Searching for axion-like particles under strong gravitational lenses
  We establish strong gravitational lens systems as robust probes of axion-like
particles (ALPs) -- a candidate for dark matter. A tiny interaction of photons
with ALPs induces birefringence. Multiple images of gravitationally lensed
polarised objects allow differential birefringence measurement, alleviating
systematics and astrophysical dependencies. We apply this novel method to the
lens system CLASS B1152+199 and constrain ALP-photon coupling $\le 9.2\times
10^{-11}\, {\rm GeV}^{-1} \textrm{ to } 7.7\times 10^{-8}\, {\rm GeV}^{-1}$
($95\%$ C.L.) for ALP mass between $3.6\times 10^{-21} \,{\rm eV}$ and $4.6
\times 10^{-18} \,{\rm eV}$. A larger sample will improve the constraints.
