# Tag Info

## New answers tagged decoherence

3

As it turns out, the excited states of an atom are not quite eigenstates. That is, they're eigenstates of the atomic hamiltonian, but they are not eigenstates of the atom-plus-EM-field hamiltonian. How do we know? If you prepare them (excited atom, empty field) and then you leave them alone, they change, so they can't be eigenstates. What they are is ...

1

If I understand your question correctly, you are confusing between two definitions of "width". One is the spread of the spectral line and the other is the lifetime of the atom in excited state. Well, these two are related(as you guessed) by uncertainty principle. $\Delta E\,\Delta t = h/4\pi$. So if the time the atom spends in excited state is less, the ...

2

I doubt you'll be able to get useful bounds without restricting the class of channels. Consider the following quantum channel (for arbitrary dimension $d,n$): $$T:\mathbb{C}^{d\times d}\to \mathbb{C}^{n\times n}: \rho \mapsto \operatorname{tr}(\rho) \sigma$$ where $\sigma$ is any quantum state. Using the Choi-Jamiolkowski isomorphism, you can easily see ...

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