High-fidelity controlled-σ Z gate for resonator-based superconducting quantum computers

摘要

A possible building block for a scalable quantum computer has recently been demonstrated [Mariantoninet al., Science 334, 61 (2011)]. This architecture consists of superconducting qubits capacitively coupled both tonindividual memory resonators as well as a common bus. In this work we study a natural primitive entangling gatenfor this and related resonator-based architectures, which consists of a controlled-σz (CZ) operation between anqubit and the bus. The CZ gate is implemented with the aid of the noncomputational qubit |2u0002 state [Strauch et al.,nPhys. Rev. Lett. 91, 167005 (2003)]. Assuming phase or transmon qubits with 300 MHz anharmonicity, we shownthat by using only low frequency qubit-bias control it is possible to implement the qubit-bus CZ gate with 99.9%n(99.99%) fidelity in about 17 ns (23 ns) with a realistic two-parameter pulse profile, plus two auxiliary z rotations.nThe fidelity measure we refer to here is a state-averaged intrinsic process fidelity, which does not include anyneffects of noise or decoherence. These results apply to a multiqubit device that includes strongly coupled memorynresonators.We investigate the performance of the qubit-bus CZ gate as a function of qubit anharmonicity, identifynthe dominant intrinsic error mechanism and derive an associated fidelity estimator, quantify the pulse shapensensitivity and precision requirements, simulate qubit-qubit CZ gates that are mediated by the bus resonator,nand also attempt a global optimization of system parameters including resonator frequencies and couplings. Ournresults are relevant for a wide range of superconducting hardware designs that incorporate resonators and suggestnthat it should be possible to demonstrate a 99.9% CZ gate with existing transmon qubits, which would constitutenan important step towards the development of an error-corrected superconducting quantum computer.