Fixed-frequency superconducting qubits demonstrate remarkable success as platforms for stable and scalable quantum computing. Cross-resonance gates have been the workhorse of fixed-coupling,fixed-frequency superconducting processors, leveraging the entanglement generated by driving one qubit resonantly with a neighbor’s frequency to achieve high-fidelity, universal CNOTs. Here, we use on-resonant and off-resonant microwave drives to go beyond cross-resonance, realizing natively interesting two-qubit gates that are not equivalent to CNOTs. In particular, we implement and benchmark native ISWAP, SWAP, ISWAP‾‾‾‾‾‾‾√, and BSWAP gates. Furthermore, we apply these techniques for an efficient construction of the B-gate: a perfect entangler from which any two-qubit gate can be reached in only two applications. We show these native two-qubit gates are better than their counterparts compiled from cross-resonance gates. We elucidate the resonance conditions required to drive each two-qubit gate and provide a novel frame tracking technique to implement them in Qiskit.
Qubit control electronics composed of CMOS circuits are of critical interest for next generation quantum computing systems. A CMOS-based application specific integrated circuit (ASIC)fabricated in 14nm FinFET technology was used to generate and sequence qubit control waveforms and demonstrate a two-qubit cross resonance gate between fixed frequency transmons. The controller was thermally anchored to the T = 4K stage of a dilution refrigerator and the measured power was 23 mW per qubit under active control. The chip generated single–side banded output frequencies between 4.5 and 5.5 GHz with a maximum power output of -18 dBm. Randomized benchmarking (RB) experiments revealed an average number of 1.71 instructions per Clifford (IPC) for single-qubit gates, and 17.51 IPC for two-qubit gates. A single-qubit error per gate of ϵ1Q=8e-4 and two-qubit error per gate of ϵ2Q=1.4e-2 is shown. A drive-induced Z-rotation is observed by way of a rotary echo experiment; this observation is consistent with expected qubit behavior given measured excess local oscillator (LO) leakage from the CMOS chip. The effect of spurious drive induced Z-errors is numerically evaluated with a two-qubit model Hamiltonian, and shown to be in good agreement with measured RB data. The modeling results suggest the Z-error varies linearly with pulse amplitude.
We present an improvement to the cross resonance gate realized with the addition of resonant, target rotary pulses. These pulses, applied directly to the target qubit, are simultaneousto and in phase with the echoed cross resonance pulses. Using specialized Hamiltonian error amplifying tomography, we confirm a reduction of error terms with target rotary — directly translating to improved two-qubit gate fidelity. Beyond improvement in the control-target subspace, the target rotary reduces entanglement between target and target spectators caused by residual quantum interactions. We further characterize multi-qubit performance improvement enabled by target rotary pulsing using unitarity benchmarking and quantum volume measurements, achieving a new record quantum volume for a superconducting qubit system.