Probing Virtual Axion-Like Particles by Precision Phase Measurements
  We propose an experiment for detecting Axion-Like Particles (ALPs) based on
the axion-photon interaction in the presence of a non-uniform magnetic field.
The impact of virtual ALPs on the polarization of the photons inside a cavity
is studied and a detection scheme is proposed. We find that the cavity normal
modes are dispersed differently owing to their coupling to the ALPs in the
presence of a background magnetic field. This birefringence, in turn, can be
observed as a phase difference between the cavity polarization modes. The
signal is considerably enhanced close to the resonance frequencies of the
cavity and further enhanced for a squeezed light source. We propose to scan the
resonances with a variable frequency source. We argue that the amplified signal
allows for exclusion of a broad range of axion mass $10^{-3}\text{eV} \lesssim
m_{a} \lesssim 1 \text{eV}$ even at very small axion-photon coupling constant
with the potential to reach sensitivity to the QCD axion. Our scheme allows for
the exclusion of a range of axion masses that has not yet been covered by other
experimental techniques.
