Low-temperature upper limit of the photon mass: Experimental null test of Ampère’s law

We describe a new method to set an upper limit of the photon mass m_γ at low temperatures, based on a null test of Amp\`ere's law. The result of our measurements, expressed as the upper limit of the photon mass at 1.24 K, is m_γ{≤}(8.4±{}0.8)×{}10^-46 g.