Superconducting circuits host two-level systems with lifetimes exceeding one second

arXiv:2610.21853 · quant-ph · Submitted 2026-10-15 · Read on arXiv

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Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: Today's paper: "Superconducting circuits host two-level systems with lifetimes exceeding one second".

Mira: The gist: The authors perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits to identify microscopic degrees of freedom that outlive the qubit by orders of magnitude,

Kai: First, who's behind it and why it matters.

Paper summary: Kai: So we’re looking at this paper, "Superconducting circuits host two-level systems with lifetimes exceeding one second." It’s about how these parasitic two-level systems in superconducting qubits limit qubit performance through decoherence, frequency drifts, and leakage Kai The main point here is that recently they’ve shown that long-lived TLSs with lifetimes longer than the state-of-the-art qubits actually cause correlated qubit dynamics which has direct implications for quantum error correction Kai They do this by performing wide-band quantum jump correlation spectroscopy on flux-tunable transmon qubits to find these microscopic degrees of freedom that outlive the qubit by orders of magnitude Kai

Mira: Right, so what they are claiming is that these TLSs are not just noise but they can actually cause correlated dynamics with the qubit which is a significant thing for how we try to build error correction codes Mira They find microscopic degrees of freedom that last much longer than the qubit itself and they identify them using this wide-band spectroscopy technique Mira It sets up the idea that these things are a real problem in quantum systems, not just some minor background noise Mira

Lev: From an engineering standpoint, if these TLSs live for seconds or more that’s a huge headache because you need your qubits to be coherent for the whole error correction process Lev It means we have this unintended memory channel that the qubit is coupled to and that’s messing with the coherence of the whole system Lev We gotta figure out how much overhead this kind of coupling adds when we try to run actual quantum error correction algorithms Lev

Kai: Exactly, Lev. The paper shows they did pump-probe measurements at varying detunings from these objects and found intrinsic lifetimes exceeding one second in some cases Kai They specifically looked at three long-lived TLSs in two tantalum electrode transmon qubits with standard aluminum Josephson junctions Kai This is a concrete experimental setup that gives us something real to look at, not just theory Kai

Mira: And the methodology they use involves this wide-band quantum jump correlation spectroscopy, which they deploy to reveal the non-Markovian part of the environment Mira They see these TLS lines within a frequency span of eighty MHz and then they use pump–probe experiments to measure the intrinsic relaxation time Mira This is how they pin down those long lifetimes Mira

Lev: When you look at that pump probe sequence, you’re essentially using the Solomon equations to infer the TLS population during the experiment because the qubit population changes after each interaction Lev The paper mentions fitting with Solomon equations where they get relaxation rates like Gammaqt equal to seventy microseconds to a minus one and Gammaq equal to fifty microseconds to a minus one Lev That gives you those intrinsic decay times we are talking about Lev <ref:2610.21853#pg3>

Paper summary: Kai: And the interpretation is interesting because they found that the scaling of the measured coupling and intrinsic relaxation time is consistent with tunneling positional defects described by the standard tunneling model Kai They write down an excitation energy as epsilon equal to root delta squared plus lambda squared where delta is potential asymmetry and lambda is the tunnel splitting Kai This suggests that these positional defects, which are common, can account for lifetimes in the seconds range they measured Kai

Mira: That’s a key assumption then, isn't it? They are assuming the standard tunneling model can fully explain why these TLS lifetimes stay so long Mira But they also note that their weak coupling to the qubit is in the kilohertz range which is consistent with positional defects but doesn't rule out other microscopic things like magnetic spin impurities or trapped quasiparticles Mira So it’s a bit of an open question what else could be causing these long lifetimes Mira

Lev: From my side, if they are really those kind of TLS, then the overhead for error correction isn't just simple decay; it's this correlated dynamics you mentioned in the abstract Lev That means when you try to do quantum error correction, these slow, correlated events could introduce errors in a way that’s hard to model with standard models Lev We need to know how much overhead that specific coupling adds for real hardware implementations Lev

Kai: So what this all boils down to is that superconducting qubits are incredibly sensitive probes of these exceptionally long-lived microscopic degrees of freedom Kai The paper opens a route toward controlling access to them and functionally using them in some way Kai It’s not just noise anymore; it’s something we can potentially study and maybe even harness Kai

Mira: Exactly. It establishes a quantitative framework for characterizing this TLS-induced decoherence Mira And it suggests these long-lived TLSs could be used as quantum memory elements in superconducting circuits Mira They are a coherent but unintended memory channel for the qubit, which is kind of interesting from a physics standpoint Mira

Lev: I’m interested in the functionalization part. If we can control access to these TLSs, maybe we can engineer them to be useful instead of just being decoherence sources Lev But getting that level of control over something that lasts for seconds is going to be a big challenge for the hardware design Lev

Paper summary: Kai: So, looking at the full picture of this paper, "Superconducting circuits host two-level systems with lifetimes exceeding one second," it’s about identifying these long-lived TLSs in transmon qubits Kai It claims they are causing correlated dynamics and outliving the qubit by orders of magnitude Kai It uses pump–probe measurements to show intrinsic lifetimes exceeding one second across three different TLS objects Kai

Mira: The authors use this wide-band quantum jump correlation spectroscopy to find these objects and then they interpret the data by scaling it with the standard tunneling model for positional defects Mira They also point out that their weak coupling is consistent with those defects but leaves room for magnetic impurities or trapped quasiparticles as alternatives Mira

Lev: For me, what this means is that we have to account for this slow relaxation channel when designing error correction schemes because it’s not just simple decay Lev And the paper flags that their measurement method, which is wide-band correlation spectroscopy, only shows these TLS lines within an eighty MHz frequency span Lev

Kai: That limitation on the frequency span is something to keep in mind when trying to find all the microscopic degrees of freedom Kai But overall, this work establishes superconducting qubits as a sensitive probe of these exceptionally long-lived environments Kai It really opens a route toward controlled access and functionalization for these things Kai

Mira: So, we’re seeing that TLSs aren't just fleeting noise but they can be coherent memory elements if we can manage them properly Mira This is a big piece of the puzzle for understanding how to make qubits more robust over longer timescales Mira

Lev: I think it makes me think about the practical implications for scaling up, because if these TLSs are this persistent, we need better ways to suppress or characterize that noise before we even try to encode information Lev That characterization framework they built is useful for that kind of engineering work Lev

Kai: So, that’s the gist of this paper on "Superconducting circuits host two-level systems with lifetimes exceeding one second," it shows these TLS objects are real and long-lived Kai They show correlated dynamics and their lifetime can be over a second with pump probe experiments Kai It connects these microscopic environments to the performance limitations of superconducting qubits Kai

Mira: And it suggests that we need to consider how to control or utilize them instead of just trying to get rid of them completely Mira This is where the research takes us next, looking at how we can make superconducting circuits more robust over time Mira

Conclusion: Kai: So, we’re wrapping up this look at the paper titled "Superconducting circuits host two-level systems with lifetimes exceeding one second."

Mira: Basically, they found these parasitic two-level systems in transmon qubits that live much longer than the qubit itself, and they claim these things cause correlated dynamics which matters for error correction.

Lev: Yeah, so it’s about this slow noise channel being there even when you think you’ve made a high-fidelity system.

Kai: Exactly. The authors used wide-band quantum jump correlation spectroscopy on flux-tunable transmon qubits to find these microscopic degrees of freedom that outlive the qubit by orders of magnitude.

Mira: And they actually found intrinsic lifetimes exceeding one second in some of these objects using pump probe measurements at different detunings.

Lev: That’s a big deal because it means this isn't just instantaneous noise; it’s something that takes time to decay, which is tough for error correction protocols.

Kai: Right, so the paper shows that superconducting qubits are a very sensitive probe for these exceptionally long-lived microscopic environments.

Mira: They establish a quantitative framework for characterizing this TLS-induced decoherence and suggest that these long-lived TLSs could potentially be used as quantum memory elements in superconducting circuits.

Lev: That idea of using them as memory, even if they’re unintended, changes how we have to think about the hardware design and what noise we need to suppress.

Kai: And the paper opens a route toward controlled access and functionalization of these microscopic degrees of freedom, which is pretty exciting for future research.

Mira: So next time, we’ll look at how this long-lived memory channel could actually be used in practice, or maybe what other sources are out there that might be causing this slow decay.

Mathieu Féchant, *Philipp Lenhard, *Nicolas Gosling, Ritika Dhundhwal, Denis Bêtre, Wolfgang Wernsdorfer, Thomas Reisinger, Ioan M. Pop

Karlsruhe Institute of Technology · Stuttgart University

quant-ph

Submitted: 2026-10-15

Updated: 2026-10-15

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 78/100

The gist: The gist: The authors perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits to identify microscopic degrees of freedom that outlive the qubit by orders of magnitude,

Key concepts

Two-Level Systems (TLS): These are microscopic defects within superconducting circuits that act like tiny two-level systems. They limit qubit performance by causing decoherence, frequency shifts, and leakage of the qubit's state. The study focuses on identifying specific TLS types that persist for very long times.
Quantum-Jump Correlation Spectroscopy: This advanced measurement technique is used to probe the non-Markovian aspects of the environment. It helps researchers identify specific microscopic degrees of freedom by analyzing how they correlate with the qubit's state during interaction, revealing subtle environmental influences.
Tunneling Model Defects: The standard tunneling model describes positional defects in superconducting materials. The measured coupling strengths and intrinsic relaxation times found in this study align well with predictions from this model, suggesting that these specific long-lived TLSs are likely related to structural imperfections like position variations.
Transmon Qubit: A transmon qubit is a type of superconducting circuit used for quantum computing. It's highly sensitive to its environment, and this research investigates how long-lived microscopic defects interact with the transmon's energy states.

Summary

The gist: The authors perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits to identify microscopic degrees of freedom that outlive the qubit by orders of magnitude, revealing intrinsic lifetimes exceeding one second.

Introduction and Motivation

Parasitic two-level systems (TLSs) are ubiquitous in superconducting circuits and limit qubit performance through decoherence, frequency drifts, and leakage Superconducting circuits host two-level systems with lifetimes exceeding one second > The authors perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits and identify microscopic degrees of freedom that outlive the qubit by orders of magnitude

Experimental Setup and Measurement Techniques

The study focuses on three long-lived TLSs in two tantalum electrode transmon qubits with standard Al/AlOx/Al Josephson junctions The intrinsic TLS lifetime is measured as the saturation of the Purcell loss rate with qubit-TLS detuning, in the range of seconds To reveal the non-Markovian part of the environment, correlation spectroscopy is deployed using Ref. [26] This method erases most of the lines visible in Fig. 2a, revealing three long-lived TLSs within the frequency span of 80 MHz

TLS Spectroscopy and Lifetime Determination

The authors perform a pump–probe experiment to measure the intrinsic relaxation time By resetting the qubit to 1⟩ before each resonant interaction with the TLS, they progressively pump population into the TLS (see Fig 3b) The analysis of this evolution using Solomon equations allows them to infer the TLS population during the experiment They find two TLSs with intrinsic lifetimes exceeding one second

Microscopic Interpretation and Model Constraints

The scaling of the measured coupling and intrinsic relaxation time is consistent with tunneling positional defects described by the standard tunneling model The excitation energy is written as ϵ = √∆2+Λ2 where ∆ is the potential asymmetry and Λ the tunnel splitting Extrapolating to kHz coupling, they expect intrinsic decay times in the seconds, consistent with their measurement This suggests that the standard tunneling model for positional defects can account for long lifetimes

Conclusion and Implications

The work establishes a quantitative framework for characterizing TLS-induced decoherence and suggests that long-lived TLSs could be harnessed as quantum memory elements in superconducting circuits These long-lived TLSs constitute a coherent but unintended memory channel for the qubit, while their weak coupling makes them difficult to detect with conventional spectroscopy The results establish superconducting qubits as a sensitive probe of exceptionally long-lived microscopic degrees of freedom and open a route toward their controlled access and functionalization

How it works

**: The authors perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits to identify microscopic degrees of freedom that outlive the qubit by orders of magnitude **

**: Pump–probe measurements at varying detunings from these objects reveal intrinsic lifetimes exceeding one second **

Measurement Setup and Calibration

The measurement setup is shown in Fig. 6 The Stark shift induced by a strong off-resonant drive follows, to leading order in the transmon’s weak anharmonicity α based on the first two levels, ∆f = 1/2π [omega2s / (4(∆s) -omega2s / (4(∆s+α)))] = 1/2π α Ω2s / 4 ∆s(∆s+α)

Measurement Setup and Calibration

The measurement setup is shown in Fig.

Improvements for AI systems

textbf Stronger Qubit Coherence Modeling for Error Correction Pipelines: The improved AI system can incorporate hidden temporal correlations in the qubit state by using machine learning models trained on the long-lived TLS spectroscopy data to predict future qubit states based on past quantum jumps, which directly addresses the potential role of these correlations in the dynamics of error codes.
textbf Novel Decoherence Mitigation Strategies: The system can implement adaptive pulse sequences that dynamically adjust for the measured Purcell decay and intrinsic lifetimes, utilizing the formula ΓTLS(δ) = Γ0 TLS + 2g squared / (4Γ + δdelay 2) to optimize qubit operations and reduce decoherence induced by TLS interaction.
textbf Enhanced Microscopic Defect Identification: The AI can use the generic Hamiltonian of a positional defect tunneling between two wells and the scaling laws described in Section V to rapidly distinguish between different microscopic origins (e.g., positional defects vs. magnetic spin impurities) by analyzing measured coupling strengths and lifetime scaling with qubit-TLS detuning.

Abstract

Understanding the microscopic environment of superconducting qubits is a central challenge for quantum technologies. Parasitic two-level systems (TLSs) of various microscopic origins are ubiquitous and limit qubit performance through decoherence, frequency drifts, and leakage. Recently, long-lived TLSs with lifetimes exceeding those of state-of-the-art qubits have been shown to induce correlated qubit dynamics, with direct implications for quantum error correction. Here, we perform wide-band quantum-jump correlation spectroscopy on flux-tunable transmon qubits and identify microscopic degrees of freedom that outlive the qubit by orders of magnitude. Pump-probe measurements at varying detunings from these objects reveal intrinsic lifetimes exceeding one second. Their weak coupling to the qubit, in the kilohertz range, is consistent with positional defects described by the standard tunneling model, without excluding other microscopic origins such as magnetic spin impurities or trapped quasiparticles. More broadly, these results establish superconducting qubits as a sensitive probe of exceptionally long-lived microscopic degrees of freedom and open a route toward their controlled access and functionalization.