# How does dynamic casimir effect generate correlated photons?

There is a recent paper on arxiv receiving lot of acclaim http://arxiv.org/abs/1105.4714

The authors experimentally show that moving a mirror of a cavity at high speeds produces light from high vacuum. The usual doubts about the experimental techniques seem to be very clearly addressed and reviewed (as per Fred Capasso's comments) http://www.nature.com/news/2011/110603/full/news.2011.346.html)

My question is: Can someone explain how correlated/squeezed photons are generated in this process? I can get a feel for how a moving mirror can generate real photons by imparting energy to the vacuum (correct me if this is not consistent with the detailed theory). But, I don't see how photons are generated in pairs. Could someone describe the parametric process happening here?

• Nice research-related question! Maybe you want to add some quantum electrodynamic tag. Greets Jun 30, 2011 at 20:42
• Great question +1. I'm left wondering if measuring the flux of the photons produced could be used as a measure of some part of the vacuum energy spectrum? Jun 30, 2011 at 23:47
• Nice question. Are you familiar with the second quantization? Are you familiar with the squeezed states? Jul 8, 2011 at 18:16

In a [parametric amplifier], an intense laser beam at frequency $2\Omega$ —the pump beam— illuminates a suitable nonlinear medium. The nonlinearity couples the pump beam to other modes of the electromagnetic field in such a way that a pump photon at frequency $2\Omega$ can be annihilated to create "signal" and "idler" photons at frequencies $\Omega\pm\epsilon$ and, conversely, signal and idler photons can be annihilated to create a pump photon.
One way to think of the present situation would be as a dual of this description. That is, the medium, the Josephson junction, oscillates at a pump frequency $2\Omega$, and interacts nonlinearly with the vacuum state.
Quantum theory allows us to make more detailed predictions than just that photons will simply be produced. If the boundary is driven sinusoidally at an angular frequency $\omega_d = 2\pi f_d$, then it is predicted that photons will be produced in pairs such that their frequencies, $\omega_1$ and $\omega_2$, sum to the drive frequency, i.e., we expect $\omega_d = \omega_1 + \omega_2$.