The MIR experiment: quantum vacuum and dynamical Casimir effect

This thesis concerns one of the few low energy experimental efforts aiming to test Quantum Electrodynamics. The experiment MIR (Motion Induced Radiation) studies the quantum vacuum in the presence of accelerated boundaries. According to Quantum Electrodynamics, a non-uniformly accelerated mirror in vacuum feels a friction force due to the interaction with the vacuum photons. As a consequence, real photons are produced in the process, which is known as dynamical Casimir effect. The radiated energy is emitted at the expense of the mechanical energy of the mirror. The effect has never been observed experimentally, since it is very feeble. Only recently a few experimental approaches have been proposed. The theory of the dynamical Casimir effect has been treated extensively in literature. According to the models proposed, for harmonic oscillations the effect is proportional to the oscillation frequency. As all the papers refer to frequencies of the order of a gigahertz, it is not possible to tackle the problem of obtaining a moving boundary with a purely mechanical approach, for example employing piezoelectric transducers or acoustic excitations, due to the large amount of energy required to keep a massive object in motion. A solution to this problem was proposed at the end of the 80's and has been adopted in the MIR experiment. In this framework the moving boundary is a semiconductor slab that switches periodically from complete transparency to total reflection when illuminated by a train of laser pulses. In this way one obtains a time variable mirror which mimics a physical oscillation, without the burden of overcoming the inertia of the mirror. Even so, the number of photons expected is extremely small. The MIR experimental strategy to enhance the photon production is to have the mirror as the wall of a resonating cavity. In this case, if the repetition rate of the laser is about twice a resonance frequency of the cavity, a parametric amplification process occurs, resulting in an enhancement of the number of photons by a factor which depends on the Q-value of the cavity. To this end, superconducting cavities are employed in the experiment.