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**Why is free field called "free"?**

For example the Klein-Gordon field, consider if we have no spatial dimension(0-dimensional QFT should be identical to QM, is it correct?), then the field hamiltonian reduces to something like p^2+ m^2q^2, which is a hamiltonian for a harmonic oscillator in QM, not a free particle, then why we call it a free field?

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