Description
The terahertz band remains underused in quantum technology. Microwave and optical frequencies underpin most quantum computing, communication and sensing platforms, yet the THz spectrum is where rare-earth crystal-field transitions, phonon modes and low-energy spin excitations all reside has largely been treated as a region to be avoided rather than exploited. Progress depends less on new physics than on instrumentation: sources with the linewidth, absolute frequency accuracy and stability that quantum measurements demand. Photonic generation is the technology best placed to change this: photomixing in Uni-travelling-carrier photodiodes, driven by comb-referenced or optically phase-locked laser (OPLL) pairs, offers continuous tunability across 0.1– 2 THz with absolute frequency accuracy and spectral purity inherited from the optical domain, and can be delivered by fibre into a cryostat with negligible thermal load. Here, we set out what quantum experiments demand of a THz source in terms of linewidth, power, and absolute frequency accuracy and discuss how comb and OPLL-stabilised photomixing meets those requirements. We will demonstrate swept continuous-wave spectroscopy of the $^5$I$_8$ manifold in LiYF₄:Ho³⁺, a system whose hyperfine structure has previously been resolved only with synchrotron-based Fourier-transform spectroscopy. We resolve the hyperfine structure of the Γ$_{3,4}$→ Γ$_2$ magnetic-dipole transition and a first experimental determination of the magnetic dipole matrix element |〈Γ$_2$ |$m ̂ $| Γ$_{3,4}$〉|=3.88 ±0.56 μ$_B$. We conclude with the instrumental developments needed to serve quantum experiments at these frequencies, including operation of photomixer sources at cryogenic temperatures.
| I am the presenting author | No |
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| If you are not the presenting author, please give the presenting author's name: | James Seddon |