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The Journal of Chemical Physics

Anisotropic activations controlling doublet–quartet spin conversion of linked chromophore-radical molecular qubits in fluid

Yasuhiro Kobori, Yuya Kokado, Kevin Lars Kopp, Tsubasa Okamoto, Masaaki Fuki

Published Feb 4, 2025• 5 citations
Publisher

Abstract

Light-energy conversion processes causing alternations in spin multiplicity are attracting attention, but the development of quantum sensing technology applicable to fluid environment such as inside cells has been unexploited. How to achieve efficient energy conversion with controlling spin quantum coherence in a noisy condensed system is challenging. In this study, we investigate the effect of molecular motion on electron spin polarization to control quantum information of three-spin qubits in a fluid environment by using steric effects of organic molecules at room temperature. Using time-resolved electron paramagnetic resonance to observe light-induced generation and transfer of quantum entanglement, we directly observed a photoexcited quartet state generated in a radical-chromophore coupled system and clarified details of the electron spin polarization mechanism including a decoherence effect by activation of anisotropic molecular motion by the steric effects.

DOI
10.1063/5.0246608

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Light-energy conversion processes causing alternations in spin multiplicity are attracting attention, but the development of quantum sensing technology applicable to fluid environment such as inside cells has been unexploited. How to achieve efficient energy conversion with controlling spin quantum coherence in a noisy condensed system is challenging. In this study, we investigate the effect of molecular motion on electron spin polarization to control quantum information of three-spin qubits in a fluid environment by using steric effects of organic molecules at room temperature. Using time-resolved electron paramagnetic resonance to observe light-induced generation and transfer of quantum entanglement, we directly observed a photoexcited quartet state generated in a radical-chromophore coupled system and clarified details of the electron spin polarization mechanism including a decoherence effect by activation of anisotropic molecular motion by the steric effects.
Molecular systems with optically addressable electron spins are emerging as versatile quantum sensing platforms, offering tunable qubit properties and molecular-scale spatial control. Owing to their longer lifetimes, coupled nuclear spins provide an attractive resource for enhancing sensing performance. For molecular spin systems, the ability to initialise, coherently control and readout nuclear spins at room temperature is crucial for realising such benefits. Here, we present the first optical detection of nuclear magnetic resonance and coherent nuclear spin control with a molecular system at room temperature. Specifically, we demonstrate optically detected Rabi oscillations of 14^{14}14N nuclear spins in the photoexcited triplet state of the organic chromophore 6,13-diazapentacene. The strong hyperfine coupling between the electronic spin and 14^{14}14N nuclei enhances the nuclear Rabi frequency, enabling fast coherent nuclear-spin rotations. This work lays the foundation for leveraging nuclear spins as memories for enhanced molecular-based quantum sensing.
Abstract Recently, spin-bearing molecules have been experimentally demonstrated to have great potential as building blocks for quantum information processing due to their substantial advantages including tunability, portability, and scalability. Here, we propose a theoretical model based on the theory of open quantum systems for spin dynamics in a molecule containing one radical, which can interact with the triplet state arising from another part of the molecule owing to optical excitation and intersystem crossing. With the initial state being a classical mixture of a radical 12\frac{1}{2}21​ 1 2 -spin, the exchange interaction between the radical and the triplet produces a spin coherent state, which could potentially be used for a qubit-qutrit quantum entangling gate. Our calculations for the time-resolved electron paramagnetic resonance spectra showed good qualitative agreement with the related experimental results for radical-bearing molecules at high temperature (~77 K, the boiling point of liquid nitrogen). This work therefore lays a solid theoretical cornerstone for optically driven quantum gate operations in radical-bearing molecular materials, aiming toward high-temperature quantum information processing.
Optically readable molecular spins are attractive as quantum sensors due to their nanoscale modularity, synthetic tunability, and scope for sensitive readout. In particular, photoexcited spin-triplet states in organic molecules are appealing in supporting high optical-spin contrast at room temperature. Their utility is underpinned by their coherence, which warrants a detailed understanding of it. Here, from first principles, we systematically explore the Hahn-echo decoherence of the benchmark molecular system for room-temperature optically detected spin coherence---pentacene guest molecules coupled to a para-terphenyl host. Using generalized cluster-correlation expansion methods, we investigate the mechanisms of nuclear-spin-induced decoherence from zero to high magnetic field, exploring the role of guest vs host molecules, specific nuclei, zero-field splitting interactions, and hyperfine parameters. We describe how zero-field decoherence is driven by ~6 nuclei on the guest, while high-field decoherence is driven by ~600 nuclei in the host; how the longitudinal zero-field splitting parameter, DDD, can prolong T2T_2T2​; and the magnetic-field-dependent processes which drive decoherence. These results advance our understanding of decoherence in optically readable molecular spins, providing insight for their synthetic enhancement and deployment as quantum probes.
Abstract Controlling molecular spin quantum bits optically offers the potential to effectively reduce decoherence and raise the working temperature of quantum computers. Here, exchange interactions and spin dynamics, as mediated by an optically driven triplet state, are calculated for a molecule that consists of a pair of radicals and represents a potential quantum-circuit building block. Consistent with the previous experimental observation of spin coherence induced by the triplet state, our work demonstrates an optically driven quantum gate operation scheme in a molecule. A technological blueprint combining a two-dimensional molecular network and programmable nanophotonics, both of which are sufficiently developed, is proposed. We thus realize computational exploration of chemical databases to identify suitable candidates for molecular spin quantum bits and couplers to be hybridized with nanophotonic devices. The work presented here is proposed to realize a new approach for exploring molecular excited states and click chemistry, toward advancing molecular quantum technology.
This thesis is concerned with studying the fascinating quantum properties of real-world nanostructures embedded in a noisy condensed matter environment. The interaction with light is used for controlling and manipulating the quantum state of the systems considered here. In some instances, laser pulses also provide a way of actively probing and controlling environmental interactions. The first two research chapters assess two different ways of performing all-optical spin qubit gates in self-assembled quantum dots. The principal conclusion is that an `adiabatic' control technique holds the promise of achieving a high fidelity when all primary sources of decoherence are taken into account. In the next chapter, it is shown that an optically driven quantum dot exciton interacting with the phonons of the surrounding lattice acts as a heat pump. Further, a model is developed which predicts the temperature-dependent damping of Rabi oscillations caused by bulk phonons, finding an excellent agreement with experimental data. A different system is studied in the following chapter: two electron spin qubits with no direct interaction, yet both exchange-coupled to an optically active mediator spin. The results of this study show that these general assumptions are sufficient for generating controlled electron spin entanglement over a wide range of parameters, even in the presence of noise. Finally, the Radical Pair model of the avian compass is investigated in the light of recent experimental results, leading to the surprising prediction that the electron spin coherence time in this molecular system seems to approach the millisecond timescale.
This thesis is concerned with studying the fascinating quantum properties of real-world nanostructures embedded in a noisy condensed matter environment. The interaction with light is used for controlling and manipulating the quantum state of the systems considered here. In some instances, laser pulses also provide a way of actively probing and controlling environmental interactions. The first two research chapters assess two different ways of performing all-optical spin qubit gates in self-assembled quantum dots. The principal conclusion is that an `adiabatic' control technique holds the promise of achieving a high fidelity when all primary sources of decoherence are taken into account. In the next chapter, it is shown that an optically driven quantum dot exciton interacting with the phonons of the surrounding lattice acts as a heat pump. Further, a model is developed which predicts the temperature-dependent damping of Rabi oscillations caused by bulk phonons, finding an excellent agreement with experimental data. A different system is studied in the following chapter: two electron spin qubits with no direct interaction, yet both exchange-coupled to an optically active mediator spin. The results of this study show that these general assumptions are sufficient for generating controlled electron spin entanglement over a wide range of parameters, even in the presence of noise. Finally, the Radical Pair model of the avian compass is investigated in the light of recent experimental results, leading to the surprising prediction that the electron spin coherence time in this molecular system seems to approach the millisecond timescale.