所谓量子纠缠(Quantum entanglement)就是当粒子联系如此密切时,以至于影响一个粒子也必然会直接影响到另一个。量子纠缠被广泛用于由量子密码学(quantum cryptography)到量子隐形传态(quantum teleportation)的许多过程。量子纠缠是粒子在由2个或2个以上粒子组成系统中相互影响的现象,虽然粒子在空间上可能分开。纠缠是关于量子力学理论最著名的预测。它描述了2个粒子互相纠缠,即使相距遥远距离,一个粒子的行为将会影响另一个的状态。当其中一个被操作(例如量子测量)而状态发生变化,另一个也会即刻发生相应的状态变化。爱因斯坦将量子纠缠称为“鬼魅似的远距作用”(spooky action at a distance)。但这并不仅仅是个诡异的预测,而是已经在实验中获得的现象,比如科学家通过向2个处于室温的纠缠的小钻石发射激光。科学家希望能够建造量子计算机,利用粒子纠缠进行超高速计算[4]。
东安格利亚大学化学学院的大卫·安德鲁斯教授(Prof David Andrews)说:“当自发纠缠光子对平分输入能量时,被称为退化下转换(degenerate down-conversion, DDC)。直到现在,一直以为这样的双光子来自相同的位置。现在,一种新的非定域化机制识别显示,每一个光子对都可以从空间上分离点发出,引入一种新的基本量子起源位置上的不确定性。”
The achievement of optimum conversion efficiency in conventional spontaneous parametric down conversion requires consideration of quantum processes that entail multi-site electrodynamic coupling, actively taking place within the conversion material. The physical mechanism, which operates through virtual photon propagation, provides for photon pairs to be emitted from spatially separated sites of photon interaction; occasionally pairs will be produced in which each photon emerges from a different point in space. The extent of such non-localized generation is influenced by individual variations in both distance and phase correlation. Mathematical analysis of the global contributions from this mechanism provides a quantitative measure for a degree of positional uncertainty in the origin of down-converted emission.