Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
1] Department of Physics, MIT-Harvard Center for Ultracold Atoms, and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA [2] Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia.
Nature. 2015 Mar 26;519(7544):439-42. doi: 10.1038/nature14293.
Quantum-mechanically correlated (entangled) states of many particles are of interest in quantum information, quantum computing and quantum metrology. Metrologically useful entangled states of large atomic ensembles have been experimentally realized, but these states display Gaussian spin distribution functions with a non-negative Wigner quasiprobability distribution function. Non-Gaussian entangled states have been produced in small ensembles of ions, and very recently in large atomic ensembles. Here we generate entanglement in a large atomic ensemble via an interaction with a very weak laser pulse; remarkably, the detection of a single photon prepares several thousand atoms in an entangled state. We reconstruct a negative-valued Wigner function--an important hallmark of non-classicality--and verify an entanglement depth (the minimum number of mutually entangled atoms) of 2,910 ± 190 out of 3,100 atoms. Attaining such a negative Wigner function and the mutual entanglement of virtually all atoms is unprecedented for an ensemble containing more than a few particles. Although the achieved purity of the state is slightly below the threshold for entanglement-induced metrological gain, further technical improvement should allow the generation of states that surpass this threshold, and of more complex Schrödinger cat states for quantum metrology and information processing. More generally, our results demonstrate the power of heralded methods for entanglement generation, and illustrate how the information contained in a single photon can drastically alter the quantum state of a large system.
多粒子的量子力学相关（纠缠）态在量子信息、量子计算和量子计量学中很感兴趣。已经在实验上实现了具有大原子集合的有用的计量学纠缠态，但这些态显示出具有非负维格纳准概率分布函数的高斯自旋分布函数。在小离子集合中产生了非高斯纠缠态，并且最近在大原子集合中也产生了。在这里，我们通过与非常弱的激光脉冲相互作用在大原子集合中产生纠缠；值得注意的是，单个光子的检测就可以将数千个原子准备到纠缠态中。我们重建了一个负值的维格纳函数——非经典性的一个重要标志——并验证了 3100 个原子中有 2910±190 个原子处于纠缠态。对于包含几个以上粒子的集合，达到如此负的维格纳函数和几乎所有原子的相互纠缠是前所未有的。尽管所实现的态的纯度略低于纠缠诱导计量增益的阈值，但进一步的技术改进应该允许生成超过此阈值的态，以及用于量子计量学和信息处理的更复杂的薛定谔猫态。更一般地，我们的结果证明了用于纠缠态生成的预示方法的有效性，并说明了单个光子中包含的信息如何可以极大地改变大系统的量子态。
