Chip-based digital readout of a superconducting qubit

Fast, high-fidelity, and scalable readout of qubits is essential to realize fault-tolerant quantum computers (QC) and take full advantage of their disruptive computational paradigm. Existing experimental approaches to readout the state of a superconducting qubit necessitate microwave components at both room and cryogenic temperatures, providing significant technological and economic barriers to system scalability. It is highly desirable to implement a fast and high-fidelity chip-based readout scheme directly inside the fridge to reduce the overall system footprint and allow for low-latency operations. Here, we demonstrate high-fidelity on-chip digital readout of superconducting qubits based on the Josephson Digital Phase Detector (JDPD) [1] in both stand-alone and flip-chip configurations. When properly excited by an external flux, the JDPD can quickly switch from a single-minima to a double-minima potential and, consequently, relax in one of the two stable configurations discriminating between two phase values of a coherent input tone at GHz frequencies carrying information on the qubit state. The output of the JDPD is further digitized with superconducting single flux quantum (SFQ) comparator circuit [2] to provide the full digital readout solution. The presented innovative technology promises transformative impacts in quantum computing, facilitating scalability and integration with classical high-performance computing (HPC).