Curator's Take
This article tackles one of the most stubborn error channels in superconducting processors—qubit leakage—and shows that even when tunable couplers suppress exchange and ZZ interactions, residual hopping at 0.8‑10 MHz remains because of transmon anharmonicity. By quantifying how frequency detuning localizes leakage and identifying a modest 1‑4 MHz spread needed to block long‑range tunneling, the work provides concrete design rules that complement recent efforts in active leakage detection and error‑corrected gates. The proposed passive removal units, which exploit either a pumped transmon or a junction readout scheme, could be integrated into near‑term devices to continuously mop up leaked excitations without added control overhead.
— Mark Eatherly
Summary
Qubit leakage is a noticeable source of errors for quantum computing. In quantum processors, leakage excitations traveling between qubits generate correlated errors and perturb gate implementations. Leakage mobility can also be utilized for creating dedicated leakage removal pathways and removal units. To quantitatively characterize leakage mobility and to guide better design of processor architectures, we study here leakage dynamics in transmons with tunable couplers through numerical and analytical methods. Even if the couplers are tuned to cancel the single-excitation exchange or the ZZ interaction, the leakage hopping rates still persists in the range of 0.8-10 MHz due to transmon nonlinearity. In typical operation regimes, however, transmon frequency detuning localizes leakage excitations. The next-nearest-neighbor transmons can be still be near-resonant opening leakage tunneling channels. To suppress longer-range hopping, we find that the frequency spread of the next-nearest-neighbor transmons needs to be in the range of 1-4 MHz. Utilizing leakage mobility, we propose two passive leakage removal units. One is based on a tunable coupler and a pumped transmon, and another on a junction readout scheme. Based on realistic experimental parameters, our results on selectively mobilizing or localizing leakage excitations are readily applicable in superconducting quantum devices.