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arXiv:2512.18037

Stability studies on subtractively-fabricated CMOS-compatible superconducting transmon qubits

Chawki Dhieb, Johannes Weber, Samuel Taubenberger, Carla Moran Guizan, Simon J. K. Lang, Zhen Luo, Emir Music, Alwin Maiwald, Wilfried Lerch, Lars Nebrich, Marc Tornow, Thomas Mayer, Daniela Zahn, Rui N. Pereira, Christoph Kutter

quant-phcond-mat.supr-conSubmitted Dec 19, 2025• Announced Dec 23, 2025• 0 citations
v1 · Dec 19, 2025v2 · Jul 13, 2026
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Abstract

Developing fault-tolerant quantum processors with error correction demands large arrays of physical qubits whose key performance metrics (coherence times, control fidelities) must remain within specifications over both short and long timescales. Here we investigated the temporal stability of subtractively fabricated CMOS-compatible superconducting transmon qubits. During a single cooldown and over a period of 95 hours, we monitored several parameters for 8 qubits, including coherence times $T_1$ and $T_2^*$, which exhibit fluctuations originating primarily from the interaction between two-level system (TLS) defects and the host qubit. We also demonstrate that subtractively-fabricated superconducting quantum devices align with the theoretical predictions that higher mean lifetimes $T_1$ correspond to larger fluctuations. To assess long-term stability, we tracked two representative qubits over 10 cooldown cycles spanning more than one year. We observed an average total downward shift in both qubit transition frequencies of approximately 61 MHz within the thermal cycles considered. In contrast, readout resonator frequencies decreased only marginally. Meanwhile, $T_1$ exhibits fluctuations from cycle to cycle, but maintains a stable baseline value.

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Developing fault-tolerant quantum processors with error correction demands large arrays of physical qubits whose key performance metrics (coherence times, control fidelities) must remain within specifications over both short and long timescales. Here we investigated the temporal stability of subtractively fabricated CMOS-compatible superconducting transmon qubits. During a single cooldown and over a period of 95 hours, we monitored several parameters for 8 qubits, including coherence times T1T_1T1​ and T2∗T_2^*T2∗​, which exhibit fluctuations originating primarily from the interaction between two-level system (TLS) defects and the host qubit. We also demonstrate that subtractively-fabricated superconducting quantum devices align with the theoretical predictions that higher mean lifetimes T1T_1T1​ correspond to larger fluctuations. To assess long-term stability, we tracked two representative qubits over 10 cooldown cycles spanning more than one year. We observed an average total downward shift in both qubit transition frequencies of approximately 61 MHz within the thermal cycles considered. In contrast, readout resonator frequencies decreased only marginally. Meanwhile, T1T_1T1​ exhibits fluctuations from cycle to cycle, but maintains a stable baseline value.
Abstract We benchmark the decoherence of superconducting transmon qubits to examine the temporal stability of energy relaxation, dephasing, and qubit transition frequency. By collecting statistics during measurements spanning multiple days, we find the mean parameters T1‾\overline {T_1}T1​​ T 1 ¯ = 49 μs and T2∗‾\overline {T_2^ \ast }T2∗​​ T 2 * ¯ = 95 μs; however, both of these quantities fluctuate, explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine sub-millihertz switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability produces a 0.8 ms limit on the pure dephasing which we also observe. These findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.
Materials improvements in transmon qubits have yielded remarkable advances in qubit lifetimes (T1)and coherence times (T2). However, state-of-the-art superconducting qubits experience temporal fluctuations in their performance, which limit gate fidelities and quantum processor performance. We observe correlated increases in qubit relaxation and dephasing rates that last for minutes to hours, which we call T1 and T2 excursions. Using interleaved lifetime and coherence time measurements, noise spectroscopy, and two-level-system (TLS) spectroscopy, we find that these excursions are caused by single or few strongly-coupled TLS defects that spectrally diffuse into resonance with the qubit frequency. Our measurement protocol provides a powerful tool for understanding the noise sources that cause temporal fluctuations in superconducting qubit performance.
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.
Quantum information processing holds the promise of a paradigm shift in computational power for specific problem classes. While contemporary quantum processors operate in the Noisy Intermediate-Scale Quantum (NISQ) regime and remain highly susceptible to errors, ongoing experimental efforts on these near-term devices are crucial for advancing the underlying technology. Among the various hardware implementations, superconducting quantum circuits stand out as one of the most robust architectures for scalable quantum technologies. Nevertheless, the primary bottleneck in these systems is the fragility of quantum information: interactions with the environment inevitably perturb the quantum states, a phenomenon known as decoherence. This rapid loss of stored information imposes a fundamental limit on the time available to reliably execute computations on these devices. Within this context, this thesis focuses on the comprehensive experimental characterization of a planar chip featuring isolated, fixed-frequency superconducting transmon qubits. The measurements were conducted in a cryogenic environment, relying on a dilution refrigerator to maintain the device at millikelvin temperatures. To probe the quantum system under these conditions, custom measurement scripts were developed to operate state-of-the-art microwave control electronics (Keysight Quantum Control System), establishing full control over pulse generation, signal acquisition, and digital demodulation. The experimental workflow systematically mapped the physical properties of the device through a progressive series of characterization steps. Initial spectroscopic techniques probed the coupled resonator-qubit system, identifying its fundamental transition frequencies and relevant energy scales. Building upon these frequency-domain measurements, coherent control was achieved by calibrating resonant microwave pulses to drive single-qubit rotations, enabling the reliable preparation of the qubit in excited or coherent superposition states. Finally, specific time-domain protocols were implemented to evaluate the operational limits of the device. By executing standard pulse sequences, the characteristic decoherence metrics of the system, namely the longitudinal energy relaxation time (T1) and the inhomogeneous dephasing time (T2*), were successfully extracted, providing a quantitative assessment of the qubit's overall performance and establishing a baseline for future implementations of more advanced quantum control protocols.
Quantum error correction using erasure qubits offers higher fault-tolerant thresholds and improved scaling by converting dominant physical errors into detectable erasures. In superconducting circuits, erasure qubits can be constructed using the dual-rail approach, which, however, requires additional qubit-count overhead and tailored coupling elements. Here, we demonstrate a hardware-efficient scheme that operates transmon qutrits as erasure qubits, which is compatible with standard superconducting circuit-QED hardware. The logical states ∣0L⟩\ket{0_\text{L}}∣0L​⟩ and ∣1L⟩\ket{1_\text{L}}∣1L​⟩ are represented by the ground and second excited states, while the dominant relaxation errors can be detected via an ancilla qubit using a microwave-activated two-qutrit SWAP gate. We demonstrate a logical qubit T1T_1T1​ lifetime exceeding 500 μs500\,μ\mathrm{s}500μs, post-selected with repeated mid-circuit erasure detection, which is ten times longer than the T1T_1T1​ time of the transmon physical qubit. Coherence times beyond 300 μs300\,μ\mathrm{s}300μs are achieved using dynamical decoupling. Single-qubit gate operations reach average Clifford gate infidelity on the order of 10−410^{-4}10−4. We further demonstrate dual-purposing an ancilla qubit for both erasure detection and parity checking, showing heralded generation of Bell states between erasure qubits. These results suggest that mainstream architectures of transmon qubit arrays may already be capable of implementing erasure-based QEC strategies for hardware-efficient fault-tolerant quantum computing.
The coherence of superconducting transmon qubits is often disrupted by fluctuations in the energy relaxation time (T1), limiting their performance for quantum computing. While background magnetic fields can be harmful to superconducting devices, we demonstrate that both trapped magnetic flux and externally applied static magnetic fields can suppress temporal fluctuations in T1 without significantly degrading its average value or qubit frequency. Using a three-axis Helmholtz coil system, we applied calibrated magnetic fields perpendicular to the qubit plane during cooldown and operation. Remarkably, transmon qubits based on tantalum-capped niobium (Nb/Ta) capacitive pads and aluminum-based Josephson junctions (JJs) maintained T1 lifetimes near 300 μs even when cooled in fields as high as 600 mG. Both trapped flux up to 600 mG and applied fields up to 400 mG reduced T1 fluctuations by more than a factor of two, while higher field strengths caused rapid coherence degradation. We attribute this stabilization to the polarization of paramagnetic impurities, the role of trapped flux as a sink for non-equilibrium quasiparticles (QPs), and partial saturation of fluctuating two-level systems (TLSs). These findings challenge the conventional view that magnetic fields are inherently detrimental and introduce a strategy for mitigating noise in superconducting qubits, offering a practical path toward more stable and scalable quantum systems.