Optimized Single Flux Quantum Pulse Trains for High-fidelity Qubit Control

The hardware overhead associated with microwave control is a major obstacle to scale-up of superconducting quantum computing. An alternative approach to qubit control involves irradiation of the qubits with trains of Single Flux Quantum (SFQ) pulses, pulses of voltage whose time integral is precisely quantized to the magnetic flux quantum. SFQ pulses can be generated and delivered to the qubit via a proximal classical Josephson digital circuit, offering the possibility of a streamlined, low-footprint classical coprocessor for monitoring errors and feeding back to the qubit array. In this talk, we describe an approach for the derivation and validation of complex SFQ pulse sequences in which classical bits are clocked to the qubit at a frequency that is a factor of a few higher than the qubit oscillation frequency, allowing for variable pulse-to-pulse timing in the qubit control sequence. Optimized sequences allow fast, coherent rotations in the qubit 0-1 subspace while suppressing leakage out of the computational manifold. We present simulation results and demonstrate that the performance of optimized SFQ control sequences is comparable to that of microwave-based sequences, with significantly reduced hardware requirements.