Senior Participants (*=coordinators): D. Awschalom*, A. Cleland, A. Clerk, D. Freedman, G.Galli, A. High, D. I. Schuster*, T. Zhong
The goal of this superseed is to develop materials that can coherently transfer quantum information between electrical circuits and optical photons. To enable coherent electrical-optical coupling, we must identify and study suitable materials systems that can coherently support both types of excitation. Color centers, atom-like structures in wide-bandgap semiconductors, can serve as an ideal coherent interface between these domains. They have spin excitations whose energies can be tuned anywhere in the microwave frequency range, and also spin-dependent optical transitions, often in the telecom band. However, the very properties that make them coherent (localized spin states, small sizes) make them challenging to couple to, in both the microwave and optical domains. Rare earth ions such as erbium also must be embedded in specific host materials to preserve their properties, materials that are not always optimal for patterning into metamaterials or for integration with other systems. Here we will address two materials science pathways to realize coherent electrical-optical coupling via color centers: development of novel color centers, optimized host materials and electrical qubit coupling.
Novel color centers for coherent optical interfaces. We will develop new color centers that have advantageous properties compared to existing color centers. The first color centers explored for quantum applications, such as the nitrogen vacancy (NV) center in diamond, were selected for their magnetic sensing properties. For quantum information applications, however, we require color centers with exceptional coherence (tens of microseconds) and low disorder optical transitions (transform limited linewidths).
Materials for enhanced coupling of electrical qubits to color centers. Color centers are of nanometer size, whereas the natural length scale of a microwave photon is one centimeter, creating a disappointingly small overlap of energy density. To address this challenge, we will develop materials platforms that enable significantly improved magnetic, electrical and acoustic coupling, using theoretical operating protocols.
Three physical modalities of coherent coupling between spins and microwaves. (a) Magnetic coupling to spins using a low impedance superconducting circuit (schematic and device images shown), and preliminary ESR signal. (b) Photoluminescence of electrically biased divacancy spins in 4H-SiC, in between two electrodes. (c) X-ray strain measurements of surface acoustic waves in a gaussian used to acoustically control spins.