We present a comprehensive study of internal quality factors in superconducting stubgeometry 3-dimensional cavities made of aluminum. We use wet etching, annealing and electrochemichal polishing to improve the as machined quality factor. We find that the dominant loss channel is split between two-level system loss and an unknown source with 60:40 proportion. A total of 17 cavities of different purity, resonance frequency and size were studied. Our treatment results in reproducible cavities, with ten of them showing internal quality factors above 80 million at a power corresponding to an average of a single photon in the cavity. The best cavity has an internal quality factor of 115 million at single photon level.
We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips - one quantum chip and one control chip - that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.
Decoherence and gate errors severely limit the capabilities
of
state-of-the-art quantum computers. This work introduces a strategy
for reference-state error mitigation (REM) of quantum chemistry that
can be straightforwardly implemented on current and near-term devices.
REM can be applied alongside existing mitigation procedures, while
requiring minimal postprocessing and only one or no additional measurements.
The approach is agnostic to the underlying quantum mechanical ansatz
and is designed for the variational quantum eigensolver. Up to two
orders-of-magnitude improvement in the computational accuracy of ground
state energies of small molecules (H2, HeH+,
and LiH) is demonstrated on superconducting quantum hardware. Simulations
of noisy circuits with a depth exceeding 1000 two-qubit gates are
used to demonstrate the scalability of the method.
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