This work develops a phenomenological model of residual decoherence in superconducting transmon qubits within the framework of State Locality Theory (SLT‑2). The model treats a qubit as a stable localized field configuration and introduces a frequency‑dependent localization rate that increases at lower transition energies. Using phenomenologically calibrated parameters, the model predicts a fundamental coherence‑time ceiling in the millisecond range and yields a falsifiable experimental signature: after subtracting all known noise mechanisms, the remaining decoherence rate should scale linearly with the inverse qubit frequency. Comparison with recent state‑of‑the‑art devices suggests that the SLT‑2 contribution may already constitute a significant fraction of the residual decoherence budget in low‑frequency qubits. The work also outlines an experimentally accessible protocol for testing this prediction using tunable‑frequency transmons. Additional SLT‑2 materials, extensions, and working notes are available in the associated OSF project: 10.17605/OSF.IO/CMVJ5.
This work develops a phenomenological model of residual decoherence in superconducting transmon qubits within the framework of State Locality Theory (SLT‑2). The model treats a qubit as a stable localized field configuration and introduces a frequency‑dependent localization rate that increases at lower transition energies. Using phenomenologically calibrated parameters, the model predicts a fundamental coherence‑time ceiling in the millisecond range and yields a falsifiable experimental signature: after subtracting all known noise mechanisms, the remaining decoherence rate should scale linearly with the inverse qubit frequency. Comparison with recent state‑of‑the‑art devices suggests that the SLT‑2 contribution may already constitute a significant fraction of the residual decoherence budget in low‑frequency qubits. The work also outlines an experimentally accessible protocol for testing this prediction using tunable‑frequency transmons. Additional SLT‑2 materials, extensions, and working notes are available in the associated OSF project: 10.17605/OSF.IO/CMVJ5.
TSR-Q-1 presents a preregistered and falsifiable experimental protocol designed to test for structured late-time coherence residuals in superconducting transmon qubits. The protocol does not treat generic recoherence, non-exponential decay, or non-Markovian behavior as evidence for the proposed framework, since such phenomena may arise from conventional open-system mechanisms. Instead, it targets a specific normalized temporal scaling law, tₙ/T₂ = nᵏ, with the preregistered theoretical value k* = 1.4706, corresponding to the first two target locations t₂/T₂ = 2.7714 and t₃/T₂ = 5.0310. If the associated amplitude-scaling hypothesis is retained, the predicted ratio is A₃/A₂ ≈ 0.551. The accompanying computational validation package develops the analysis pipeline through five stages: preliminary signal injection, blind null testing, blind injection-recovery, multi-qubit global-k inference, and sensitivity mapping. The synthetic null models include stretched-exponential decay, 1/f^α correlated noise, random-telegraph/TLS-like contributions, slow drift, white noise, and device-to-device variability. These simulations validate the detectability, specificity, and sensitivity of the proposed analysis procedure under the tested synthetic noise families. They do not constitute experimental evidence for the existence of the predicted physical effect. The next stage is a preregistered hardware implementation using superconducting qubits, with device-specific null calibration, blind analysis, systematic controls, held-out replication, and, ultimately, independent experimental replication.
Decoherence and parameter fluctuations are two of the mayor obstacles for solid-state quantum computing. In this work, decoherence in superconducting qubits of the transmon type is investigated. For this purpose, a time-multiplexed measurement protocol was developed and applied in long-term measurements. The resulting simultaneous measurement of the qubit's relaxation and dephasing rate, as well as its resonance frequency enables analysis of correlations between these parameters. A spectral noise analysis complements these measurements. Together, the results agree well with the interacting defect model of two-level-systems and yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our measurements show inherent correlations between dephasing and fluctuations in qubit frequency on the timescale of seconds to days, which is attributed to the influence of individual defects, located close to conductor edges. Cross-correlation and spectral noise analysis confirm this interpretation and ascribe the source of fluctuation to interactions between thermal fluctuators and surface defects. Single defects reducing the coherence of qubits by up to one order of magnitude are a major challenge for future quantum computers. Non-tunable qubits are intrinsically insensitive to some decoherence channels and thus ideal for this fundamental analysis. However, to widen the focus and contrast the results of different material systems, we pursue the fabrication of voltage controlled gatemon qubits. In the course of this work, the theoretical foundation and technical implementation of transmon qubits based on regular Josephson weak links, and semiconducting nanowires is given. The experimental design and measurement setup are explained in detail. Our findings make continuous re-calibration a necessity in today's solid-state qubits, although new materials or processing techniques might mitigate the problem. However, the results of this work imply that fundamental improvements of qubit parameter stability are necessary in order to realize scalable and coherent qubit circuits.
Transmon qubits experience open-system effects that manifest as noise at a broad range of frequencies. We present a model of these effects using the Redfield master equation with a hybrid bath consisting of low- and high-frequency components. We use two-level fluctuators to simulate 1/f -like noise behavior, which is a dominant source of decoherence for superconducting qubits. By measuring quantum state fidelity under free evolution with and without dynamical decoupling (DD), we can fit the low- and high-frequency noise parameters in our model. We train and test our model using experiments on quantum devices available through IBM quantum experience. Our model accurately predicts the fidelity decay of random initial states, including the effect of DD pulse sequences. We compare our model with two simpler models and confirm the importance of including both high frequency and 1/f noise in order to accurately predict transmon behavior. Published by the American Physical Society 2024
The tunnelling of quasiparticles across Josephson junctions in superconducting quantum circuits is an intrinsic decoherence mechanism for qubit degrees of freedom. Understanding the limits imposed by quasiparticle tunnelling on qubit relaxation and dephasing is of theoretical and experimental interest, particularly as improved understanding of extrinsic mechanisms has allowed crossing the 100 microsecond mark in transmon-type charge qubits. Here, by integrating recent developments in high-fidelity qubit readout and feedback control in circuit quantum electrodynamics, we transform a state-of-the-art transmon into its own real-time charge-parity detector. We directly measure the tunnelling of quasiparticles across the single junction and isolate the contribution of this tunnelling to qubit relaxation and dephasing, without reliance on theory. The millisecond timescales measured demonstrate that quasiparticle tunnelling does not presently bottleneck transmon qubit coherence, leaving room for yet another order of magnitude increase. Superconducting circuits are promising for quantum computing, but quasiparticle tunnelling across Josephson junctions introduces qubit decoherence. Ristè et al. convert a transmon qubit into its own real-time quasiparticle tunnelling detector and accurately measure induced decoherence in the millisecond range.
Quasiparticles, especially the ones near the Josephson junctions in the superconducting qubits, are known as an important source of decoherence. By injecting quasiparticles into a quantum chip, we characterized the diffusion feature by measuring the energy relaxation time and the residual excited-state population of a transmon qubit. From the extracted transition rates, we phenomenologically modeled the quasiparticle diffusion in a superconducting circuit that contained “hot” nonequilibrium quasiparticles in addition to low-energy ones.
Fault-tolerant quantum computing requires quantum gates that combine short duration, high fidelity, and robustness to noise. As quantum processors scale, robustness becomes increasingly important because exhaustive characterization and frequent recalibration of individual gates become impractical. We present a comprehensive optimal-control framework for controlled-Z (CZ) gates between directly coupled, frequency-tunable transmons that systematically accounts for intrinsic Hamiltonian errors and realistic decoherence mechanisms, including energy relaxation and dephasing. Using smooth, single-shot flux pulses, we obtain 12-ns CZ gates---close to the theoretical speed limit of 11.6 ns---with intrinsic errors suppressed to the 2×10−5 level. We construct dephasing noise models, calibrated against published experimental measurements, that capture fast dephasing from room-temperature control electronics and slow dephasing from 1/f flux noise. Using these models, we identify dephasing sweet spots that suppress sensitivity to dephasing noise. We further exploit transmon asymmetry as a design parameter to access distinct noise-resilient operating regimes. These results establish an experimentally applicable framework for balancing gate speed, fidelity, and noise robustness while reducing characterization and recalibration overhead.