The key to the microwave-to-optical conversion is the usage of InAs quantum dots (QDs), which act as a single photon emitter and also a transducer because they are sensitive to local strain in a host crystal. A benefit of the approach is that the exciton energy shift due to strain is about two orders of magnitude more sensitive (~10 GHz pm–1) than that of small optical cavities (~100 MHz pm–1), which are also used for spectral conversion. Furthermore, GaAs is a convenient material to couple SAWs to superconducting circuits piezo-electrically.
In the integrated structure built by the authors, InAs QDs are sandwiched in a distributed-Bragg-reflector (DBR) optical cavity fabricated on a GaAs substrate. Then, the substrate is processed to add SAW cavities and interdigitated transducers (IDTs) made from superconducting niobium (pictured). Both the DBR and IDT structures are designed for microwave frequencies around 3.6 GHz. The IDT drives the SAW cavity to generate optical photons from microwave photons. The phonons interact with optical photons from the InAs QDs via the piezoelectric effect. As a result, the optically excited QDs exhibit sidebands in the scattering spectrum.
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