Strong Light-matter Coupling

Cavity Quantum Electrodynamics (QED) is an area of study that focuses on the interaction between light and matter in the environment of a resonance cavity. It is a technique developed for quantum computation to coherently control quantum states and entanglement using light. In the contect of this project, both the spin ensemble and microwave photons are quantum systems, which when coupled together can be treated as two coupled harmonic oscillators. When the two systems are brought to resoant together, strong coupling can form. In the strong coupling regime, the decay rate of the coupled system is slower than their individual decay rate, therefore information can be coherently exchanged between the microwave photons and the spin ensemble. This technique enables the possibility to control and read the quantum state of the spin system using microwave photons.

In the strong coupling experiment, we have three control parameters which are the quality factor of the cavity, the mode volume of the cavity and the number of spins. Mode volume corresponds to the spatical confinment of the electromagnetic wave in the cavity, which for example is smaller in two-dimensional coplanar waveguide (left) and bigger in the three dimensional microwave caviity (right). The higher the Q-factor, the smaller the mode volume, the larger the number of spins, the stronger the coupling rate.

In our laboratory, we explore both types of cavityes depending on the physical size and the resonance frequency of the molecular ensemble. These includes the two-dimensional cavity made from superconducting Ti-NbTiN with quality factors as high as 20,000, and the three-dimensional microwave cavity made from oxygen free copper with quality factors as high as 15,000.