On-chip triple hybrid quantum systems: coupling microwaves to magnon-phonon polarons

"Quantum systems" describes physical systems in which the elementary excitations are quantised as small packets of energy. Examples include individual photons of light, vibrations of crystals (phonons), and excitations of magnetic systems (magnons). The ability to control these elementary excitations can lead to the development of revolutionary new technologies such as quantum computers, secure communication through quantum encryption, and sensing technologies for navigation, geophysical exploration and medical imaging. Crucial to these technologies is the ability to transfer the quanta of energy between different physical systems in a coherent way that preserves the information encoded within the quantum states. This is achieved by "overlapping" or hybridising the quantum states of the systems. Many research groups are working on ways to achieve quantum hybridisation in different physical systems. Recently, a field of research known as "Cavity Spintronics" has achieved quantum hybridisation between microwave photons, magnons and phonons. However, these experiments require bulky, centimetre-size microwave cavities and large, millimetre-size magnetic spheres, which are not suitable for the development of technological applications. The project we propose will develop a fully on-chip architecture for coupling microwave photons with magnons and phonons in a micron-size ferromagnetic element. The on-chip architecture will lend itself more easily to integration with other physical systems such as optical cavities, acoustic resonators and superconducting qubits (the building blocks of quantum computers).

Our proposal will build upon two recent developments. Firstly, we have developed a novel method to create a large overlap between magnon and phonon states in thin magnetic layers and have demonstrated the first hybrid magnon-phonon state in a micron-scale extended magnetic structure. This was achieved by patterning the layer's surface with a shallow periodic stripe pattern, which created confinement of the phonon and magnon modes and caused them to overlap. Secondly, research groups, including our project partners at the Hitachi Cambridge laboratory, have recently developed methods to overlap microwave photons with micron-scale magnetic structures in on-chip architectures. This proposal will build upon these two key developments by fabricating microwave circuits on electronic chips containing micron-size patterned magnetic structures, in which hybrid magnon-phonon states are formed. The overlap with the photons in the on-chip microwave circuit will lead to hybridisation between all three systems (photons, magnons and phonons). Furthermore, microwave circuits can be readily controlled and detected using standard laboratory measurement instruments. This will allow us to excite and probe the magnons and phonons using the microwaves.

Our proposal to make fully on-chip hybrid photon-magnon-phonon systems will yield significant technological advantages that could lead to new applications in the realms of quantum computing, communications and sensor technology. It will enable investigations of the fundamental properties of photons, magnons and phonons and of the interactions between the different quantum systems.