Electron spin echo envelope modulation (ESEEM) has been observed for the first time from a coupled heterospin pair of electron and nucleus in liquid solution. Previously, modulation effects in spin-echo experiments have only been described in liquid solutions for a coupled pair of homonuclear spins in nuclear magnetic resonance or a pair of resonant electron spins in electron paramagnetic resonance. We observe low-frequency ESEEM (26 and 52kHz) due to a new mechanism present for any electron spin with S>12 that is hyperfine coupled to a nuclear spin. In our case these are electron spin (S=32) and nuclear spin (I=1) in the endohedral fullerene N@C60. The modulation is shown to arise from second-order effects in the isotropic hyperfine coupling of an electron and N14 nucleus.

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The N@C60 molecule has been proposed as an electron spin-based qubit in several quantum information processing schemes.17,18 At the very least, the slow evolution within the sublevels of the S=32 system, which is responsible for the observed ESEEM must be taken into account when designing pulse sequences to perform a quantum algorithm. However, it could also be exploited to provide a separate family of gates for performing operations between sublevels, increasing the potential of N@C60 as a single quantum bit.

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