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http://peeterjoot.com/2018/09/26/phy2403h-quantum-field-theory-lecture-6-canonical-quantization-simple-harmonic-oscillators-symmetries-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 6: Canonical quantization, Simple Harmonic Oscillators, Symmetries. Taught by Prof. Erich Poppitz