Characterization and benchmarking of a phase-sensitive two-qubit gate using direct digital synthesis*

We implemented an iSWAP gate with two transmon qubits using a flux-tunable coupler.  Precise control of the relative phase of the qubit-control pulses and the parametric coupler drive was achieved with a multi-channel instrument called Presto [1], using direct digital synthesis (DDS).  We describe the process of tuning and benchmarking the iSWAP gate, where the relative phase of the pulses is controlled via software.  Traditional iSWAP gates require multiple local oscillators and mixers with advanced synchronization methods based on external triggering.  Presto synchronizes and generates all outputs using two-step digital mixing, where pulse envelopes are defined as arbitrary waveforms at 1 GS/s. In the first step the envelopes are mixed with a digital carrier where phase (and frequency) can be set arbitrarily each time a pulse is generated. In the second step the signal is upconverted to the sampling rate of the converter and digitally mixed to the target frequency.  With Presto we perform the iSWAP gate in 254 ns, validate it with quantum-state tomography, and evaluate its fidelity with interleaved randomized benchmarking.

[1] Tholén et al., arXiv:2205.15253 [quant-ph] (2022).