UC Berkeley

In harnessing quantum advantages for computation, there is a need for developing high fidelity operations on qubits. An algorithm can be broken down into single qubit operations and multi-qubit entangling gates. However, as the leading quantum processors today are limited to 50-100 qubits and each qubit is sensitive to decoherence noise (often referred to as NISQ era devices), running algorithms with long gate depth is difficult. Understanding the errors that plague existing gates and also expanding the dictionary of available gates is an important part of building a quantum computer. In this thesis we demonstrate two multiqubit gate experiments.

In the first experiment we demonstrate a multiqubit entangling gate for superconducting qubits on an all-to-all connected processor that draws upon the advantages of Rabi driven qubits. We also take inspiration from the ion qubit community by using a Mølmer-Sørensen-like interaction through the use of a shared coplanar waveguide (CPW) resonator driven superconducting qubits. We perform sensitivity analysis to understand the parameters that limit our gate fidelities.

In the second experiment we introduce and demonstrate a technique for scalable RB of many universal and continuously parameterized gate sets, using a class of circuits called randomized mirror circuits. The technique can be applied to a gate set containing an entangling Clifford gate and the set of arbitrary single-qubit gates, as well as gate sets containing controlled rotations about the Pauli axes. We use our technique to benchmark universal gate sets on four qubits, including a gate set containing a controlled-S gate and its inverse, and we investigate how the observed error rate is impacted by the inclusion of non-Clifford gates. We also show that our technique scales to many qubits with experiments on a 27-qubit IBM Q processor. We use our technique to quantify the impact of crosstalk on this 27-qubit device, and we find that it contributes approximately 2/3 of the total error per gate in random many-qubit circuit layers.