A non-interacting quantum field ˆϕ(x) can be decomposed into ak and a†k. This enables us to calculate the variance of a free field. For example, the variance of the free real scalar field, in the vacuum |0⟩ of the theory, is computed to be (without a momentum cut-off) Var(ϕ)0=⟨0|ϕ2|0⟩−(⟨0|ϕ|0⟩)2=∫d3k(2π)31√k2+m2→∞.
Friday, 5 June 2020
Variance of an interacting quantum field in its vacuum state
Now, consider an interacting quantum field theory described by the Hamiltonian H and |Ω⟩ is the vacuum state of the interaction theory i.e. H|Ω⟩=0|Ω⟩(also, Pμ|Ω⟩=0|Ω⟩).
Decomposing the field into creation and annihilation operators is no longer possible. So how does one compute the variance Var(ϕ)Ω=⟨Ω|ϕ2|Ω⟩−(⟨Ω|ϕ|Ω⟩)2
in λϕ4 theory?
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