Digital qubit readout with a flux-switchable superconducting circuit
Quantum computing platforms based on superconducting qubits have emerged as one of the most promising candidates in the race to build a large scale quan- tum computer . Controllability, standard chips fabrication techniques com- bined with the possibility of exploring unconventional hybrid systems  are well established advantages of superconducting qubits architectures as quantum processors. However, while the performance of small superconducting quantum processors has advanced the threshold necessary for fault tolerance, the current technique to control and readout the qubit state imposes severe system scaling challenges . Within this framework, digital control based on cryogenic energy- efficient superconducting Single Flux Quantum (SFQ) logic is being adapted to perform qubit control and readout for scalable quantum 3D-architectures . This is leading to the development of innovative concepts for quantum proces- sor control and benchmarking in this integrated digital-quantum hybrid system.
Here, we propose an SFQ-compatible approach to accomplish diabatic read- out of superconducting qubits based on a Josephson Digital Phase Detector (JDPD). When properly excited by flux bias pulse, the JDPD is able to 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 frequency. The basic concepts behind this new readout scheme have been experimentally verified with a preliminary version of the JDPD. The capability to work as a phase detector has been demon- strated up to 100kHz tone with a remarkable agreement between the experi- mental outcomes and simulations .
By choosing design parameters, the JDPD will be sensitive at frequency in the range of GHz, the typical frequency of superconducting qubits. These character- istics make the JDPD suitable for the implementation of a high speed platform integrated with superconducting digital electronics for both control and readout the qubit’s state directly at 20 mK, providing a solid solution for highly scalable superconducting quantum processors.
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