The gravitational time delays of macrolenses can be used to constrain the rest mass of the photon with high accuracy. Assuming a point mass + external shear lens model, we prove that an upper limit of the photon mass can be derived directly from two observables—the time delay $\mathrm{\Delta }t$ and the leading-to-trailing flux ratio $R$ of strongly lensed fast radio bursts (FRBs). Using the observed values of $\mathrm{\Delta }t$ and $R$ of a lensed FRB candidate, i.e., FRB 20190308C, as a reference, we obtain a strict upper limit of the photon mass between $m_{\gamma }<5.3\times {10}^{-42}\mathrm{kg}$, for a given external shear strength of ${\gamma }^{\prime }=0.01$, and $m_{\gamma }<2.1\times {10}^{-41}-2.4\times {10}^{-42}\mathrm{kg}$, within the external shear range of $0<{\gamma }^{\prime }<1$. This provides the most stringent limit to date on the photon mass through gravitational lensing time delays, improving by 1 to 2 orders of magnitude the previous results obtained from lensed active galactic nuclei.

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• Gravitational lenses
• Particle astrophysics
• Radio, microwave, & sub-mm astronomy