近日，瑞士苏黎世联邦理工学院的Rebekka Garreis&Chuyao Tong及其研究团队取得一项新进展。经过不懈努力，他们揭示双层石墨烯量子点的长寿命谷态。相关研究成果已于2024年1月17日在国际知名学术期刊《自然—物理学》上发表。

该研究团队测量了门定义双层石墨烯量子点器件中自旋态和谷态的特征弛豫时间。他们发现，不同的谷状态可以彼此区分，保真度超过99%。更令人惊讶的是，谷三重态和单重态之间的弛豫时间超过500ms，比自旋态的弛豫时间长一个数量级以上。这项工作为未来对谷量子比特相干性的测量奠定了基础，并证明了双层石墨烯是一种实用的平台，可以容纳电控、长寿命的谷量子比特。

据悉，双层石墨烯是一种具有前景的二维材料，可作为电可控量子比特的平台。特别令人关注的是其在谷自由度中编码量子信息的能力，这是由六方晶体结构的对称性产生的双重轨道简并。使用谷态可能是有利的，因为已知的自旋和轨道混合机制不太可能在谷态中起作用，从而提供更鲁棒的量子比特。与谷态相关的贝里曲率允许对其能量进行电控制，为相干量子比特操纵提供了途径。然而，谷态的弛豫时间尚不清楚，这最终限制了这些量子比特的相干性，从而限制了它们作为实用量子比特的适用性。

附：英文原文

Title: Long-lived valley states in bilayer graphene quantum dots

Author: Garreis, Rebekka, Tong, Chuyao, Terle, Jocelyn, Ruckriegel, Max Josef, Gerber, Jonas Daniel, Gchter, Lisa Maria, Watanabe, Kenji, Taniguchi, Takashi, Ihn, Thomas, Ensslin, Klaus, Huang, Wei Wister

Issue&Volume: 2024-01-17

Abstract: Bilayer graphene is a promising platform for electrically controllable qubits in a two-dimensional material. Of particular interest is the ability to encode quantum information in the valley degree of freedom, a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure. The use of valleys could be advantageous, as known spin- and orbital-mixing mechanisms are unlikely to be at work for valleys, promising more robust qubits. The Berry curvature associated with valley states allows for electrical control of their energies, suggesting routes for coherent qubit manipulation. However, the relaxation time of valley states—which ultimately limits these qubits’ coherence properties and therefore their suitability as practical qubits—is not yet known. Here we measure the characteristic relaxation times of these spin and valley states in gate-defined bilayer graphene quantum dot devices. Different valley states can be distinguished from each other with a fidelity of over 99%. The relaxation time between valley triplets and singlets exceeds 500ms and is more than one order of magnitude longer than for spin states. This work facilitates future measurements on valley-qubit coherence, demonstrating bilayer graphene as a practical platform hosting electrically controlled, long-lived valley qubits.

DOI: 10.1038/s41567-023-02334-7

Source: https://www.nature.com/articles/s41567-023-02334-7