Abstract
Implementation of high-fidelity 2-qubit operations is a key ingredient for scalable quantum error correction. In superconducting qubit architectures, tunable buses have been explored as a means to higher-fidelity gates. However, these buses introduce new pathways for leakage. Here we present a modified tunable bus architecture appropriate for fixed-frequency qubits in which the adiabaticity restrictions on gate speed are reduced. We characterize this coupler on a range of 2-qubit devices, achieving a maximum gate fidelity of 99.85%. We further show the calibration is stable over one day.
Highlights
Achieving high-fidelity 2-qubit (2Q) gates is one of the largest obstacles toward fault-tolerant quantum computation
Many approaches have been developed based on either fixed-frequency or tunable transmons
Tunable transmons naturally allow for fast iSWAP gates between the j10i and j01i states or controlled-Z (CZ) gates by utilizing the interaction between the j11i and j20i states [1,2]
Summary
With the bus far-detuned from qubits, we can approximate the effective coupling due to the tunable bus as Jbus ≈ 1⁄2ðgQ1;busgQ2;busÞ=Δ, where 1=Δ 1⁄4 1⁄21=ðωQ1 − ωbusÞþ 1=ðωQ2 − ωbusÞ. The direct capacitor between Q1 and Q2 adds a positive coupling term gQ1;Q2; the total effective coupling becomes Jtot ≈ Jbus þ gQ1;Q2 [12]. When ωbus > ωQ1;Q2, the farther the tunable bus is detuned from the qubits, the closer to zero the (negative) effective. For any positive small gQ1;Q2, it is always possible to find a zero in Jtot by adjusting ωbus.
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