Observation of period doubling and higher multiplicities in a driven single-spin system

One of the prime features of quantum systems strongly driven by external time-periodic fields is the subharmonic response with integer multiples of the drive period $k{T}_d$. Here we demonstrate experimentally based on a careful theoretical analysis period doubling and higher multiplicities ($k=2,\dots 5$) for one of the most fundamental systems, namely, an individual spin-$1\mathrm{/}2$. Realized as a nitrogen vacancy center in diamond, the particular coherence properties under ambient conditions, strong field sensitivity, and optical addressability allow to monitor coherent $k$-tupling oscillations over a broad set of driving parameters in the vicinity of the ideal manifolds. We verify an enhanced sensitivity within this domain which provides new means for improved sensing protocols.