基于Zeeman态编码的单原子量子比特研究

The quantum bit (qubit) is the units of the quantum information. The qubit can be stored in any two state systems which can accomplish unitary transformation. Neutral atoms also exhibit favorable properties for storing and processing quantum information and represent an alternative physical system to perform quantum computation. Their hyperfine ground states are readily prepared in pure quantum states including state superpositions and can realize the writing-in and reading-out of information. Using the entanglement of photon and atom, the interaction between the quantum state of fly qubit for transferring information and the quantum state of carrier qubit， which can be applied for storage and transfer of quantum information and meaningful for quantum network and quantum communication. Moreover, by processing qubits with quantum gates, a quantum computer is fundamentally superior to classical computers in certain cases. As the CNOT gate of quantum bit of single atom is basic component of quantum logical calculation, a large number of quantum logical gate can realize quantum calculation. Based on the previous work of a single cesium atom prepared in a large-magnetic-gradient magneto-optical trap (MOT), we study the initialization, readout, and high-speed manipulation of a qubit stored in a single cesium atom trapped in a micrometer-sized optical trap. The main contents of the study including: (1) Studying microscopic magic-wavelength optical dipole trap; (2) Preparing the trapped atom into specific hyperfine states or Zeeman states by optical pumping. The state selective detection is performed by a laser which is resonant to the atomic transition and thus pushes out the FORT only it is in the specific state; (3) Performing the single qubit operations using two-photon Raman transitions; Measuring the coherence time of the qubit using Ramsey spectroscopy and presenting an analytical model to interpret the experimental data and identify the homogeneous and inhomogeneous dephasing mechanisms；（4）Studying on the extesion and appliciation of the qubit.

量子信息科学以量子比特作为信息单元，量子比特的物理载体是任何可执行幺正变换的两能态量子系统。碱金属原子拥有长寿命的超精细基态，其可以作为量子比特的逻辑基矢实现信息的写入和读出；利用光与原子的纠缠，可以实现传递信息的飞行量子比特（光子）与存储信息的量子比特（原子）的相互作用；基于单原子量子比特的受控非门（CNOT门）是量子逻辑运算的基本构成单元，宏观数量的量子逻辑门可以实现量子计算。本项目在我们小组已掌握的单原子冷却与俘获的实验基础上，实现单原子量子态的制备和探测，进而实现基于单原子量子比特的可控操作。主要内容包括：(1) 魔术波长微米尺度光学偶极阱系统的研究；（2）光学阱中单原子超精细基态以及Zeeman磁量子态的制备及识别实验技术研究；（3） 基于单原子量子比特的相干操控实验技术研究及量子比特的退相干机制研究；（4）量子比特的扩展性研究探索。

量子信息需要量子比特来存储和处理信息，单个中性原子作为量子比特有着独特的优势。基于单原子的量子比特可以产生非经典的单光子源用于保密通讯，单粒子本身作为完美的量子系统可以演示非经典物理效应；基于中性原子的单比特经过扩展后可以实现量子寄存器或量子门应用于量子计算和量子通讯。近年来，随着对量子信息和量子存储研究的深入，对单原子的操控及其量子态的研究成为热门。.我们实验实现了单原子的俘获和观测，实验可以分辨1-10个原子信号；研究小组已经建立了可以反馈控制的单原子磁光阱实验装置，实现了微米尺度1064nm光学阱中单原子的稳定俘获，并实现了微米尺度光学阱中单原子的进一步光学冷却和温度测量；该实验系统可以进行闭环的初始化工作，提供光学阱中大于99%的单原子装载概率。实现了光学阱中单原子的超精细态制备和Zeeman态的相干操控，获得5.4秒的超精细基态寿命；实现了激发态和基态之间的量子态制备和操控。基于在光学阱中俘获的单原子，我们实验演示了中性原子的量子比特；同时，基于基态和激发态的量子态操控，我们实验演示了触发式单光子源。我们理论计算出构建铯原子魔术波长光学阱的波长为935nm，在实验上搭建了相关系统，并在单原子水平上测量了光学阱中的单原子光频移。.该项目执行的三年期间，围绕“单原子的高效制备、探测以及量子态的操控”这一主题，依照项目申请书和项目计划任务书中预定的研究内容，先后进行了以下几方面的工作：（1）单原子的制备和探测；（2）单原子超精细态的高效制备单原子以及激发态的制备和检测；（3）基态量子比特的相干操控；（4）激发态量子比特的研究；（5）基于单量子比特的可控单光子源。.单原子量子比特的相干操控，一方面可以扩展实现多比特用于量子信息的存储，另一方面，则是实现比特间的相互作用，实现量子门。