Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits

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

Kai: Today's paper: "Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits".

Mira: The gist: High-sensitivity SWAP spectroscopy enables the detection of Rabi chevrons at qubit-TLS coupling rates typically associated with the incoherent regime,

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

Title and authors: Kai: So we're looking at this paper today, "Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits." It’s from Minguzzi and Clai Owens and others. This paper is about finding ways to see two-level systems, or TLS defects, that we usually think are too weakly coupled to measure coherently.

Mira: Yeah, it tackles a problem where we can only reliably see the most strongly coupled TLS defects with standard Rabi spectroscopy. This work is trying to extend that coherent detection down to much lower coupling rates.

Lev: From an experimental standpoint, I’m curious if this actually means we can pull more information out of these defects than before. If they are so weakly coupled that we usually just see incoherent decay, can this technique find something new?

Kai: Exactly. The core idea here is using a specific pulse sequence to get better frequency resolution and longer interaction times without needing those long flux pulses we usually have to calibrate for. They’ve managed to get detection down to swap rates as low as zero point one MHz, which is about ten times lower than what we've reported before <ref:2610.11125#pg1,swap rates as low as 0.1 MHz>.

Mira: That level of sensitivity, that zero point one MHz step size, is quite significant when you think about the timescale of these defects <ref:2610.11125#pg1>. They find that even at these low coupling rates, the median TLS energy-relaxation time they measure is around ten microseconds <ref:2610.11125#pg2>.

Lev: Ten microseconds sounds much longer than we've seen for these types of defects in similar systems where we were talking about strongly coupled ones. That suggests a different kind of defect physics might be involved here.

Kai: Right, and this isn't just about seeing a slower decay; they are using the dressed-state picture to model it. They fit the coherent swap oscillations to extract that TLS energy-relaxation time directly from the data they collect.

Mira: The way they describe it is interesting because they find that for these sub-MHz coupling strengths, the exponential fit only captures the initial decay on a timescale of about one over two times g, which is just a few microseconds <ref:2610.11125#pg1>.

Lev: So what does that mean when you look at real hardware? If the coherent oscillation only lasts a few microseconds before it transitions into that faster exponential decay, how do you use that to distinguish between different types of defects?

Kai: They use this modeling to extract parameters like an oscillation frequency, omega osc, which they found is around zero point three two MHz for one resonance. That frequency is tied directly to the coupling strength between the qubit and the TLS, g, where g corresponds to a Rabi splitting of about zero point three two MHz.

Title and authors: Mira: And they use that same model to describe how noise affects those oscillations, specifically how low-frequency ones cause dephasing by making that oscillation frequency fluctuate over time.

Lev: That fluctuating frequency due to noise is what really matters for coherence, right? If the gap frequency omega osc is fluctuating because of one/f noise, it adds pure dephasing to the system <ref:2610.11125#pg1>.

Kai: Right, and they use the dressed-state model to show how this gap frequency stays far below the thermal energy scale at twenty mK, which is around four hundred MHz. That means the noise acting on that gap is effectively classical in terms of its impact on the coherence.

Mira: They also identify a rate, v, which quantifies that excess decoherence of the dressed gap beyond just energy relaxation. This rate comes from transitions between dressed states and non-Markovian dephasing from those low-frequency one/f noises they mentioned <ref:2610.11125#pg1>.

Lev: So they're saying this specific noise coupling to the low-frequency flux noise is actually protected by this dressed gap structure, which is a key point for understanding how we can suppress decoherence in dual-rail architectures or spin-locked driven qubits.

Kai: They show that the linear coupling to that low-frequency flux noise gets suppressed by the dressed gap, which acts as a shield against it, and they mention this protection is analogous to what you see in other architectures like transmon dual-rail systems.

Mira: The implication is that TLS defects that we previously categorized as incoherent might actually be able to cross into the coherent regime if we tune the qubit parameters correctly. That's a big statement about how tunable these things are.

Lev: If they can tune them into coherence, then characterizing these defects isn't just about finding where they fail; it’s about understanding how to make them work in our quantum computers.

Kai: So to wrap up the paper "Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits," we see that high-sensitivity SWAP spectroscopy can detect Rabi chevrons at coupling rates as low as zero point one MHz, yielding a median TLS energy-relaxation time of ten microseconds.

Mira: The key finding is that the dressed-state picture allows them to map these measured dynamics to physical quantities, giving us a way to understand the underlying physics of these weakly coupled TLS defects.

Lev: For running this on real hardware, the results suggest that for seventy-seven of the eighty-eight resolved TLS defects, this rate v is actually less than one/T* two which is a good sign for maintaining qubit coherence.

Kai: So what we have here is a method that extends coherent detection to these lower coupling regimes and provides a way to use noise protection mechanisms to understand how these defects behave. The paper "Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits" opens up new ways to probe the behavior of TLS defects.

The paper's summary: Kai: So to recap, this paper is about using a really sensitive SWAP spectroscopy technique to find TLS defects that we usually think are too weakly coupled to see coherently, pushing detection down to rates as low as zero point one MHz.

Mira: Right, and what's interesting is how they handle the physics underneath those numbers. They use this dressed-state picture because it lets them map the measured oscillations directly onto physical coupling strengths and relaxation times.

Kai: Exactly, because that mapping shows them that even in these weaker coupling regimes, they can pull out things like a median TLS energy-relaxation time of about ten microseconds. That’s a lot longer than what we usually see for these kinds of defects.

Mira: And the way they model the decay is telling; for those weak couplings, the exponential part only describes the very first few microseconds before something else happens. They extract an oscillation frequency, like zero point three two MHz in one case, which is directly related to that coupling strength g.

Kai: So it’s not just finding a slow decay; they are using that modeling to figure out the actual interaction strength between the qubit and the TLS. That tells us how strongly they are actually linked.

Mira: And beyond just measuring that link, they use this dressed-state model to show how low-frequency noise—that one/f noise we always worry about—actually affects the coherence of that coupling gap.

Kai: Because they found this gap frequency stays well below the thermal energy at twenty millikelvin, so the noise acting on it is behaving in a way that’s more manageable, even if it's not perfectly quiet.

Mira: They quantify this extra noise effect with a rate v, and they show that because of this gap structure, the linear coupling to low-frequency flux noise gets suppressed. That’s a neat physical mechanism for protecting the coherence in these systems.

Kai: It suggests that these TLS defects we’ve been classifying as "incoherent" might actually be on the edge of being coherent, and tuning can push them over that line.

Mira: That's a big implication because if you can tune them into coherence, then characterizing these defects isn't just about finding where they fail; it’s about understanding how to make them work in our quantum computers.

Kai: So the next thing we need to look at is what this means for actually building those qubits. Does this technique actually help us design better hardware?

The paper's improvements: Tom: So we’re looking at what the authors are suggesting they should do next with this SWAP spectroscopy work today. It’s about taking these findings and turning them into better hardware or better characterization tools for these defects.

Kai: The paper points out that to really get the most out of this, we need to resolve coherent dynamics with frequency precision that's even lower, like below one megahertz over longer time scales.

Mira: That’s right, and they suggest focusing on strain tuning and biasing the qubit right near that flux-insensitive sweet spot. They want to reduce dephasing noise that just blurs out those slow chevron oscillations.

Kai: It means if you want to see those subtle interactions in real hardware, you need better ways to control the qubit parameters to minimize that noise floor.

Mira: And they think the tools they introduced, like this high-sensitivity SWAP spectroscopy sequence, are going to be really important when we try to characterize these defects on much larger device volumes.

Kai: So it sounds like the next big step is tying this detection method into a practical tuning strategy for noise reduction.

Mira: Exactly, and they want to make sure that as we scale up the number of qubits, we don't lose the ability to see these subtle coherent effects anymore.

Conclusion: Kai: So we're wrapping up "Sub-MHz SWAP spectroscopy of two-level systems in superconducting qubits." Basically, this paper shows that we can use high sensitivity to find coherent interactions between qubits and defects even when those defects are usually considered too weak to measure coherently.

Mira: It’s a solid piece of work because they don't just report a slow decay; they use the dressed-state picture to explain the physics behind those measured rates, which gives us real insight into how these TLS defects behave under certain conditions.

Lev: From an error correction view, knowing that these defects have a measurable relaxation time of about ten microseconds is useful because it tells us exactly how long we have before we need to worry about decoherence affecting our logic gates.

Kai: That's right, and they’ve managed to show that the qubit Ramsey rate is often higher than the decoherence rate for most of these detected defects, which means this technique could be a useful tool for maintaining qubit quality.

Mira: They also establish a physical mechanism—that dressed gap—that protects the coherence against low-frequency noise, which is super important because it’s analogous to protection we see in other architectures like dual-rail systems.

Lev: If that protection holds up, it means we might be able to design qubits where these specific types of TLS defects don't immediately destroy our quantum information.

Kai: So the main implication is that TLS defects classified as incoherent might actually be tunable into a coherent regime if we can find the right control settings for the qubit.

Mira: It changes how we classify these materials, moving them from just "bad" to potentially "useful" if you can tune them correctly.

Lev: But they did flag a limitation: this method needs very high frequency precision and longer time windows, which means it’s not something you can do easily on every single chip right now.

Kai: Exactly, so the next challenge is taking these findings and figuring out how to make this technique practical for larger chips or more complex noise environments.

Mira: We'll keep an eye on how they suggest tuning and biasing the qubit because that seems like the most promising path forward for actually seeing these coherent dynamics in a lab setting.