Description Superconducting qubit relaxation rates are known to fluctuate on millisecond timescales due to time-varying system–environment couplings. Standard characterisation methods average over these fluctuations, obscuring regime transitions in effective coherence. This work introduces a task-calibrated boundary-geometry formulation of qubit stability in which the time-dependent decoherence-per-gate ratio ρ(t) defines a stability margin relative to a declared performance threshold ρ*. A threshold-conditional scheduling policy is derived that parks computation when the stability margin becomes negative. Ensemble stochastic telegraph simulations (N = 200 accepted realisations) demonstrate 78.7% per-gate error reduction at equal useful-gate count and 19.7% active-time reduction, in agreement with analytical prediction (79.2%). Estimator-delay sensitivity is quantified via a dimensionless parameter τ̂ = τ_est · k↓, yielding a falsifiable requirement τ̂ ≲ 0.2 for sustained benefit. The framework does not claim microscopic mechanism closure. It provides a governance-constrained, falsifiable control architecture for stabilising coherence under rapid relaxation fluctuations and defines explicit promotion/demotion conditions under TRIDENT/USDOP v1.2. Keywords Superconducting qubits Qubit relaxation fluctuations Real-time adaptive estimation Decoherence mitigation Threshold-conditional scheduling Coherence stabilisation Boundary-state formalism Stability margin Telegraph noise model Stochastic control Estimator latency Quantum hardware control architecture TRIDENT governance USDOP v1.2 SHRF framework Related corpus works • TRIDENT-Z0 Diagnostic Framework: USDOP v1.2https://doi.org/10.5281/zenodo.18721255 • Optimization as Fragility: Boundary Over-Control and Coherence Collapsehttps://doi.org/10.5281/zenodo.18686580
Description Superconducting qubit relaxation rates are known to fluctuate on millisecond timescales due to time-varying system–environment couplings. Standard characterisation methods average over these fluctuations, obscuring regime transitions in effective coherence. This work introduces a task-calibrated boundary-geometry formulation of qubit stability in which the time-dependent decoherence-per-gate ratio ρ(t) defines a stability margin relative to a declared performance threshold ρ*. A threshold-conditional scheduling policy is derived that parks computation when the stability margin becomes negative. Ensemble stochastic telegraph simulations (N = 200 accepted realisations) demonstrate 78.7% per-gate error reduction at equal useful-gate count and 19.7% active-time reduction, in agreement with analytical prediction (79.2%). Estimator-delay sensitivity is quantified via a dimensionless parameter τ̂ = τ_est · k↓, yielding a falsifiable requirement τ̂ ≲ 0.2 for sustained benefit. The framework does not claim microscopic mechanism closure. It provides a governance-constrained, falsifiable control architecture for stabilising coherence under rapid relaxation fluctuations and defines explicit promotion/demotion conditions under TRIDENT/USDOP v1.2. Keywords Superconducting qubits Qubit relaxation fluctuations Real-time adaptive estimation Decoherence mitigation Threshold-conditional scheduling Coherence stabilisation Boundary-state formalism Stability margin Telegraph noise model Stochastic control Estimator latency Quantum hardware control architecture TRIDENT governance USDOP v1.2 SHRF framework Related corpus works • TRIDENT-Z0 Diagnostic Framework: USDOP v1.2https://doi.org/10.5281/zenodo.18721255 • Optimization as Fragility: Boundary Over-Control and Coherence Collapsehttps://doi.org/10.5281/zenodo.18686580
The fidelity of operations on a solid-state quantum processor is fundamentally bounded by environmental decoherence. Characterizing environmental fluctuations is challenging because the acquisition time of nonadaptive experimental protocols limits temporal precision and can average out rapid features of the underlying dynamics. Here, we overcome this temporal-resolution limit by 2 orders of magnitude using a field-programmable gate-array powered classical controller that adaptively and continuously tracks the relaxation-time fluctuations of two fixed-frequency superconducting transmon qubits, which exhibit average relaxation times of approximately 0.17 ms and occasionally exceed 0.5 ms. We report events in which the relaxation time switches by nearly an order of magnitude over timescales of just tens of milliseconds, rather than minutes or hours as previously reported. Our real-time Bayesian estimation protocol estimates relaxation times within a few milliseconds, close to the decoherence timescale itself. Our statistical analysis further suggests that some of these fast fluctuations arise from two-level systems switching at rates up to 10 Hz, 4 orders of magnitude faster than earlier reports. These results redefine the timescales relevant for calibration in superconducting quantum processing units, establish a reference for rapid relaxation-rate characterization in device screening, and improve our understanding of fast relaxation dynamics.
Superconducting qubit parameters drift on sub-second timescales, motivating calibration and benchmarking techniques that can be executed on millisecond timescales. We demonstrate an on-FPGA workflow that co-locates pulse generation, data acquisition, analysis, and feed-forward, eliminating CPU round trips. Within this workflow, we introduce sparse-sampling and on-FPGA inference tools, including computationally efficient methods for estimation of exponential and sine-like response functions, as well as on-FPGA implementations of Nelder-Mead optimization and golden-section search. These methods enable low-latency primitives for readout calibration, spectroscopy, pulse-amplitude calibration, coherence estimation, and benchmarking. We deploy this toolset to estimate T1 in 10 ms, optimize readout parameters in 100 ms, optimize pulse amplitudes in 1 ms, and perform Clifford randomized gate benchmarking in 107 ms on a flux-tunable superconducting transmon qubit. Running a closed-loop on-FPGA recalibration protocol continuously for 6 hours enables more than 74,000 consecutive recalibrations and yields gate errors that consistently retain better performance than the baseline initial calibration. Correlation analysis shows that recalibration suppresses coupling of gate error to control-parameter drift while preserving a coherence-linked performance. Finally, we quantify uncertainty versus time-to-decision under our sparse sampling approaches and identify optimal parameter regimes for efficient estimation of qubit and pulse parameters.
The fidelity of operations on a solid-state quantum processor is fundamentally bounded by environmental decoherence. Characterizing environmental fluctuations is challenging because the acquisition time of nonadaptive experimental protocols limits temporal precision and can average out rapid features of the underlying dynamics. Here, we overcome this temporal-resolution limit by 2 orders of magnitude using a field-programmable gate-array powered classical controller that adaptively and continuously tracks the relaxation-time fluctuations of two fixed-frequency superconducting transmon qubits, which exhibit average relaxation times of approximately 0.17 ms and occasionally exceed 0.5 ms. We report events in which the relaxation time switches by nearly an order of magnitude over timescales of just tens of milliseconds, rather than minutes or hours as previously reported. Our real-time Bayesian estimation protocol estimates relaxation times within a few milliseconds, close to the decoherence timescale itself. Our statistical analysis further suggests that some of these fast fluctuations arise from two-level systems switching at rates up to 10 Hz, 4 orders of magnitude faster than earlier reports. These results redefine the timescales relevant for calibration in superconducting quantum processing units, establish a reference for rapid relaxation-rate characterization in device screening, and improve our understanding of fast relaxation dynamics.
Superconducting qubits are an attractive platform for quantum computing since they have demonstrated high-fidelity quantum gates and extensibility to modest system sizes. Nonetheless, an outstanding challenge is stabilizing their energy-relaxation times, which can fluctuate unpredictably in frequency and time. Here, we use qubits as spectral and temporal probes of individual two-level-system defects to provide direct evidence that they are responsible for the largest fluctuations. This research lays the foundation for stabilizing qubit performance through calibration, design, and fabrication.
This technical note applies the TRIDENT-Z0 diagnostic framework to environment-induced decoherence in standard Markovian open quantum systems. When a quantum system couples to an external environment, off-diagonal elements of its reduced density matrix decay in the environment-selected pointer basis, suppressing interference and destroying macroscopic coherence. The dynamics are governed by Lindblad master equations under the Born-Markov approximation.--- SCOPE BOUNDARY ---The evaluation is explicitly restricted to weak-coupling, memoryless (Markovian) dynamics satisfying:tau_env << tau_sysgamma << 1 / tau_envNon-Markovian dynamics, objective collapse models (GRW, Penrose), many-body localization, cosmological decoherence, and active quantum error correction protocols are excluded from scope.--- QUANTITATIVE STRUCTURE ---Decoherence rates are determined by system-environment coupling strength g, environmental spectral density J(omega), and temperature T. A dimensionless decoherence ratio is defined:eta = tau_d / tau_syswhere tau_d is the decoherence (T2) timescale and tau_sys is the intrinsic dynamical timescale of the isolated system.Coherence is preserved when eta >> 1.Coherence collapses when eta <= 1.The boundary is a continuous crossover governed by a dimensionless ratio, not a sharp thermodynamic phase transition.--- TRIDENT GATE ASSESSMENT ---G1 - Reproducibility: PASSG2 - Mechanistic grounding: PASSG3 - Quantitative predictability: PASSG4 - Boundary-defined coherence dependency: CONDITIONAL PASSCoherence is extrinsic and boundary-dependent. Restoration requires boundary modification (isolation, cooling, reduction of coupling g, or error correction). Unlike equilibrium quantum ordered systems, the coherent state is not the ground state and must be maintained against environmental interaction.This case card provides the open-system quantum coherence collapse counterpart to the intrinsic equilibrium coherence anchor established in PHYS-BCS-001.v1 and completes the quantum coherence axis of the PHYS node within the TRIDENT corpus.Keywords Quantum decoherenceOpen quantum systemsLindblad master equationBorn-Markov approximationEinselectionPointer basisQuantum coherenceDephasing T2Density matrix dynamicsSystem-environment couplingSpectral density J(omega)Ohmic environmentQuantum-to-classical transitionDimensionless timescale ratioBoundary-defined coherenceTRIDENT diagnostic frameworkSHRF corpusCoherence collapseMacroscopic quantum coherenceEvidence-bound analysis
Superconducting qubit coherence has a precision problem. Relaxation time T1and decoherence time T2are by far the most common benchmarks of single qubit performance in superconducting quantum circuits, and are heavily relied upon for engineering next-generation devices. However, these metrics have strong environmental sensitivities and are temporally unstable, and thus alone are not robust metrics for describing individual qubits and comparing them against one another.In this tutorial, the measurement and interpretation of superconducting qubit coherence metrics is detailed, and useful qubit metrics are outlined that consider current knowledge of T1and T2variation sources.