Liu Shengshuai, Lou Yanbo, Chen Yingxuan, Jing Jietai
State Key Laboratory of Precision Spectroscopy, Joint Institute of Advanced Science and Technology, School of Physics and Electronic Science, East China Normal University, Shanghai 200062, China.
Department of Physics, Zhejiang University, Hangzhou 310027, China.
Phys Rev Lett. 2021 Feb 12;126(6):060503. doi: 10.1103/PhysRevLett.126.060503.
The laws of quantum mechanics forbid the perfect copying of an unknown quantum state, known as the no-cloning theorem. In spite of this, approximate cloning with imperfect fidelity is possible, which opens up the field of quantum cloning. In general, quantum cloning can be divided into discrete variable and continuous variable (CV) categories. In the CV regime, all-optical implementation of the optimal N→M quantum cloning has been proposed in two original parallel works, which involves a parametric amplifier and a set of beam splitters and thus avoids the optic-electro and electro-optic conversions in the current CV quantum cloning technologies. However, such original proposal of all-optical CV optimal N→M quantum cloning scheme has never been experimentally implemented. Here, we show that optimal N→M quantum cloning of coherent states can be realized by utilizing a parametric amplifier based on four-wave mixing process in a hot atomic vapor and a set of beam splitters. In particular, we realize 1→M, 2→M, and 4→M quantum cloning. We find that the fidelity of N→M quantum cloning increases with the decrease of clone number M and the increase of original replica number N. The best cloning fidelity achieved in our experiment is about 93.3% ±1.0% in the 4→5 case. Our results may find potential applications in realizing all-optical high-fidelity quantum state transfer and all-optical high-compatibility eavesdropping attack in quantum communication networks.
量子力学定律禁止对未知量子态进行完美复制，这就是所谓的不可克隆定理。尽管如此，实现保真度不完美的近似克隆是可行的，这开启了量子克隆领域。一般来说，量子克隆可分为离散变量和连续变量（CV）两类。在连续变量体系中，两篇开创性的并行论文提出了最优N→M量子克隆的全光实现方案，该方案涉及一个参量放大器和一组分束器，从而避免了当前连续变量量子克隆技术中的光电和电光转换。然而，这种全光连续变量最优N→M量子克隆方案的原始提议从未在实验中实现。在此，我们表明，利用热原子蒸汽中基于四波混频过程的参量放大器和一组分束器，可以实现相干态的最优N→M量子克隆。特别是，我们实现了1→M、2→M和4→M量子克隆。我们发现，N→M量子克隆的保真度随着克隆数M的减少和原始复制数N的增加而提高。在我们的实验中，4→5情况下实现的最佳克隆保真度约为93.3%±1.0%。我们的结果可能在量子通信网络中实现全光高保真量子态转移和全光高兼容性窃听攻击方面找到潜在应用。
