Details of the performance, capabilties, physics and scaling of the state of the art in superconducting circuits for quantum computers

Superconducting Circuits for Quantum Information: An Outlook – The performance of superconducting qubits has improved by several orders of magnitude in the past decade. These circuits benefit from the robustness of superconductivity and the Josephson effect, and at present they have not encountered any hard physical limits. However, building an error-corrected information processor with many such qubits will require solving specific architecture problems that constitute a new field of research. For the first time, physicists will have to master quantum error correction to design and operate complex active systems that are dissipative in nature, yet remain coherent indefinitely. We offer a view on some directions for the field and speculate on its future.

Superconducting qubits: Desired parameter margins for scalability and the corresponding demonstrated values. Desired capability margins are numbers of successful operations or realizations of a component before failure. For the stability of the Hamiltonian, capability is the number of Ramsey shots that meaningfully would provide one bit of information on a parameter (e.g., the qubit frequency) during the time when this parameter has not drifted. Estimated current capability is expressed as number of superconducting qubits, given best decoherence times and success probabilities. Demonstrated successful performance is given in terms of the main performance characteristic of successful operation or Hamiltonian control (various units). A reset qubit operation forces a qubit to take a particular state. A Rabi flop denotes a single-qubit p rotation. A swap to bus is an operation to make a two-qubit entanglement between distant qubits. In a readout qubit operation, the readout must be QND or must operate on an ancilla without demolishing any memory qubit of the computer. Stability refers to the time scale during which a Hamiltonian parameter drifts by an amount corresponding to one bit of information, or the time scale it would take to find all such parameters in a complex system to this precision. Accuracy can refer to the degree to which a certain Hamiltonian symmetry or property can be designed and known in advance, the ratio by which a certain coupling can be turned on and off during operation, or the ratio of desired to undesired couplings. Yield is the number of quantum objects with one degree of freedom that can be made without failing or being out of specification to the degree that the function of the whole is compromised. Complexity is the overall number of interacting, but separately controllable, entangled degrees of freedom in a device. Question marks indicate that more experiments are needed for a conclusive result. Values given in rightmost column are compiled from recently published data and improve on a yearly basis.

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