{d\rho}{dt} = -i[H, \rho] + \sum_k \left( L_k \rho L_k^\dagger - \tfrac{1}{2}\{ L_k^\dagger L_k, \rho \} \right),
\end{equation}
the dynamics ...
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Spontaneous symmetry breaking in quantum mechanics
I am learning QM and trying to understand the (absence of) SSB in QM. I have read statements from several posts in the forum about the absence of SSB in few-body QM (and I'm not sure if they are ...
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