WisheDeom
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Homework Statement
I must prove that the set of coherent states \left\{ \left| \lambda \right\rangle \right\} of the quantum simple harmonic oscillator (SHO) is a complete set, i.e. it forms a basis for the Hilbert space of the SHO.
Homework Equations
The coherent states are defined as eigenkets of the creation operator with eigenvalue \lambda; in terms of the energy eigenkets they can be written
\left| \lambda \right\rangle = \exp \left( -\frac{|\lambda|^2}{2} \right) \sum_n \frac{\lambda^n}{\sqrt{n!}} \left| n \right\rangle
Completeness means the sum (infinite series in this case)
\sum_{\left\{ \left| \lambda \right\rangle \right\}} \left| \lambda \right\rangle \left\langle \lambda \right|
converges and is non-zero. Sites have told me the sum should converge to \pi, but I don't know how to compute that.
The Attempt at a Solution
I'm not even quite sure how to start. The eigenvalues are complex numbers, so I know the sum (integration) must be over the complex plane, but how should I do this? I tried parametrizing \lambda = x + iy, and then separately by \lambda = r e^{i \theta}, but both got very messy quickly, and I'm not sure what to do. Am I on the right track at all?