Taneli Tolppanen

Leakage Mobility and Passive Leakage Removal in Transmons with Tunable Couplers

  1. Taneli Tolppanen,
  2. Gonzalo Martín-Vázquez,
  3. Sasu Tuohino,
  4. and Matti Silveri
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.
arxiv pdf

Passive leakage removal unit based on a disordered transmon array

  1. Gonzalo Martín-Vázquez,
  2. Taneli Tolppanen,
  3. and Matti Silveri
Leakage out from the qubit subspace compromises standard quantum error correction protocols and is a challenge for practical quantum computing. We propose a passive leakage removal
unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. The transmons have parametric disorder both in frequency and anharmonicity such that the qubit subspace is protected by localization through energy level mismatch while the energy levels for leakage excitations are in resonance for maximized leakage mobility. Leakage excitations propagate through the idle transmons until reaching the last site with feedback-measurement or dissipation removing them. For removing leakage excitations, we find two optimal measurement rates, which are comprehensively understood through two distinct timescales between the propagation and disintegration of leakage excitations. Based only on an array of standard transmon devices, our approach is readily compatible with existing superconducting quantum processor designs under realistic conditions.
arxiv pdf