arXiv subjects

Taneli Tolppanen

Publications and source records attributed to Taneli Tolppanen.

3 recordsLinked to original sources

arXiv2026

Leakage Mobility and Passive Leakage Removal in Transmons with Tunable Couplers

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.

quant-ph↗

arXiv2025

Passive leakage removal unit based on a disordered transmon array

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.

quant-ph↗

arXiv2024

Phase transitions induced by standard and predetermined measurements in transmon arrays

The confluence of unitary dynamics and non-unitary measurements gives rise to intriguing and relevant phenomena, generally referred to as measurement-induced phase transitions. These transitions have been observed in quantum systems composed of trapped ions and superconducting quantum devices. However, their experimental realization demands substantial resources, primarily owing to the classical tracking of measurement outcomes, known as post-selection of trajectories. In this work, we first describe the statistical properties of an interacting transmon array which is repeatedly measured, and predict the behavior of relevant quantities in the area-law phase using a combination of the replica method and non-Hermitian perturbation theory. We show numerically that a transmon array, modeled by an attractive Bose-Hubbard model, in which local measurements of the number of bosons are probabilistically interleaved, exhibits a phase transition in the entanglement entropy properties of the ensemble of trajectories in the steady state. Furthermore, by using deterministic feedback operations after the local number measurements, the distribution of the number of bosons measured at a single site carries information on the phase in the entanglement of individual trajectories. Interestingly, we can extract information about the phase and the phase transition from simple observables without considering an absorbing state in the feedback pattern. This implies that the feedback measurement approach might be a viable experimental option to use simple observables to study some aspects of the entanglement phase transition in individual trajectories.

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