Strong coupling of magnons in a YIG sphere to photons in a planar superconducting resonator in the quantum limit

Abstract

We report measurements made at millikelvin temperatures of a superconducting coplanar waveguide resonator (CPWR) coupled to a sphere of yttrium-iron garnet. Systems hybridising collective spin excitations with microwave photons have recently attracted interest for their potential quantum information applications. In this experiment the non-uniform microwave field of the CPWR allows coupling to be achieved to many different magnon modes in the sphere. Calculations of the relative coupling strength of different mode families in the sphere to the CPWR are used to successfully identify the magnon modes and their frequencies. The measurements are extended to the quantum limit by reducing the drive power until, on average, less than one photon is present in the CPWR. Investigating the time-dependent response of the system to square pulses, oscillations in the output signal at the mode splitting frequency are observed. These results demonstrate the feasibility of future experiments combining magnonic elements with planar superconducting quantum devices.

Introducing coherent time control to cavity magnon-polariton modes

“…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. Despite this necessity for fast manipulation, the literature about time resolved experiments with either an yttrium iron garnet (YIG) waveguide [20] or CMPs [2,7,21,22] is scarce and confined to cavity-pulsing. A simultaneous and coherent control over both subsystems has yet to be demonstrated, which is the subject of this work.…”

Coherent Conversion Between Microwave and Optical Photons—An Overview of Physical Implementations

“…The inclusion of optical cavity modes in the form of WGMs has not led to a significant improvement, principally because of the small overlap between the spatially uniform Kittel mode, which occupies the entire YIG sphere, and the WGMs, which are confined to the equator, giving a small magnon–photon coupling. Possible routes to improve this include using higher‐order magnetostatic modes, which are concentrated near the surface of the sphere (although these are harder to excite with microwaves), or use a ferromagnetic disc or oblate spheroid (although this is likely to have negative consequences for the linewidth of the magnetic modes). Alternatively, Mie resonances in magnetic dielectrics with length scales similar to the optical wavelength could be used instead of WGMs, with the increased filling factor improving the coupling to spin waves …”

Quantum electrodynamics with magnetic textures

“…In these cases, however, our approach underestimates the resulting ΔM n by a factor ∼1.5 only, as checked numerically. The coupling is finally calculated using equation (11). Finally, it remains to compute the rate Γ (ω n ) appearing in the transmission equation (8).…”

“…Different magnonic excitations have been proposed or have already been coupled to quantum light experimentally [9]. Most studies have focused on Yttrium-Iron-Garnet (YIG) films or spheres coupled to either superconducting coplanar waveguide (CPW) resonators [10,11] or 3D cavities [12,13]. The latter allows exciting mostly the uniform Kittel mode in which all spins precese in unison.…”

“…n n 0 c n with c w = D a a a ( ) ( ) M b r n n 0 c the magnetic susceptibility at resonance for each mode. We refer the reader to [19] where we explained in detail how to arrive to equation (11).…”