Spin decoherence in molecular crystals: Nuclear vs electronic spin baths

The loss of information about the relative phase between two quantum states, known as decoherence, strongly limits resolution in electron paramagnetic spectroscopy and hampers the use of molecules for quantum information processing. At low temperatures, the decoherence of an electronic molecular spin can be driven by its interaction with either other electron spins or nuclear spins. Many experimental techniques have been used to prolong the coherence time of molecular qubits, but these efforts have been hampered by the uncertainty about which of the two mechanisms is effectively limiting coherence in different experimental conditions. Here, we use the cluster-correlation expansion to simulate the decoherence of two prototypical molecular qubits and quantitatively demonstrate that nuclear spins become the leading source of decoherence only when the electron spin concentration is below ∼1 mM. Moreover, we show that deuterated samples, much easier to achieve than fully spin-free environments, could achieve record coherence times of ∼0.1 ms for an electron spin concentration of ∼0.1 mM. Alternatively, hydrogen-rich molecular crystals with electron spin concentrations below 1 mM can still achieve coherence times of 10 ms through dynamical decoupling, showing that the potential of molecular spins for quantum technologies is still untapped.