1. PG lectures 2004-05 Spontaneous emission

2. Outline Lectures 1-2 Introduction What is it? Why does it happen? Deriving the A coefficient. Full quantum description 3-4 Modifying the ‘environment’. Two atoms: superradiance A mirror: Cavity QED.

3. 2p(m=0) to 1s 2p(m=1) to 1s Oscillating charge

4. 2p(m=0) to 1s 2p(m=1) to 1s Dipolar radiation

5. I understand that light is emitted when an atom decays from an excited state to a lower energy state. That’s right And light consists of particles called photons Yes So the photon ‘particle’ must be inside the atom when it is in the excited state. Well no. Well how do you explain that the photon comes out of the atom when it was not there in the first place. I’m sorry. I don’t know I can’t explain it to you. Dad Feynman Dad Feynman Dad Feynman Dad Pause Feynman The light quanta has the peculiarity that it apparently ceases to exist when it is in one of its stationary states, namely the zero state….When a light quanta is absorbed it is said to jump into this zero state and when one is emitted it can be considered to jump from the zero state to one in which it is physically in evidence, so that it appears to have been created. Since there is no limit to the number of light quanta that may be created in this way we must suppose that there are an infinite number of light quanta in the zero state. (Dirac 1927) Spontaneous emission: what do the greats have to say? (Feynman, Physics Teacher 1969)

6. What is this zero state? Vacuum energy. Second quantisation of the electromagnetic field see Loudon The quantum theory of light pp. 130-143 a* a n photons destruction creation k One mode of EM field =1,2 polarisation Vacuum state The idea of a photon is most easily expressed for an EM field inside a …perfectly reflecting cavity. Loudon p. 1

7. Quantum theory of spontaneous emission University of Durham

8. Statement of the problem 1.Interpret spontaneous emission as: transition induced by the vacuum field. (a) only accounts for half the spontaneous decay rate (b) vacuum fluctuations alone also lead to the wrong sign of the electron spin anomaly g-2 2.Solution: vacuum fluctuations + self-reaction (the interaction of an electron with its own field) both contribute to observable processes. 3.However, their respective contributions cannot be uniquely defined. 4.A matter of interpretation!

9. Outline 1.Semi-classical: Fermi’s golden rule vacuum only accounts for half the decay rate. 2.QED: Radiative reaction or self reaction (electromagentic mass) 3.Decay rate and level shifts (Lamb shift) due to radiative reaction 4.Vacuum fluctuations + radiative reaction excited states decay at a rate  lowest energy state does not decay P. W. Milonni, The quantum vacuum, (Acad. Press, 1994). P. W. Milonni, Why spontaneous emission? Am. J. Phys. 52, 340 (1984).

10. where 2-level time dependent perturbation theory electric dipole approximation