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arXiv:2007.09825 · npj Quantum Information

Coherent manipulation and quantum phase interference in a fullerene-based electron triplet molecular qutrit

Ye‐Xin Wang, Zheng Liu, Yu‐Hui Fang, Shen Lin Zhou, Shang‐Da Jiang, Song Wei Gao

Published Feb 15, 2021• 35 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1038/s41534-021-00362-w

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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.
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.
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.
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.
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.
The phase of a superposition state is a quintessential characteristic that differentiates a quantum bit of information from a classical one. This phase can be manipulated dynamically or geometrically, and can be exploited to sensitively estimate Hamiltonian parameters, perform faithful quantum state tomography and encode quantum information into multiple modes of an ensemble. Here we discuss the methods that we have employed to manipulate and exploit the phase information of single-, two-, multi-qubit and multi-mode spin systems.
We propose a protocol to prepare and probe dark states in a single-molecule electron spin resonance based on atomic force microscopy (ESR-AFM), thus extending existing techniques developed to address Rabi oscillations. To this end, we simulate in analogy to atomic physics the coherent manipulation of a three-level system in the V-configuration using a multifrequency magnetic field. More specifically, we consider a neutral molecule with a zero-field-split (ZFS) first excited triplet and analyze within a pump-probe scheme the dynamics of the total triplet population. In the triplet subspace, the dark state is a coherent superposition formed by two ZFS states which does not evolve in time despite a continuous resonant driving towards the third one. We characterize the properties of this state comparing it to its fully incoherent counterpart. Within a density-matrix formalism for open systems, we then compute the expected dark state signatures both for ideal scenarios and for the realistic parameterization of ESR-AFM on pentacene.