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Results tagged with quantum-mechanics
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user 1039
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.
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Local measurement of entangled particle pairs and interpretation of state
Before measurement, we have a bipartite entanglement between the two particles. After measurement, we have a multipartite entanglement between the two particles, the measuring apparatus, and the envir …
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Entangled particles
Even with quantum entanglement, quantum information still can't be transmitted faster than light. This is the no-signaling theorem.
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Justification of ignoring large set of entanglements
Most of the time, we have decoherence, which suppresses interferences between configurations with different values for the pointer basis. On the other hand, showing a pointer basis always exists is an …
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Observing the exponential growth of Hilbert space?
This exponential growth is what allows us to have multipartite entanglement, and exponentially many Everett branches in the first place.
If in the near future, we can construct a quantum computer imp …
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Energy conservation and quantum measurement
Thinking about your question has led me to the following conclusion:
Suppose there is a a conserved quantity $Y$, and an isolated system $S$. Let's also suppose the isolation is temporarily lifted to …
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What is Timelike Quantum Entanglement?
In most quantum field theories, operators localized at regions which are timelike separated don't commute. Not only that, the state at the earlier time can affect the state at the later time, meaning …
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Accepted
Why isn't the wave equation $\nabla^2 \psi - 1/c^2 \partial_{tt} \psi = (\frac{mc}{\hbar})^2...
That's the Klein-Gordon equation, which applies to scalar fields. For fermionic fields, the appropriate relativistic equation is the Dirac equation, but that was only discovered by Dirac years after S …
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Are these two quantum systems distinguishable?
The density matrices in both cases are identical. If quantum mechanics is exactly linear, both states ought to be indistinguishable. But if there are some slight nonlinearities in the time evolution, …
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Accepted
Time travel and nuclear decay
There's a prescription by Deutsch for the quantum mechanics of closed timelike curves. It works on the level of density states, instead of Hilbert space states. Given his prescription, he showed that …
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Dirac equation on general geometries?
To write down Dirac's equation over curved spacetime, first express the geometry in terms of vierbeins and spin connections. Spinor bundles are vector bundles over spacetime transforming locally under …
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What equation describes the wavefunction of a single photon?
According to Wigner's analysis, the single photon Hilbert space is spanned by a basis parameterized by energy-momenta on the forward light cone boundary, and a helicity of $\pm 1$.
However, a manife …
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Time as a Hermitian operator in quantum mechanics
The energy spectrum is bounded from below. A time operator would contradict the Stone–von Neumann theorem. This isn't really a problem. All it means is we have limits as to how accurate clocks can be …
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What is the density of virtual photons around a unit charge?
It's infinity. This is the soft photon problem, which requires infrared regularization.
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Proving Operator identities (Quantum Physics)
$$\left\langle \chi | \left( A + B \right)^\dagger | \psi \right\rangle = \left\langle \psi | (A +B) | \chi \right\rangle^* =$$
$$=\left\langle \psi | A | \chi \right\rangle^* + \left\langle \psi | B …
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Are density states more fundamental than wavefunctions? [duplicate]
Some interpretations, like the many-worlds interpretation, treat the wavefunction (modulo an overall phase factor) as objective and fundamental.
But consider the following example for a qubit: a class …
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