Lu Hsuan-Hao, Simmerman Emma M, Lougovski Pavel, Weiner Andrew M, Lukens Joseph M
School of Electrical and Computer Engineering and Purdue Quantum Science and Engineering Institute, Purdue University, West Lafayette, Indiana 47907, USA.
Quantum Information Science Group, Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
Phys Rev Lett. 2020 Sep 18;125(12):120503. doi: 10.1103/PhysRevLett.125.120503.
Accurate control of two-level systems is a longstanding problem in quantum mechanics. One such quantum system is the frequency-bin qubit: a single photon existing in superposition of two discrete frequency modes. In this Letter, we demonstrate fully arbitrary control of frequency-bin qubits in a quantum frequency processor for the first time. We numerically establish optimal settings for multiple configurations of electro-optic phase modulators and pulse shapers, experimentally confirming near-unity mode-transformation fidelity for all fundamental rotations. Performance at the single-photon level is validated through the rotation of a single frequency-bin qubit to 41 points spread over the entire Bloch sphere, as well as tracking of the state path followed by the output of a tunable frequency beam splitter, with Bayesian tomography confirming state fidelities F_{ρ}>0.98 for all cases. Such high-fidelity transformations expand the practical potential of frequency encoding in quantum communications, offering exceptional precision and low noise in general qubit manipulation.
对二能级系统的精确控制是量子力学中一个长期存在的问题。这样的一个量子系统是频率模式量子比特：一个存在于两个离散频率模式叠加态中的单光子。在本信函中，我们首次在量子频率处理器中演示了对频率模式量子比特的完全任意控制。我们通过数值计算确定了电光相位调制器和脉冲整形器多种配置的最佳设置，实验证实了所有基本旋转的近单位模式转换保真度。通过将单个频率模式量子比特旋转到遍布整个布洛赫球的41个点，以及跟踪可调谐频率分束器输出所遵循的状态路径，验证了单光子水平的性能，贝叶斯断层扫描证实所有情况下的状态保真度F_ρ>0.98。这种高保真变换扩展了量子通信中频率编码的实际潜力，在一般量子比特操纵中提供了卓越的精度和低噪声。
