Quantifying dynamics and interactions of individual spurious low-energy fluctuators in superconducting circuits
Sebastian E. de Graaf, Sumedh Mahashabde, Sergey Kubatkin, Alexander Ya. Tzalenchuk, Andrey V. Danilov · 2021 · 14 citationsRead the paper
Understanding the nature and dynamics of material defects in superconducting circuits is of paramount importance for improving qubit coherence and parameter stability and much needed for implementing large-scale quantum computing. Here we present measurements on individual highly coherent environmental two-level systems (TLS). We trace the spectral diffusion of specific TLS and demonstrate that it originates from the TLS coupling to a small number of low energy incoherent fluctuators. From the analysis of these fluctuations, we access the relevant parameters of low energy fluctuators: dipole moments, switching energies, and, more importantly, interaction energies. Our approach opens up the possibility of deducing the macroscopic observables in amorphous glassy media from direct measurements of local fluctuator dynamics at the microscopic level---a route towards substantiating commonly accepted, but so far phenomenological, models for the decohering environment.
What the full text shows
Shown in the lab- Direct evidence of a single coherent TLS coupled to two interacting low-energy incoherent fluctuators (TLFs), manifested as a four-peak frequency histogram. (T = 30 mK)
- Extraction of interaction energy between two TLFs (gFF) and their asymmetry energies (Δ1, Δ2). — gFF = 3.9 mK, Δ1 = 12.0 mK, Δ2 = 8.6 mK (T = 30 mK)
- Estimation of the TLS dipole moment and its likely location. — p0 ≈ 1.9 eÅ; located on the metal/air (MA) interface (γ = 19 MHz/V)
- Determination of the distance between a hopping TLS and a disturbing TLF. — r0 ≈ 80 nm (δVg0 = 5 V)
- Observation of a correlation between temperature and the characteristic energies of active TLFs. (10 mK < T < 150 mK)
- Measurement of the interaction energy between a TLS and a TLF (gSF). — ~0.5 mK (T = 30 mK)
- Setup
- NbN thin film on sapphire substrate; bare resonance frequency f0 = 4.86 GHz; Qi ≈ 1.3 x 10^5; temperature range 10 mK to >300 mK.
- How
- The researchers used frequency-tunable NbN planar resonators combined with external electrostatic gating to isolate individual TLS. They employed S21 transmission measurements and Pound locking (PL) to track the temporal resonance frequency fluctuations and generate histograms of the TLS energy states.
- Limitations
- Irreversible thermal reconfiguration of TLS occurs at T ≥ 150 mK. · Sampling rate for TLF dynamics is limited to 10 Hz. · Small sample size of TLS/TLF constellations limits the ability to determine a functional form for temperature dependence.
Summarized by Gemma 4 from the open-access full text · 10 Oct 2026. Read it