Qblox Qubit Control Module: Generating high fidelity distortion-corrected flux pulses without a bias-tee

Flux-tunable superconducting qubits are at the forefront of high-fidelity quantum processors. However, the required combination of a low-drift biasing flux while idling and fast flux modulation during two qubit gates have led to the use of separate electronics for biasing and pulsing with a bias-tee connection. This crossover filter introduces distortions to the pulses leading to non-Markovian errors in repeated two-qubit gates. Predistortion filters, either precompiled in software or added in real-time by hardware, attempt to counter the effects. Nonetheless, their success is limited as a result of the non-linear characteristics exhibited by bias-tee capacitors, which fall outside the scope of linear time-invariant (LTI) assumptions in conventional filtering methods.

We present an integrated hardware solution, the Qblox Qubit Control Module, that combines the low drift and low 1/f noise of dedicated bias equipment with the speed of fast arbitrary waveform generators. Real-time IIR and FIR predistortion filters are further added to battle linear distortions like in-cable reflections, on-chip charging effects and the skin effect.This enables back-to-back operations in arbitrary control flows (feedback and feedforward), which are not possible with in-software precompiled waveform programs.

With the combination of bias-tee free operation and real-time filters, the error in amplitude of pulses for two-qubit gates is reduced to less than 0.1%, regardless of the number of preceding pulses.