Enhancing the ODMR Signal of Organic Molecular Qubits

• URL: http://arxiv.org/abs/2409.19249v3
• Date: Thu, 21 Nov 2024 00:23:20 GMT
• Title: Enhancing the ODMR Signal of Organic Molecular Qubits
• Authors: Yong Rui Poh, Joel Yuen-Zhou,
• Abstract summary: In quantum information science and sensing, electron spins are often purified into a specific polarisation through an optical-spin interface. Diamond-NV centres and transition metals are both excellent platforms for these so-called colour centres. We propose to improve the optically-detected magnetic resonance signal by moving singlet populations back into the triplet $M_S=pm1$ sublevel.
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• Abstract: In quantum information science and sensing, electron spins are often purified into a specific polarisation through an optical-spin interface, a process known as optically-detected magnetic resonance (ODMR). Diamond-NV centres and transition metals are both excellent platforms for these so-called colour centres, while metal-free molecular analogues are also gaining popularity for their extended polarisation lifetimes, milder environmental impacts, and reduced costs. In our earlier attempt at designing such organic high-spin $\pi$-diradicals, we proposed to spin-polarise by shelving triplet $M_{S}=\pm1$ populations as singlets. This was recently verified by experiments albeit with low ODMR contrasts of $<1\%$ at temperatures above 5 K. In this work, we propose to improve the ODMR signal by moving singlet populations back into the triplet $M_{S}=0$ sublevel, designing a true carbon-based molecular analogue to the NV centre. Our proposal is based upon transition-orbital and group-theoretical analyses of beyond-nearest-neighbour spin-orbit couplings, which are further confirmed by ab initio calculations of a realistic trityl-based radical dimer. Microkinetic analyses point towards high ODMR contrasts of around $30\%$ under experimentally-feasible conditions, a stark improvement from previous works. Finally, in our quest towards ground-state optically-addressable molecular spin qubits, we exemplify how our symmetry-based design avoids Zeeman-induced singlet-triplet mixings, setting the scene for realising electron spin qubit gates.

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