Magnons at low excitations: Observation of incoherent coupling to a bath of two-level systems

Rapid Communication
Open Access

Magnons at low excitations: Observation of incoherent coupling to a bath of two-level systems

Marco Pfirrmann, Isabella Boventer, Andre Schneider, Tim Wolz, Mathias Kläui, Alexey V. Ustinov, and Martin Weides

Phys. Rev. Research 1, 032023(R) – Published 21 November 2019

Abstract

Collective magnetic excitation modes, magnons, can be coherently coupled to microwave photons in the single excitation limit. This allows for access to quantum properties of magnons and opens up a range of applications in quantum information processing, with the intrinsic magnon linewidth representing the coherence time of a quantum resonator. Our measurement system consists of a yttrium iron garnet sphere and a three-dimensional microwave cavity at temperatures and excitation powers typical for superconducting quantum circuit experiments. We perform spectroscopic measurements to determine the limiting factor of magnon coherence at these experimental conditions. Using the input-output formalism, we extract the magnon linewidth $κ_{m}$ . We attribute the limitations of the coherence time at lowest temperatures and excitation powers to incoherent losses into a bath of near-resonance two-level systems (TLSs), a generic loss mechanism known from superconducting circuits under these experimental conditions. We find that the TLSs saturate when increasing the excitation power from quantum excitation to multiphoton excitation and their contribution to the linewidth vanishes. At higher temperatures, the TLSs saturate thermally and the magnon linewidth decreases as well.

Received 25 March 2019
Revised 20 August 2019

DOI:https://doi.org/10.1103/PhysRevResearch.1.032023

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Magnetization dynamics Magnons Quantum information with hybrid systems Spin dynamics

Cavity resonators Ferromagnetic resonance

Condensed Matter, Materials & Applied PhysicsQuantum Information, Science & Technology

Authors & Affiliations

Marco Pfirrmann ^1,*, Isabella Boventer^1,2, Andre Schneider¹, Tim Wolz¹, Mathias Kläui², Alexey V. Ustinov^1,3, and Martin Weides^1,4,†

¹Institute of Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
²Institute of Physics, Johannes Gutenberg-University Mainz, 55099 Mainz, Germany
³Russian Quantum Center, National University of Science and Technology MISIS, 119049 Moscow, Russia
⁴James Watt School of Engineering, University of Glasgow, Glasgow G12 8LT, United Kingdom

^*marco.pfirrmann@kit.edu
^†martin.weides@glasgow.ac.uk

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand

Issue

Vol. 1, Iss. 3 — November - December 2019

Subject Areas

Reuse & Permissions

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Color-coded absolute value of the reflection spectrum plotted against probe frequency and applied current at $T = 55 mK$ and $P = - 140 dBm$ . The resonance dips show the dressed photon-magnon states forming an avoided level crossing with the degeneracy point at $I_{0} = 2.09 A$ , corresponding to an applied field of $B_{0} = 186.98 mT$ (dashed vertical line). The inset displays the squared gradient of the zoomed-in amplitude data. The kink in the data represents a weakly coupled magnetostatic mode. This was also seen in Ref. [9]. (b) Raw data of the cross section at the center of the avoided level crossing and fit to the input-output formalism. The fit gives a magnon linewidth of $κ_{m} / 2 π = 1.82 \pm 0.18 MHz$ . The data are normalized by the field-independent background before fitting and is multiplied to the fit to display it over the raw data.
Reuse & Permissions
Figure 2
(a) Temperature dependence of the magnon linewidth $κ_{m}$ at the degeneracy point. For low probe powers, $κ_{m}$ follows a $tanh (1 / T)$ behavior (crosses), while for high probe powers (circles) the linewidth does not show any temperature dependence. (b) Power dependence of the magnon linewidth $κ_{m}$ for $T = 55$ and $200 mK$ at the degeneracy point. Both temperature curves show a similar behavior. At probe powers of about $- 90 dB m κ_{m}$ drops for both temperatures, following the ${(1 + P / P_{c})}^{- 1 / 2}$ trend of the TLS model. All linewidth data shown here are extracted from the fit at matching frequencies.
Reuse & Permissions
Figure 3
Magnetic field (magnon frequency) dependence of the the magnon linewidth $κ_{m}$ for different probe powers at $T = 55 mK$ (a) and $T = 200 mK$ (b). The shown probe powers correspond to the ones at the transition in Fig. 2. The number of excited magnons depends on the detuning of magnon and photon frequency. At matching frequencies (dashed line) the magnon linewidth has a minimum, corresponding to the highest excited magnon numbers and therefore the highest saturation of TLSs. A second minimum at about $187.25 mT$ corresponds to the coupling to an additional magnetostatic mode within the sample [inset of Fig. 1]. The insets show the ratio of excitation power within each component of the hybrid system. At matching frequencies, both components are excited equally. The magnon share drops at the plot boundaries to about 20%. The coupling to the magnetostatic mode is visible as a local maximum in the magnon excitation ratio. The $x$ axes are scaled as in the main plots. The legends are valid for both temperatures.
Reuse & Permissions

Physical Review Research