One goal of quantum information science is the engineering of sensors that take advantage of the enhanced sensitivity provided by electron spin superposition states. Such quantum sensors rely on coupling strength and coherence lifetime between the molecule of interest and its environment. Proteins contribute a macromolecular scaffold for fine control of position, charge, and spin degrees of freedom and, thus, for probing chemical and biological microenvironments. Here we use putidaredoxin, a Fe2S2 electron transfer protein with a mixed-valent S = 1/2 active site, to elucidate microenvironment effects on electron spin decoherence times. Through site-directed mutagenesis of the protein’s secondary coordination sphere and solvent exchanges, the magnitudes of intramolecular and intermolecular effects on spin relaxation were differentiated and quantified. In addition to modulations, decoherence times are also demonstrated to exhibit orientation dependence, which provides a further handle on discriminating between protein vs. solvent contributions to decoherence. This study demonstrates a proof of concept and a step towards the design and construction of biomolecular quantum sensors.

Modulating spin relaxation times in an Fe2S2 ferredoxin

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