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

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 κm. 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.

Frequency fluctuations of ferromagnetic resonances at milliKelvin temperatures

“…A circle fit 24 returns the internal linewidth (HWHM) κ i = ω r /(2Q i ), with Q i as internal Qfactor. Varying power and temperature, we find a linewidth dependence that decreases with increasing power P and temperature T in accordance to previous reports, which attributed this effect to energy loss into a bath of two-level systems (TLS) [25][26][27] (see Fig. 1 (e)).…”

Introducing coherent time control to cavity magnon-polariton modes

“…Since the recent emergence of this hybrid particle, three different models, in particular, have helped to unravel the physics of CMPs over the last years: first, the picture of two coupled oscillators, which is the most intuitive one; the underlying physics, however, is only revealed from an electromagnetic viewpoint, which is the second model and shows a phase correlation between cavity and magnon excitation [9]; and finally, the quantum description of the CMP, which has, for instance, given the theoretical framework for a coupling of magnons to a superconducting qubit [10,11]. Many spectroscopic experiments have led to new insights about loss channels [3,12,13], their temperature dependence [14][15][16], and to the observation of level attraction [17][18][19]. These spectroscopic measurements, however, are performed under continuous driving, and while they have yielded great physical insight into these hybrid systems, flexible and universal information processing requires the manipulation of such physical * tim.wolz@kit.edu † martin.weides@glasgow.ac.uk states on demand and on nanosecond timescales.…”

Steering between level repulsion and attraction: broad tunability of two-port driven cavity magnon-polaritons

“…The phenomena of (strong) coupling of magnons-the associated quanta of collective spin wave excitations-to microwave cavity photons resulting in cavity magnon-polaritons (CMPs) has been the subject of numerous works in the past few years [1][2][3][4][5][6][7][8][9]. The ability to couple magnons to different physical systems, through magnetooptical [10][11][12] to optical, or by magnetostrictive interaction to mechanical [13] and cavity photons simultaneously makes CMPs highly interesting for various applications [8].…”