Qblox Qubit Control Module: Integrated solution for bias-tee-free high-fidelity two-qubit gates

The execution of quantum algorithms is limited by the fidelity of two-qubit gates. Most high-performance superconducting quantum processors rely on fast flux control. This involves dynamically controlling the qubit and/or coupler frequency with a stable flux-bias during idle times and rapid flux modulation during gates. These conflicting requirements have led to the use of separate ultrastable slow electronics (<<1 MSPS) for DC biasing and fast (>1 GSPS) electronics for pulsing, combined via a bias-tee. However, the bias-tee introduces pulse distortions. Predistortion filters aim to counter these but achieve only limited success due to the nonlinear behaviour of bias-tee filter capacitors, resulting in uncorrectable non-Markovian errors in repeated two-qubit gates. This highlights the need to eliminate bias-tees entirely.

The Qblox Qubit Control Module (QCM) offers an integrated solution, using a single source for both stable voltage offset and fast flux pulses, eliminating the bias-tee. We show that the dephasing in flux-tunable transmons remains unchanged when using the Qblox QCM as a DC biasing instrument, demonstrating the sufficiently low drift and low 1/f noise for this application. Additionally, the module incorporates real-time IIR and FIR digital filters to compensate remaining distortions caused by in-cable reflections, on-chip charging effects, and the skin effect. The result is repeatable flux pulses and improved two-qubit gate fidelity, with pulse amplitude errors reduced to less than 0.1%, even during back-to-back operations.