Experimental implementations of cavity-magnon systems: from ultra strong coupling to applications in precision measurement

Abstract

Several experimental implementations of cavity-magnon systems are presented. First an Yttrium Iron Garnet (YIG) block is placed inside a re-entrant cavity where the resulting hybrid mode is measured to be in the ultra strong coupling (USC) regime. When fully hybridised the ratio between the coupling rate and uncoupled mode frequencies is determined to be g/ω=0.46. Next a thin YIG cylinder is placed inside a loop gap cavity. The bright mode of this cavity couples to the YIG sample and is similarly measured to be in the USC regime with ratio of coupling rate to uncoupled mode frequencies as g/ω=0.34. A larger spin density medium such as lithium ferrite (LiFe) is expected to improve couplings by a factor of 1.46 in both systems as coupling strength is shown to be proportional to the square root of spin density and magnetic moment. Such strongly coupled systems are potentially useful for cavity QED, hybrid quantum systems and precision dark matter detection experiments. The YIG disc in the loop gap cavity, is, in particular, shown to be a strong candidate for dark matter detection. Finally, a LiFe sphere inside a two post re-entrant cavity is considered. In past work it was shown that the magnon mode in the sample has a turnover point in frequency (Goryachev et al 2018 Phys. Rev. B 97 155129). Additionally, it was predicted that if the system was engineered such that it fully hybridised at this turnover point the cavity-magnon polariton transition frequency would become insensitive to both first and second order magnetic bias field fluctuations, a result useful for precision frequency applications. This work implements such a system by engineering the cavity mode frequency to near this turnover point, with suppression in sensitivity to second order bias magnetic field fluctuations shown.

Ultrastrong photon-to-magnon coupling in multilayered heterostructures involving superconducting coherence via ferromagnetic layers

“…The achieved peak coupling strength is a high value as compared to other photon-to-magnon hybrids, exceeding the coupling strength in typical cavity-based (9,13,16), split ring-based (43), and on-chip resonator-based (19,20) hybrids. This coupling strength is comparable only with the coupling in macroscopic photon-to-magnon hybrids (14,15,17,18,35), where dimensions of the ferromagnetic oscillator reach several millimeters. Along with the total coupling, the system demonstrates a strong single-spin coupling strength (9,13,14,35,43) (19,20).…”

Electromagnetic Approach to Cavity Spintronics

“…This is the case of the ultrastrong coupling regime which is when g/! > 0.1 and it is also where approaches such as the rotating wave approximation begin to breakdown here [51]. Therefore, we will briefly investigate how our approach can be applied to one of these systems: a double-post, re-entrant microwave resonator coupled to a YIG sphere as investi- gated by Goryachev and co-workers [5]).…”

A formalism for magnon gravitational wave detectors

“…Any product of two even (odd) matrices is even and a product of even (odd) and odd (even) matrices becomes odd. To take the non-relativistic limit of the Hamiltonian, we have to diagonalize the Hamiltonian (26) and expand the upper block diagonal part in powers of 1/m. More precisely, 1/m expansion is recognized as an expansion with respect to two parameters, (mx) −1 and v/c.…”

“…Let us show the ability of magnon gravitational detectors by giving constraints on high frequency gravitational waves. Recently, measurements of resonance fluorescence of magnons induced by the axion dark matter was conducted and upper bounds on an axion-electron coupling constant have been obtained [25,26]. Such an axion-magnon resonance [33] has a similar mechanism to our gravitonmagnon resonance.…”

“…As a result, we obtain a formalism for non-relativistic fermions in curved spacetime, including a gravitational wave background as a special case. Finally, as a demonstration, we will give upper limits on the spectral density of continuous gravitational waves (95% CL): 7.5 × 10 −19 [Hz −1/2 ] at 14 GHz and 8.7 × 10 −18 [Hz −1/2 ] at 8.2 GHz, respectively, by utilizing results of magnon experiments conducted recently [25,26].…”