Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Investigation of a parasitic two-level system in a superconducting qubit

Turunen, Santeri

Published Sep 5, 2025• 0 citations
OpenAlex

Abstract

Superconducting qubits are the most prevalent way to design qubits due to their relatively easy manufacturing, tunability and fast gates. However, one of their major problems is the short coherence time, which is influenced by various factors, such as charge noise, flux noise and thermal noise. Several working solutions have been found to mitigate these factors. In contrast, there is no general solution to mitigating the formation of parasitic two-level systems (TLS), which is one of the most dominant causes of error in superconducting circuits. There is no general theory for parasitic TLS, as they form randomly and their physical form varies. Due to this, it is important to be able to characterize parasitic TLSs, so that information can be obtained about their formation and effects on superconducting qubits to improve qubit design. This thesis demonstrates ways to characterize parasitic TLSs in a superconducting qubit. The frequency and the coupling strength of a TLS can be extracted by simulating the spectrum of the system and comparing it to experimental data. The states of a qubit and a TLS are possible to distinguish by utilizing an iSWAP gate to be able to characterize them separately. In this thesis a sample consisting of two fixed-frequency transmon qubits and one tunable transmon coupler was used. The cryostat measurements began by characterizing the sample and calibrating control pulses. After this, data of Rabi oscillations of the system were collected, from which simulations were made and relevant values were obtained through fitting. Then, the iSWAP gate was calibrated and used to swap qubit populations to distinguish the TLS from the qubits. The frequency and the coupling strength of the TLS were obtained from the simulations. At first, the TLS was nearly resonant with the qubit, but then its frequency had moved further away. The separation done with the iSWAP succeeded, which allowed a clear distinction of the separate modes. The results showed that these methods can be utilized to characterize parasitic TLSs. Research can be improved, for example, by measuring TLS coherence times to characterize its properties more accurately.

Related papers

Locating Two-Level Systems in a Superconducting Xmon Qubit

Xin-Xin Yang, Xiaoyan Yang, Liang-Liang Guo +5 more

One significant source of decoherence in superconducting circuits is known as two-level systems (TLSs), found in amorphous oxide layers. These circuits can, however, also be utilized as spectral and temporal TLS probes. Comprehensive investigations on the physics of TLSs are now possible thanks to recent advancements in superconducting qubits. Here, we simultaneously measure the tunable Xmon qubit decoherence time as well as the resonance frequency for more than 3 days to investigate stochastic fluctuations. Time-domain Allan deviation and frequency-domain power spectral density analysis indicate that two TLSs in near resonance with the qubit are responsible for the fluctuations. From the extracted oscillation in T1 decay, we locate the two TLSs near the junctions.

Applied Sciences•May 30, 2023•1 citation

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

Joaquin Minguzzi, John Clai Owens, Oskar Painter +2 more

An outstanding challenge in superconducting quantum circuits is mitigating loss from two-level system (TLS) defects. SWAP spectroscopy with flux-tunable qubits can detect individual TLSs, but the Rabi chevrons that signal coherent exchange have been observed only for the most strongly coupled defects. Here, we introduce a sequence with fast flux pulses only at state preparation and readout, switched off during the energy-relaxation delay for finer frequency resolution and longer interaction times. We detect Rabi chevrons at swap rates as low as 0.1 MHz, an order of magnitude lower than previously reported. In the dressed-state picture, we fit the coherent swap oscillations to extract the TLS energy-relaxation time. These Rabi chevrons reveal coherent TLSs with a median energy-relaxation time of 10 microseconds, at coupling strengths previously associated with incoherent TLSs. The swap coherence times exceed the qubit T2* limited by flux noise, indicating that the dressed gap protects them from low-frequency 1/f noise.

quant-pharXiv:2610.11125•Oct 9, 2026•0 citations

Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits

Fabrizio Berritta, David Pahl, Lukas Pahl +15 more

Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity $D \approx 0.9~\mathrm{MHz}^2/\mathrm{s}$. These timescales are about $3 \times 10^2$ times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.

quant-pharXiv:2608.02086•Aug 4, 2026•0 citations

Mapping the positions of Two-Level-Systems on the surface of a superconducting transmon qubit

Jürgen Lisenfeld, Alexander K. Händel, Etienne Daum +3 more

The coherence of superconducting quantum computers is severely limited by material defects that create parasitic two-level-systems (TLS). Progress is complicated by lacking understanding how TLS are created and in which parts of a qubit circuit they are most detrimental. Here, we present a method to determine the individual positions of TLS at the surface of a transmon qubit. We employ a set of onchip gate electrodes near the qubit to generate local DC electric fields that are used to tune the TLS’ resonance frequencies. The TLS position is inferred from the strengths at which TLS couple to different electrodes and comparing them to electric field simulations. We found that the majority of detectable surface-TLS was residing on the leads of the qubit’s Josephson junction, despite the dominant contribution of its coplanar capacitor to electric field energy and surface area. This indicates that the TLS density is significantly enhanced near shadow-evaporated electrodes fabricated by lift-off techniques. Our method is useful to identify critical circuit regions where TLS contribute most to decoherence, and can guide improvements in qubit design and fabrication methods.

•Jan 1, 2026•0 citations

Mapping the positions of Two-Level-Systems on the surface of a superconducting transmon qubit

Jürgen Lisenfeld, Alexander K. Händel, Etienne Daum +3 more

The coherence of superconducting quantum computers is severely limited by material defects that create parasitic two-level-systems (TLS). Progress is complicated by lacking understanding how TLS are created and in which parts of a qubit circuit they are most detrimental. Here, we present a method to determine the individual positions of TLS at the surface of a transmon qubit. We employ a set of on-chip gate electrodes near the qubit to generate local DC electric fields that are used to tune the TLS' resonance frequencies. The TLS position is inferred from the strengths at which TLS couple to different electrodes and comparing them to electric field simulations. We found that the majority of detectable surface-TLS was residing on the leads of the qubit's Josephson junction, despite the dominant contribution of its coplanar capacitor to electric field energy and surface area. This indicates that the TLS density is significantly enhanced near shadow-evaporated electrodes fabricated by lift-off techniques. Our method is useful to identify critical circuit regions where TLS contribute most to decoherence, and can guide improvements in qubit design and fabrication methods.

quant-pharXiv:2511.05365•Nov 10, 2025•0 citations

Two-Level System Spectroscopy from Correlated Multilevel Relaxation in Superconducting Qubits

Tanay Roy, Xinyuan You, David van Zanten +5 more

Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous $T_1$ extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by $\gtrsim 100\,\mathrm{MHz}$ from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.

quant-pharXiv:2602.11127•Feb 12, 2026•0 citations
Superconducting qubits are the most prevalent way to design qubits due to their relatively easy manufacturing, tunability and fast gates. However, one of their major problems is the short coherence time, which is influenced by various factors, such as charge noise, flux noise and thermal noise. Several working solutions have been found to mitigate these factors. In contrast, there is no general solution to mitigating the formation of parasitic two-level systems (TLS), which is one of the most dominant causes of error in superconducting circuits. There is no general theory for parasitic TLS, as they form randomly and their physical form varies. Due to this, it is important to be able to characterize parasitic TLSs, so that information can be obtained about their formation and effects on superconducting qubits to improve qubit design. This thesis demonstrates ways to characterize parasitic TLSs in a superconducting qubit. The frequency and the coupling strength of a TLS can be extracted by simulating the spectrum of the system and comparing it to experimental data. The states of a qubit and a TLS are possible to distinguish by utilizing an iSWAP gate to be able to characterize them separately. In this thesis a sample consisting of two fixed-frequency transmon qubits and one tunable transmon coupler was used. The cryostat measurements began by characterizing the sample and calibrating control pulses. After this, data of Rabi oscillations of the system were collected, from which simulations were made and relevant values were obtained through fitting. Then, the iSWAP gate was calibrated and used to swap qubit populations to distinguish the TLS from the qubits. The frequency and the coupling strength of the TLS were obtained from the simulations. At first, the TLS was nearly resonant with the qubit, but then its frequency had moved further away. The separation done with the iSWAP succeeded, which allowed a clear distinction of the separate modes. The results showed that these methods can be utilized to characterize parasitic TLSs. Research can be improved, for example, by measuring TLS coherence times to characterize its properties more accurately.
One significant source of decoherence in superconducting circuits is known as two-level systems (TLSs), found in amorphous oxide layers. These circuits can, however, also be utilized as spectral and temporal TLS probes. Comprehensive investigations on the physics of TLSs are now possible thanks to recent advancements in superconducting qubits. Here, we simultaneously measure the tunable Xmon qubit decoherence time as well as the resonance frequency for more than 3 days to investigate stochastic fluctuations. Time-domain Allan deviation and frequency-domain power spectral density analysis indicate that two TLSs in near resonance with the qubit are responsible for the fluctuations. From the extracted oscillation in T1 decay, we locate the two TLSs near the junctions.
An outstanding challenge in superconducting quantum circuits is mitigating loss from two-level system (TLS) defects. SWAP spectroscopy with flux-tunable qubits can detect individual TLSs, but the Rabi chevrons that signal coherent exchange have been observed only for the most strongly coupled defects. Here, we introduce a sequence with fast flux pulses only at state preparation and readout, switched off during the energy-relaxation delay for finer frequency resolution and longer interaction times. We detect Rabi chevrons at swap rates as low as 0.1 MHz, an order of magnitude lower than previously reported. In the dressed-state picture, we fit the coherent swap oscillations to extract the TLS energy-relaxation time. These Rabi chevrons reveal coherent TLSs with a median energy-relaxation time of 10 microseconds, at coupling strengths previously associated with incoherent TLSs. The swap coherence times exceed the qubit T2* limited by flux noise, indicating that the dressed gap protects them from low-frequency 1/f noise.
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity D≈0.9 MHz2/sD \approx 0.9~\mathrm{MHz}^2/\mathrm{s}D≈0.9 MHz2/s. These timescales are about 3×1023 \times 10^23×102 times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.
The coherence of superconducting quantum computers is severely limited by material defects that create parasitic two-level-systems (TLS). Progress is complicated by lacking understanding how TLS are created and in which parts of a qubit circuit they are most detrimental. Here, we present a method to determine the individual positions of TLS at the surface of a transmon qubit. We employ a set of onchip gate electrodes near the qubit to generate local DC electric fields that are used to tune the TLS’ resonance frequencies. The TLS position is inferred from the strengths at which TLS couple to different electrodes and comparing them to electric field simulations. We found that the majority of detectable surface-TLS was residing on the leads of the qubit’s Josephson junction, despite the dominant contribution of its coplanar capacitor to electric field energy and surface area. This indicates that the TLS density is significantly enhanced near shadow-evaporated electrodes fabricated by lift-off techniques. Our method is useful to identify critical circuit regions where TLS contribute most to decoherence, and can guide improvements in qubit design and fabrication methods.
The coherence of superconducting quantum computers is severely limited by material defects that create parasitic two-level-systems (TLS). Progress is complicated by lacking understanding how TLS are created and in which parts of a qubit circuit they are most detrimental. Here, we present a method to determine the individual positions of TLS at the surface of a transmon qubit. We employ a set of on-chip gate electrodes near the qubit to generate local DC electric fields that are used to tune the TLS' resonance frequencies. The TLS position is inferred from the strengths at which TLS couple to different electrodes and comparing them to electric field simulations. We found that the majority of detectable surface-TLS was residing on the leads of the qubit's Josephson junction, despite the dominant contribution of its coplanar capacitor to electric field energy and surface area. This indicates that the TLS density is significantly enhanced near shadow-evaporated electrodes fabricated by lift-off techniques. Our method is useful to identify critical circuit regions where TLS contribute most to decoherence, and can guide improvements in qubit design and fabrication methods.
Transmon qubits are a cornerstone of modern superconducting quantum computing platforms. Temporal fluctuations of energy relaxation in these qubits are widely attributed to microscopic two-level systems (TLSs) in device dielectrics and interfaces, yet isolating individual defects typically relies on tuning the qubit or the TLS into resonance. We demonstrate a novel spectroscopy method for fixed-frequency transmons based on multilevel relaxation: repeated preparation of the second excited state and simultaneous T1T_1T1​ extraction of the first and second excited states reveals characteristic correlations in the decay rates of adjacent transitions. From these correlations we identify one or more dominant TLSs and reconstruct their frequency drift over time. Remarkably, we find that TLSs detuned by ≳100 MHz\gtrsim 100\,\mathrm{MHz}≳100MHz from the qubit transition can still significantly influence relaxation. The proposed method provides a powerful tool for TLS spectroscopy without the need to tune the transmon frequency, either via a flux-tunable inductor or AC-Stark shifts.