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Phase Diffusion in Low-$E_J$ Josephson Junctions at milli-Kelvin Temperatures

Josephson junctions (JJs) with Josephson energy $E_J \lesssim 1K$ are widely employed as non-linear elements in superconducting circuits for quantum computing, operating at milli-Kelvin temperatures. Here we experimentally study incoherent phase slips (IPS) in low-$E_J$ Aluminum-based JJs at $T<0.2K$, where the IPS become the dominant source of dissipation. We observed strong suppression of the critical (switching) current and a very rapid growth of the zero-bias resistance with decreasing Josephson energy below $E_J \sim 1K$. This behavior is attributed to the IPSs whose rate exponentially increases with decreasing the ratio $E_J/T$. Our observations are in line with other data reported in literature. With further improvement of coherence of superconducting qubits, the observed dissipation from IPS might limit the performance of qubits based on low-$E_J$ junctions.