A programmable pulse combiner for precision control of silicon spin qubits

A pulse combiner has been developed to control silicon spin qubits, which require precisely tuned rectangular waveforms for high-fidelity operation. Typically, the AC and DC components of a pulse signal are delivered separately through coaxial cables and discrete wires in a refrigerator, then recombined near the qubit using a bias tee to prevent heat generation from the DC passing through attenuators. However, this introduces pulse distortion due to low-frequency cut-off at the bias tee capacitor, resulting in the degradation of qubit control fidelity and inaccuracy in qubit tune-ups. Conventional work employs an adjustable resistive adder to combine the low-frequency component of the pulse with the DC. However, such manual adjustment of the variable resistor affects the impedance of the overall adder, leading to instability in the DC voltage.

To address these challenges, we propose a new pulse combiner exploiting a transimpedance amplifier and an active adder circuit. This design enables precise waveform adjustments while maintaining DC voltage stability, since the tunable resistor is fully decoupled from the DC adder. The variable resistor is composed of highly programable 20-bit R-2R DAC, allowing fine-tuning of the pulse shape with high resolutions. We set up the pulse combiner with 48 channels in a qubit test environment and verified its pulse shaping functionality. This approach ensures stable and accurate gate pulsing for silicon spin qubits, enabling precision control of large-scale qubits.