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Advanced Functional Materials

Photoexcited Triplet State of Fullerene‐Palladium Porphyrin Dyad as a Spin Qutrit

Cong‐Hui Wu, Qi Xiong, Xing‐Quan Tao, G. Y. Zhang, Zheng Liu, Linan Zhou, Song Wei Gao, Ye‐Xin Wang, Shen Lin Zhou, Shang‐Da Jiang

Published Feb 25, 2025• 4 citations
Publisher

Abstract

Abstract The photoexcited triplet state of fullerene is a promising system for molecular qutrits, as it exhibits long coherence times, well‐defined multilevel quantum states in transient electron paramagnetic resonance (EPR) processes, and the advantage of easy initialization following laser radiation. However, in pristine C60, the frequency range of the EPR signal often exceeds the bandwidth of the EPR pulses due to the large zero‐field splitting effect of the localized S = 1 spin centers. This significantly hinders the selective excitation of signals at optimal positions and limits the precise quantum gating of the transient spins. To address this challenge, C60 has been modified with palladium porphyrin (PdP), forming a donor‐acceptor complex that enhances the delocalization of the triplet state's electronic spin. This modification significantly narrows the peak spacing of the triplet EPR spectrum. The quantum coherence properties of PdP‐C60 are characterized and successfully prepared a clear three‐level superposition state. Furthermore, quantum phase interference experiments validated the system's controllability as a molecular qutrit, which can achieve coherent multi‐level superposition states. This work provides new insights and methods for exploiting fullerene derivatives in quantum control and information processing applications.

DOI
10.1002/adfm.202426013

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Abstract The photoexcited triplet state of fullerene is a promising system for molecular qutrits, as it exhibits long coherence times, well‐defined multilevel quantum states in transient electron paramagnetic resonance (EPR) processes, and the advantage of easy initialization following laser radiation. However, in pristine C60, the frequency range of the EPR signal often exceeds the bandwidth of the EPR pulses due to the large zero‐field splitting effect of the localized S = 1 spin centers. This significantly hinders the selective excitation of signals at optimal positions and limits the precise quantum gating of the transient spins. To address this challenge, C60 has been modified with palladium porphyrin (PdP), forming a donor‐acceptor complex that enhances the delocalization of the triplet state's electronic spin. This modification significantly narrows the peak spacing of the triplet EPR spectrum. The quantum coherence properties of PdP‐C60 are characterized and successfully prepared a clear three‐level superposition state. Furthermore, quantum phase interference experiments validated the system's controllability as a molecular qutrit, which can achieve coherent multi‐level superposition states. This work provides new insights and methods for exploiting fullerene derivatives in quantum control and information processing applications.
Abstract High-spin magnetic molecules are promising candidates for quantum information processing because their intrinsic multiplicity facilitates information storage and computational operations. However, due to the absence of suitable sublevel splittings, their susceptibility to environmental disturbances and limitation from the selection rule, the arbitrary control of the quantum state of a molecular electron multiplet has not been realized. Here, we exploit the photoexcited triplet of C70 as a molecular electron spin qutrit with pulsed electron paramagnetic resonance. We prepared the system into 3-level superposition states characteristic of a qutrit and validated them by the tomography of their density matrices. To further elucidate the coherence of the operation and the nature of the system as a qutrit, we demonstrated the quantum phase interference in the superposition. The interference pattern is further interpreted as a map of possible evolution paths in the space of phase factors, representing the quantum nature of the 3-level system.
Paramagnetic molecules with high-spin centers can accommodate multilevel spin manipulations. In the cocrystal PdF 20 TPP·C 60 /N@C 60 ·8Benzene (ENF-PdTPP), 14 N@C 60 ( S = 3/2) exhibits distinct transition addressability in the frequency domain and a long quantum coherence time ( T 2 ≈ 5.47 μs at 250 K), making it a promising candidate for multilevel qu d it applications. A three-level superposition state was prepared at near-room temperature (250 K) and validated by quantum phase interference. By varying the evolution time (Δ τ ) of the three-level superposition state, a controllable quantum phase effect has been observed. We found that such a phenomenon can be harnessed as a resource for coherent manipulation for the realization of a fast controlled-phase (C-phase) gate (23.5 MHz), enriching the potential of this high-spin system for quantum algorithm implementations.
Entanglement is one of the most technologically important quantum phenomena and its con-trolled creation brings us a step closer to the realisation of a quantum computer. Hybrid electron and nuclear spin systems which combine long nuclear decoherence times with the high polarisation and rapid processing times of electron spins are considered reliable candidates for the representation of the fundamental building block of a quantum computer, the qubit. In the literature electron spins quite often play the role of a mediator which can access, manipulate and couple states with long coherence times, beneficial for storing quantum information. Despite the fact that an electron spin can be a useful resource for nuclear spin systems, its permanent presence can be a source of decoherence. The use of transient photoexcited electron spins provide an additional advantage and once the operations which involve the electron spin are completed, the electron spin can decay and not interfere further with the evolution of the system. In this thesis we report magnetic resonance experiments and density functional theory calculations for the demonstration of nuclear - nuclear entanglement using photoexcited triplet states. We study homonuclear and heteronuclear fullerene derivatives and we identify in each case the relevant parameters that can lead to high fidelity entangling operations. The hyperfine interaction in a homonuclear system is the key parameter which determines the degree of entanglement between the nucelar spins according to a recent theoretical proposal. We measure and calculate the hyperfine interaction in homonuclear systems with 13C nuclear spins in order to prove the feasibility of this scheme. Further experiments on a fullerene system with two nuclear spins a 31P and a 1H show that entangling operations of high fidelity which involve the demonstration of CNOT gates, are possible within the lifetime of the triplet state.
Quantum teleportation transfers quantum states between nodes in a quantum network through entanglement distribution and subsequent Bell-state measurements. Here, we report electron spin teleportation through an unexplored hole-transfer mechanism within an ensemble of covalently linked acceptor–donor–stable radical (A–D–R•) molecules. Photoexcitation of A after spin state preparation on R• results in ultrafast hole transfer to produce an entangled pair 1(A•−–D•+). A subsequent spontaneous hole transfer 1(A•−–1[D•+)–R•] → A•−–D–R+ constitutes a Bell-state measurement, projecting the spin state initially prepared on R• onto A•−. Quantum state tomography using pulse electron paramagnetic resonance spectroscopy reveals successful teleportation. A fidelity of 98% is achieved due to high entanglement purity, small Larmor frequency mismatch between sender and receiver radicals, as well as minimized delay between state preparation and teleportation. These results represent an important step in developing molecular materials that can transfer information coherently between nanoscale quantum devices. Recent advances with molecular qubits have enabled the implementation of spin quantum teleportation (QT), albeit with limited fidelity & applicability. Here, the authors demonstrate optimal QT of spin states via hole transfer in covalently linked acceptor-donor-radical molecules, opening towards rational design of molecular materials for coherent information transfer.
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.
We propose an alternative concept for a scalable spin quantum computer that combines aspects of other proposals with the advantageous features of endohedral fullerenes. The key advantages are that electron spins instead of nuclear spins are used and that the manipulation of fullerene molecules is fairly easy. Qubits are set and read out via pulsed electron-spin resonance. Addressing is provided by local magnetic fields or field gradients (A(A(A gate). The qubit-qubit interaction is mediated by magnetic dipolar coupling and can be controlled via the direction of the magnetic field with respect to the distance vector of the qubits (J(J(J gate). Molecular as well as solid-state architectures are discussed.