Measuring Quantum Noise Limits in Superconducting Digital Circuits

A fundamental question in Josephson junction physics is the temperature at which quantum tunneling becomes the dominant source of junction escape as opposed to thermal excitation. This quantum crossover temperature is important to the design of classical superconducting circuits, as it dictates the minimum error rate that can be achieved for a given junction size. This property has been previously described and measured for over forty years, but the measurement technique is limited to single junctions and not extensible to junctions embedded in a larger circuit. Here we demonstrate a technique for measuring the quantum crossover temperature for a superconducting digital circuit by examining the width of the transition from operation to failure in a basic Reciprocal Quantum Logic digital circuit. This transition width is extracted from the broadband noise generated from the circuit errors and further, disambiguates from spectral noise, such as line noise, that can broaden this transition and artificially increase the crossover temperature. Application of this technique to a medium sized circuit demonstrates that the quantum crossover temperature of a junction embedded in a larger circuit is indistinguishable from the crossover temperature extracted from an isolated junction.