Balanced Comparator for RSFQ Qubit Readout

摘要

The balanced comparator is an essential part of RSFQ-based readout for superconducting qubits. In this paper we report measurements of the sensitivity of the balanced comparator fabricated in a specialized 30 ${rm A/cm}^{2}$ process with 3.6 $mu{rm A}$ critical currents and measured at 30–500 mK. At sampling frequency of 3.5 GHz the measured gray zone is $Delta I=104 {rm nA}$. Due to electron overheating in the shunt resistors the effective noise temperature is 140 mK. The gray zone has a strong dependence on sampling frequency above 5 GHz due to over-clocking. Related measurements have been reported where cooling fins and reduced critical current density of 10 ${rm A/cm}^{2}$ were used, and a quantum limited gray zone of 40 nA was achieved. This makes single shot readout possible for the phase, RF-SQUID and persistent current qubits. The readout speed is 30-50 ps, which is quite favorable for error correction schemes.