A molecular switch, triggered by a flash of light, could make quantum computers more reliable by replacing error-prone microwave pulses with optical ones. Quantum computers rely on precise operations called quantum gates, which manipulate qubits to perform calculations. To build a useful quantum computer, researchers need a "universal set" of gates that can implement any possible computation. This project tackles a specific bottleneck: the entangling gate, which links two qubits together. Currently, microwave pulses used to entangle molecular electron spin qubits often miss some molecules, lowering the gate's fidelity—the accuracy of the operation. The team previously measured fidelity too low for practical quantum algorithms. This research explores two fixes. First, orienting molecules in liquid crystals or doped crystals to better control the coupling between qubits. Second, using an optical pulse to activate a light-sensitive chromophore that switches the coupling strength between two qubits. Light pulses can reach every molecule in a sample, potentially achieving higher fidelity than microwaves. If successful, this work could enable more reliable quantum gates, a step toward building a universal quantum computer. The project is fundamental science, but similar advances in controlling molecular spins have previously underpinned technologies like magnetic resonance imaging.
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Quantum Information Processing (QIP) is a major target worldwide, with many implementation schemes being examined. In order to test the outcome and fidelity of any gate operations quantum state tomography is required. This measures the expectation values of a set of operators which form a complete basis of the Hilbert space, and thus can also detect the quantum states of quantum bits (qubits). It has already been demonstrated that appropriately designed Electron Paramagnetic Resonance (EPR) sequences can be used for tomography (Nat. Commun., 2023, 14, 7029). We have previously investigated the fidelity of a two-qubit entangling gate, by which a separable state is converted to a maximally entangled state, on a randomly orientated molecular electron spin qubit system containing two nitroxide moieties a disordered frozen solution matrix. In this experiment it was found that the fidelity achieved was significantly lower than that required for implementing QIP algorithms. However, simulations have indicated that the fidelity could be improved by orienting molecular electron spin qubit systems to better control the strength of the coupling interaction between the qubits, as this has a dependence on the angle of the inter-qubit vector and the applied magnetic field. This project aims to investigate mechanisms of orienting molecules, for example in liquid crystals or partially doped single crystals, and the impact this has on the fidelity of a two-qubit entangling gate; initially following our previously defined methodology for measuring and analysing this fidelity. As all useful two-qubit gates can be implemented by a combination of an entangling gate and single-qubit gates, the demonstration of single-qubit gates, an entangling gate, and subsequent quantum state tomography is sufficient to demonstrate a universal set of gates. In addition, we look to also control the coupling between two molecular electron spin qubits using an optically-activated molecular switch. Here the strength of the coupling between the qubits is mediated by the electronic state (ground or excited) of a light sensitive chromophore moiety. The use of an optical pulse to control the entangling of two qubits is predicted to have advantages in terms of sensitivity compared to entangling microwave pulses, provided that the quantum yield of the desired state is high. The action of microwave pulses is limited by the bandwidth of the microwave pulse, and is frequently insufficient to excite a complete EPR spectrum, thus only acting on some of the molecules in the system. In contrast with careful choice of the sample concentration and volume, a light pulse can act on all molecules in the sample. With modification of the tomography methodology, it may also be possible the consider and compare the fidelity of entangling gates based on combinations of microwave and optical pulses. The project will run alongside the EPSRC grant, "Light Switchable Molecular Qugates" (EP/Y01748X/1, https://gow.epsrc.ukri.org/NGBOViewGrant.aspx?GrantRef=EP/Y01748X/1).
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