Chemists are building molecular-scale switches that could turn individual quantum bits on and off, using light-sensitive molecules called rotaxanes to control how these qubits interact. Today’s quantum computers rely on fragile, hard-to-scale hardware. This project tackles a fundamental bottleneck: how to selectively connect qubits in a controllable way. The team targets [3]rotaxanes—molecules where a single light-activated switch links two qubits—then moves to [5]rotaxanes with two independent switches, allowing selective coupling between specific qubit pairs. The switch works by flipping the molecule between a spin singlet (no unpaired electrons) and a spin triplet (two unpaired electrons) when hit with light. If successful, this work establishes a synthetic framework for molecular quantum information processing. It is fundamental science—no immediate practical application is claimed. But past fundamental work on molecular switches and spin states underpins advances in sensing, data storage, and quantum computing hardware. A molecular approach could eventually lead to quantum processors that are chemically assembled, scalable, and reconfigurable by light, rather than etched in silicon or trapped in vacuum.
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Overall aim: We target a synthetic chemistry approach to switchable arrays of molecular quantum bits (qubits), using photo-active oligomers as colour-sensitive spin-triplet switches. Quantum information processing (QIP) could revolutionise how computing is performed and used across science and society. Here we propose a molecular approach to QIP, using molecules as qubits and as switches between the qubits. It builds on world-leading synthetic expertise at Glasgow in photo-active oligomers, with the transition between a spin singlet ground state and a spin triplet photo-excited state as the switch, and on world-leading chemistry in Manchester in making linked qubit arrays. The project is strongly focused on synthesis, intending to establish the framework for a molecular approach. The initial targets are [3]rotaxanes, where the qubits are linked by a single switch. These are an important milestone on the path towards [5]rotaxanes, where the qubits are linked by two distinct switches, which will allow us to switch on interactions selectively between qubit pairs. The characterisation of these very large supramolecules will be challenging, and involve advanced techniques including X-ray diffraction (if the supramolecules crystallise) and techniques such as small-angle X-ray scattering, NMR, ion-mobility mass spectrometry, pulsed EPR spectroscopy, when the supramolecules do not crystallise. The solution structures will also be calculated using molecular dynamic simulations. Physical studies of the [3]- and [5]rotaxanes will involve electrochemistry, which is a complementary technique allowing study of the supramolecules when the switch is a spin doublet (i.e. one unpaired electron) and Laser Induced Pulsed Dipolar Spectroscopy to study the supramolecules switch in the phototriplet state.
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