Superconducting quantum computing is now at an important crossroad, where "proof of concept" experiments involving small numbers of qubits can be transitioned to more challenging and systematic approaches that could actually lead to building a quantum computer. Our optimism is based on two recent developments: a new hardware architecture for error detection based on "surface codes", and recent improvements in the coherence of superconducting qubits. I will explain how the surface code is a major advance for quantum computing, as it allows one to use qubits with realistic fidelities, and has a connection architecture that is compatible with integrated circuit technology. We have also recently demonstrated a universal set of logic gates in a superconducting Xmon qubit that achieves single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up to 99.4%. This places Josephson quantum computing at the fault-tolerant threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbor coupling. Using this device we have further demonstrated generation of the five-qubit Greenberger-Horne-Zeilinger (GHZ) state using the complete circuit and full set of gates, giving a state fidelity of 82% and a Bell state (2 qubit) fidelity of 99.5%. These results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.
Superconducting Qubits Poised for Fault-Tolerant Quantum Computation - A Review