Charting the circuit QED design landscape using optimal control theory

Abstract

With recent improvements in coherence times, superconducting transmon qubits have become a promising platform for quantum computing. They can be flexibly engineered over a wide range of parameters, but also require us to identify an efficient operating regime. Using state-of-the-art quantum optimal control techniques, we exhaustively explore the landscape for creation and removal of entanglement over a wide range of design parameters. We identify an optimal operating region outside of the usually considered strongly dispersive regime, where multiple sources of entanglement interfere simultaneously, which we name the quasi-dispersive straddling qutrits regime. At a chosen point in this region, a universal gate set is realized by applying microwave fields for gate durations of 50 ns, with errors approaching the limit of intrinsic transmon coherence. Our systematic quantum optimal control approach is easily adapted to explore the parameter landscape of other quantum technology platforms.Quantum computers: global sweet spot for superconducting circuit designThe optimal working regime for so-called transmon qubits exploits a subtle interplay of interaction and quantum interference. Their flexibility in design places superconducting qubits among the hottest contestants in the current race for a quantum computer. Researchers from the University of Kassel, Germany, and the University of California at Berkeley have used optimal control techniques to map out the full range of design parameters with which a superconducting circuit of two transmission-line-coupled qubits can be engineered. Each parameter point was analyzed for its ability to support all the operations required for a universal quantum computer, and the optimum is found in a regime where the qubits straddle each other. These results will guide the design choices for a superconducting quantum computer operating at maximum speed and precision.