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arXiv:1706.09259

Experimental protection of the coherence of a molecular qubit exceeding a millisecond

Yingqiu Dai, Zhifu Shi, Yue Fu, Xi Qin, Shiwei Mu, Yang Wu, Ji‐Hu Su, Lei Qin, Yuan‐Qi Zhai, Yi‐Fei Deng, Xing Rong, Jiangfeng Du

Published Jun 28, 2017• 2 citations
arXivHTMLPDF

Abstract

There are several important solid-state systems, such as defects in solids, superconducting circuits and molecular qubits, for attractive candidates of quantum computations. Molecular qubits, which benefit from the power of chemistry for the tailored and inexpensive synthesis of new systems, face the challenge from decoherence effect. The decoherence effect is due to the molecular qubits' inevitable interactions to their environment. Strategies to overcome decoherence effect have been developed, such as designing a nuclear spin free environment and working at atomic clock transitions. These chemical approaches, however, have some fundamental limitations. For example, chemical approach restricts the ligand selection and design to ligands with zero nuclear magnetic dipole moment, such as carbon, oxygen, and sulfur. Herein, a physical approach, named Dynamical decoupling (DD), is utilized to combat decoherence, while the limitations of the chemical approaches can be avoided. The phase memory time $T_2$ for a transition metal complex has been prolonged to exceed one millisecond ($1.4~$ms) by employing DD. The single qubit figure of merit $Q_M $ reaches $ 1.4\times 10^5$, which is $40$ times better than that previously reported value for such system. Our results show that molecular qubits, with milliseconds $T_2$, are promising candidates for quantum information processing.

DOI
10.48550/arxiv.1706.09259

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Molecular qubits are promising as they can benefit from tailoring and versatile design of chemistry. It is essential to reduce the decoherence of molecular qubits caused by their interactions with the environment. Herein the dynamical decoupling (DD) technique is utilized to combat such decoherence. The coherence time for a transition-metal complex (PPh4)2[Cu(mnt)2] is prolonged from 6.8 μs to 1.4 ms. The ratio of the coherence time and the length of π/2 pulse, defined as the single qubit figure of merit (Q M), reaches 1.4 × 105, which is 40 times greater than what previously reported for this molecule. Our results show that molecular qubits, with milliseconds coherence time, are promising candidates for quantum information processing.
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Conventional computing faces a huge technical challenge as traditional transistors will soon reach their size limitations. This will halt progress in reaching faster processing speeds and to overcome this problem, require an entirely new approach. Quantum computing (QC) is a natural solution offering a route to miniaturisation by, for example, storing information in electron or nuclear spin states, whilst harnessing the power of quantum physics to perform certain calculations exponentially faster than its classical counterpart. However, QCs face many difficulties, such as, protecting the quantum-bit (qubit) from the environment and its irreversible loss through the process of decoherence.Hybrid systems provide a route to harnessing the benefits of multiple degrees of freedom through the coherent transfer of quantum information between them. In this thesis I show coherent qubit transfer between electron and nuclear spin states in a 15N@C60 molecular system (comprising a nitrogen atom encapsulated in a carbon cage) and a solid state system, using phosphorous donors in silicon (Si:P). The propagation uses a series of resonant mi- crowave and radiofrequency pulses and is shown with a two-way fidelity of around 90% for an arbitrary qubit state. The transfer allows quantum information to be held in the nuclear spin for up to 3 orders of magnitude longer than in the electron spin, producing a 15N@C60 and Si:P ‘quantum memory’ of up to 130 ms and 1.75 s, respectively. I show electron and nuclear spin relaxation (T1), in both systems, is dominated by a two-phonon process resonant with an excited state, with a constant electron/nuclear T1 ratio. The thesis further investigates the decoherence and relaxation properties of metal atoms encapsulated in a carbon cage, termed metallofullerenes, discovering that exceptionally long electron spin decoherence times are possible, such that these can be considered a viable QC candidate.
Molecular qubits have emerged as versatile candidates for quantum information technologies.Their appeal lies in the chemical tunability, the possibility of large-scale synthetic control and their integration with solid-state and solution-phase architectures (Bayliss et al., 2022;Wojnar et al., 2024).In contrast to solid-state defects or superconducting qubits, molecular qubits can be engineered at the atomic level, allowing researchers to systematically tailor their electronic, magnetic, and vibrational properties (Laorenza & Freedman, 2022).However, the practical implementation of molecular qubits is limited by decoherence and relaxation processes, which shorten the time window over which quantum information can be stored and manipulated.
Dynamical decoupling (DD) and Decoherence free subspace (DFS) are powerful techniques in quantum computing aimed at enhancing fidelity by mitigating the detrimental effects of noise and environmental interactions. In quantum systems, noise and decoherence can degrade the coherence of quantum states, leading to errors and reduced computational accuracy. DD employs carefully designed sequences of control pulses to periodically intervene and counteract these undesired interactions, while DFS utilizes subspace encoding techniques to protect quantum information from decoherence. To evaluate and compare the fidelity achieved by DD and DFS, this paper presents a series of experiments considering the effect of time and the number of qubits. The experiments provide valuable insights into the performance of both methods under different conditions. By systematically examining the fidelity results, the impact of time and the scaling behavior with increasing qubits can be assessed. The experimental findings showcase the effectiveness of DD in reducing the impact of noise and extending the coherence time of quantum states. By strategically manipulating the quantum system, DD minimizes error accumulation and preserves the integrity of fragile quantum information. It is observed that the decrease in fidelity with increasing t is relatively small and gradual. Despite being outperformed by DD, the DFS method exhibits robustness and maintains a high degree of fidelity even in the presence of extended time durations.