Department of Chemistry and Institute for Sustainability and Energy at Northwestern , Northwestern University , Evanston , Illinois 60208-3113 , United States.
J Am Chem Soc. 2019 Feb 6;141(5):2152-2160. doi: 10.1021/jacs.8b13155. Epub 2019 Jan 25.
The ability to prepare physical qubits in specific initial quantum states is a critical requirement for their use in quantum information science (QIS). Subnanosecond photoinduced electron transfer in a structurally well-defined donor-acceptor system can be used to produce an entangled spin qubit (radical) pair in a pure initial singlet state fulfilling this criterion. Synthetic DNA is a promising platform on which to build spin qubit arrays with fixed spatial relationships; therefore, we have prepared a series of DNA hairpins in which naphthalenediimide (NDI) is the chromophore/acceptor hairpin linker, variable-length diblock A- and G-tracts are intermediate donors, and a stilbenediether (Sd) is the terminal donor. Photoexcitation of NDI in these DNA hairpins generates high-yield, long-lived, entangled spin qubit pairs at 85 K, and time-resolved and pulse electron paramagnetic resonance (EPR) spectroscopies are used to probe their spin dynamics. Specifically, measurements of the distance-dependent dipolar coupling between the two spins are used to obtain the average spin qubit pair distance in the absence of the terminal Sd donor and reveal that one of the spins is fully delocalized across up to five adjacent guanines in a G-tract on the EPR time scale. We have recently shown that extensive spin hopping between degenerate sites accessible to one spin of the pair may result in spin decoherence. However, we observe a strong out-of-phase electron spin echo envelope modulation (OOP-ESEEM) signal from the NDI-Sd spin qubit pair in DNA hairpins showing that spin coherence is maintained across a 2 adenine A-tract followed by a 2-4 guanine G-tract as a result of rapid spin transport to Sd. These results demonstrate that pulse-EPR can manipulate coherent spin states in DNA hairpins, which is essential for quantum gate operations relevant to QIS applications.
制备处于特定初始量子态的物理量子比特的能力是将其应用于量子信息科学（QIS）的关键要求。在结构上定义明确的供体-受体系统中，亚纳秒光诱导电子转移可用于产生处于纯初始单态的纠缠自旋量子比特（自由基）对，满足这一标准。合成 DNA 是构建具有固定空间关系的自旋量子比特阵列的有前途的平台；因此，我们已经制备了一系列 DNA 发夹，其中萘二酰亚胺（NDI）是发色团/受体发夹接头，可变长度的 A-和 G-嵌段是中间供体，而二苯并二噁烷（Sd）是末端供体。这些 DNA 发夹中 NDI 的光激发在 85 K 时产生高产量、长寿命、纠缠的自旋量子比特对，并且使用时间分辨和脉冲电子顺磁共振（EPR）光谱来探测它们的自旋动力学。具体来说，测量两个自旋之间的距离相关偶极耦合来获得在没有末端 Sd 供体的情况下两个自旋的平均自旋量子比特对距离，并揭示其中一个自旋在 EPR 时间尺度上完全分布在相邻 G-嵌段中的五个相邻鸟嘌呤上。我们最近表明，一对中一个自旋可访问的简并位点之间的广泛自旋跳跃可能导致自旋退相干。然而，我们观察到来自 DNA 发夹中 NDI-Sd 自旋量子比特对的强反相电子自旋回波包络调制（OOP-ESEEM）信号，表明由于快速自旋传输到 Sd，自旋相干性在跨越 2 个腺嘌呤 A-嵌段和 2-4 个鸟嘌呤 G-嵌段后得以保持。这些结果表明，脉冲-EPR 可以操纵 DNA 发夹中的相干自旋态，这对于与 QIS 应用相关的量子门操作至关重要。
