Nuclear spin-spin coupling is an essential property in the NMR analysis. Direct (nuclear dipolar) couplings provide information on the local structures of crystals and molecules. In condensed matter physics, indirect couplings are used to probe electronic states in solids, especially magnetic excitations. In both cases, the nuclear spin-spin couplings reflect the inherent characteristics of the materials of interest, which warrants these analyses.
In NMR quantum computation, by contrast, nuclear spin-spin couplings are no longer regarded as the inherent characteristics, or rather, they should be intentionally controlled to work for gate operations between nuclear spin qubits. For this purpose, direct couplings may be unsuitable, as it seems formidable to control hundreds of inter-qubit couplings by decoupling/recoupling. Our approach is to use optical means to control indirect couplings. Adding photons to an electronic system can bring about excitations, which may alter the indirect couplings. Inter-band photo-excitations in semiconductors are typical examples of this kind. Optically induced indirect coupling in semiconductors was first demonstrated in cross-polarization experiments under light illumination in GaAs [1], and subsequently confirmed in spin-echo double-resonance (SEDOR) experiments [2]. We are now working on the elucidation of this effect through systematic and thorough investigations. Some recent progress of these experiments will be discussed in this presentation.
The optically induced indirect coupling reminds us of the well-known nuclear spin polarization effect by inter-band photo-excitations called “optical pumping NMR.” In fact, these two effects are closely related to each other, and they both are the essential ingredients of the NMR quantum computation.
We are indebted to the staff of NMR Station (currently, High-Magnetic Field Characterization Unit) of NIMS for their helpful support. This work was supported by JSPS KAKENHI Grant Numbers JP21K18897, JP23H01131.