Abstract

Carbon and silicon materials can support quantum superposition and entanglement for practical technologies. Superposition incorporates a phase with information content surpassing any classical mixture. Entanglement offers correlations stronger than any which would be possible classically. Together these give quantum computing its spectacular potential, but earlier applications may be found in metrology and sensing. Quantum interference in molecules offers low power switches and thermovoltaic energy scavenging.

Carbon nanomaterials can be structurally characterised with resolution approaching half the length of a carbon-carbon bond. Fullerene molecules can be assembled in carbon materials for quantum technologies. N@C60 contains a single nitrogen atom in a cage of sixty carbon atoms, whose spin superposition states are coherent for hundreds of microseconds. Other endohedral fullerenes can be almost as good. Information can be transferred from electron to nuclear spins and back again to give even longer memory times, and can be stored and retrieved holographically in collective spin states. Small tip-angle excitations can be used to demonstrate many of the fundamental principles. Correlated spins can be used for magnetic field sensors that surpass the standard quantum limit. Devices can be made with sensitivity to a coupled electron spin. In silicon entanglement has been demonstrated between electron and nuclear spins, and remarkably long coherence times are possible.

The big development over the next five years in materials for quantum technologies will be a transition from ensembles to devices. Many of the ingredients are already available: atomic-scale fabrication and characterization of materials, theoretical designs for architectures, exquisite control of electron and nuclear spins, and measurement of spin states [1]. The challenge ahead is to harness these together in devices with hierarchies of memory times and of controllability. The materials and techniques for quantum technologies also provide the means to investigate foundational quantum questions such as realism in the Leggett-Garg inequality, which in turn stretch the bounds of non-classical behaviour for technology.

[1] Ardavan, A. & Briggs, G. A. D. Quantum control in spintronics. Phil. Trans R. Soc. A 369 , 3229-3248, doi:10.1098/rsta.2011.0009 (2011).