High Cooperativity Cavity QED with Magnons at Microwave Frequencies (1408.2905v3)

Abstract: Using a sub-millimetre sized YIG (Yttrium Iron Garnet) sphere mounted in a magnetic field-focusing cavity, we demonstrate an ultra-high cooperativity of $10^5$ between magnon and photon modes at millikelvin temperatures and microwave frequencies. The cavity is designed to act as a magnetic dipole by using a novel multiple-post approach, effectively focusing the cavity magnetic field within the YIG crystal with a filling factor of 3%. Coupling strength (normal-mode splitting) of 2 GHz, (equivalent to 76 cavity linewidths or $0.3$ Hz per spin), is achieved for a bright cavity mode that constitutes about 10% of the photon energy and shows that ultra-strong coupling is possible in spin systems at microwave frequencies. With straight forward optimisations we demonstrate that with that this system has the potential to reach cooperativities of $10^7$, corresponding to a normal mode splitting of 5.2 GHz and a coupling per spin approaching 1 Hz. We also observe a three-mode strong coupling regime between a dark cavity mode and a magnon mode doublet pair, where the photon-magnon and magnon-magnon couplings (normal-mode splittings) are 143 MHz and 12.5 MHz respectively, with HWHM bandwidth of about 0.5 MHz.

• The paper demonstrates a breakthrough in achieving ultra-high cooperativity between magnon and photon modes through an innovative multiple-post cavity design.
• It reports a coupling strength of 2 GHz and cooperativity exceeding 10^5 by integrating a sub-millimeter YIG sphere within a 3D microwave cavity at millikelvin temperatures.
• The study paves the way for advanced quantum information applications by enabling robust hybrid magnon-photon interactions in a strongly coupled regime.

High Cooperativity Cavity QED with Magnons at Microwave Frequencies: A Review

This paper presents a significant advancement in the domain of cavity quantum electrodynamics (QED) by demonstrating ultra-high cooperativity between magnon and photon modes using a Yttrium Iron Garnet (YIG) sphere positioned within a specially designed resonant cavity. The authors achieve this by applying a novel multiple-post field-focusing cavity design that facilitates the interaction between magnons and photons with unprecedented efficiency at microwave frequencies.

Overview of Techniques and Results

Utilizing a sub-millimeter-sized YIG sphere and operating at millikelvin temperatures, the paper achieves a cooperativity exceeding $10^5$, a considerable feat in the field of quantum information. This was realized with a system that combines 3D microwave cavity technology with the high spin density inherent in ferromagnetic YIG crystals, effectively operating in a regime between magnon and photon modes.

The innovative design of the cavity uses a multiple-post structure to focus the magnetic field into a small spatial region where the YIG sphere resides. With this arrangement, an effective coupling strength—a manifestation of the normal mode splitting—of 2 GHz was observed. This represents a remarkable improvement over conventional configurations as the coupling constant per spin was found to be 0.3 Hz, with straightforward calculations suggesting that with further optimization, cooperativities could potentially reach $10^7$.

Strong and Ultra-Strong Coupling Regimes

The significance of ultra-strong coupling in this paper lies in the fact that the coupling strength $g$ becomes comparable to the system’s natural frequency, challenging the conventional Jaynes-Cummings model and leading to complex dynamical behavior. By achieving such coupling, the paper opens venues for systems where counter-rotating terms become significant, allowing for research in domains previously constrained to optomechanical systems and superconducting qubits.

Experimental Observations

The extensive set of experiments carried out highlights various magnon-photon interaction regimes. Notably, the observation of three-mode interactions characterized by a dark cavity mode and a doublet magnon mode reflect the diversity and complexity inherent in these high-cooperativity systems. The high Q-factor of the cavity and the low magnetic losses of the YIG sphere underscore the effectiveness of the system design.

Implications for Quantum Information Systems

The paper emphasizes the potential of high-spin density YIG crystals combined with precisely engineered microwave cavities for quantum technological applications. Such systems could serve as foundational components in hybrid quantum information networks, where information storage and transfer require robust coherence and high fidelity.

Future Directions

As outlined, improving the cavity's surface treatment and further reducing dimensions could substantially enhance both coupling strength and cooperativity. This progress places the work at the frontier of achieving a robust, ultra-strong coupling regime at lower gigahertz frequencies, conducive to even broader magnetic and photonic applications.

In conclusion, this paper not only reports on important experimental achievements but also sets the stage for ongoing research into cavity QED systems where the merging of electronic and photonic states offers a wealth of new phenomena to be explored, with promising implications for the future of quantum computation and communication.