Electron paramagnetic resonance spectrometer based on lumped element superconducting resonators
Electron paramagnetic resonance (EPR) spectroscopy is a widely used tool to detect and characterize paramagnetic species, such as some metal ions and organic radicals, in various science fields such as physics, chemistry, material science and medicine. However, conventional EPR systems only allow studying macroscopic samples due to the weak coupling between the spins and the microwave photons . The detection and identification of certain proteins and molecules in samples of biological and environmental interest is then limited, due to the low spin concentration. New alternatives are being investigated for pushing the detection limit to its minimum.
Among them, using superconducting circuits as quantum sensors opens a new path for these developments. In fact, lumped element resonators (LERs), which consist of a series inductance-capacitance circuit , can act as very high efficient microwave cavities. By a correct circuit design, the microwave photon within the cavity can be localized within a microscopic volume pushing the limits of sensitivity of the EPR spectroscopy by several orders of magnitude. Moreover, frequency multiplexing and tuneability of LERs allow characterizing spin transitions in the low magnetic field regime and the simultaneous characterization of different samples in a single chip.
As a proof of concept, we have developed an on-chip EPR device based on Nb superconducting LERs to sense the iron- and oxygen-binding protein metmyoglobin (Fe(III)-porphyrin) (from equine heart). We have checked the spectrometer sensitivity by depositing different amounts of these proteins on different LERs, going from microdroplets to a single molecular layer, deposited by means of dip pen nanolithography based on AFM. Low temperature spectroscopy measurements show that we can get weak coupling for all myoglobin protein samples. This sensitivity opens the path to the detection and characterization of micro-sized samples and even individual proteins. Further optimization of the LERs and pulsed measurements will be performed in order to develop a fully functional spectrometer for electron paramagnetic resonance characterization of microscopic samples.