[Paper Review] Polaromechanics: cavity-magnon polaritons strongly coupled to phonons

Ruichang Shen, Jie Li+6 more|arXiv (Cornell University)|Jul 21, 2023

Mechanical and Optical ResonatorsPhysics and Astronomy3 citations

TL;DR

This paper demonstrates strong coupling between a mechanical oscillator and a cavity-magnon polariton (CMP) in a cavity magnomechanical system by using coherent perfect absorption (CPA) to reduce the polariton decay rate by four orders of magnitude. This achieves a polariton-mechanics cooperativity of $4 \times 10^3$, enabling observation of normal-mode splitting and establishing a triple-strong-coupling regime essential for coherent quantum control of photons, magnons, and phonons.

ABSTRACT

Building hybrid quantum systems is a crucial step for realizing multifunctional quantum technologies, quantum information processing, and hybrid quantum networks. A functional hybrid quantum system requires strong coupling among its components. However, couplings between distinct physical systems are typically very weak. Experimental realization of strong coupling in a hybrid system remains a long-standing challenge, especially when it has multiple components and the components are of different nature. Here we demonstrate the realization of triple strong coupling in a novel {\it polaromechanical} hybrid system, where polaritons, formed by strongly coupled ferromagnetic magnons and microwave photons, are further strongly coupled to phonons. The corresponding polaromechanical normal-mode splitting is observed. A high polaromechanical cooperativity of $9.4 imes10^3$ is achieved by significantly reducing the polariton decay rate via exploiting coherent perfect absorption. The quantum cooperativity much greater than unity is achievable if placing the system at cryogenic temperatures, which would enable various quantum applications. Our results pave the way towards coherent quantum control of photons, magnons and phonons, and are a crucial step for building functional hybrid quantum systems based on magnons.

Motivation & Objective

• To achieve strong coupling between a mechanical oscillator and a cavity-magnon polariton (CMP) in a cavity magnomechanical (CMM) system.
• To overcome the weak coupling limitation in prior CMM experiments by reducing the polariton decay rate and enhancing the coupling strength.
• To enable coherent quantum control of photons, magnons, and phonons by reaching the triple-strong-coupling regime.
• To establish a platform for studying rich strong-coupling and nonlinear effects in multipartite hybrid systems.

Proposed method

• Utilizing coherent perfect absorption (CPA) to engineer effective gain in the microwave cavity, balancing cavity loss and reducing the effective decay rate of the CMP mode.
• Operating the system at the CPA condition to achieve a four-order-of-magnitude reduction in the polariton decay rate, from ~3.43 kHz to ~0.48 kHz.
• Employing a macroscopic YIG sphere as the mechanical oscillator, whose deformation mode is coupled to the magnon via magnetostriction.
• Using microwave driving to tune the polariton frequency and induce magnon self-Kerr effects, enabling continuous frequency tuning of the upper-branch polariton.
• Measuring reflection spectra to observe normal-mode splitting and anti-crossing behavior, signatures of strong coupling.
• Quantifying the cooperativity $C_{\rm{+,b}}$ via the ratio of coupling strength to effective decay rates, achieving $4 \times 10^3$.

Experimental results

Research questions

• RQ1 Can strong coupling be achieved between a mechanical oscillator and a cavity-magnon polariton in a CMM system?
• RQ2 Can the decay rate of the cavity-magnon polariton be reduced sufficiently to enable strong coupling?
• RQ3 What is the resulting cooperativity and how does it compare to previous CMM experiments?
• RQ4 Can normal-mode splitting and anti-crossing features be observed, confirming the triple-strong-coupling regime?
• RQ5 What are the implications for preparing quantum states of phonons, photons, and magnons?

Key findings

• The polariton decay rate was reduced from ~3.43 kHz to ~0.48 kHz via CPA, a four-order-of-magnitude improvement.
• The system achieved a polariton-mechanics cooperativity of $C_{\rm{+,b}} \approx 4 \times 10^3$, representing a three-order-of-magnitude enhancement over previous CMM experiments.
• Normal-mode splitting with a splitting energy of $2G_{+}$ was observed, confirming strong coupling, with $G_{+}$ increasing to 23.19 kHz at 10.32 dBm drive power.
• Anti-crossing in the normal-mode spectrum was observed as the polariton frequency was tuned through the mechanical sideband, indicating level repulsion.
• The system operates in the triple-strong-coupling regime, where the normal modes are hybridizations of microwave photons, magnons, and phonons.
• The enhanced cooperativity significantly boosts mechanical cooling efficiency and lowers the threshold for phonon laser operation.