Department of Physics, University of California, Santa Barbara, California 93106, USA.
Nature. 2010 Sep 30;467(7315):570-3. doi: 10.1038/nature09418.
Entanglement is one of the key resources required for quantum computation, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers. In superconducting devices, two-qubit entangled states have been demonstrated and used to show violations of Bell's inequality and to implement simple quantum algorithms. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways. These are typified by the states |GHZ>= (|000+ |111>)/ sqrt [2] and |W>= (|001> + |010> + |100>)/ sqrt [3]. Here we demonstrate the operation of three coupled superconducting phase qubits and use them to create and measure |GHZ> and |W>states. The states are fully characterized using quantum state tomography and are shown to satisfy entanglement witnesses, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.
纠缠是量子计算所需的关键资源之一，因此，纠缠态的实验创建和测量对于各种量子计算机的物理实现至关重要。在超导器件中，已经演示并使用了两量子比特纠缠态来证明贝尔不等式的违背，并实现简单的量子算法。与两量子比特情况不同，所有最大纠缠的两量子比特态在局部基变换下是等价的，而三个量子比特可以以两种基本不同的方式纠缠。这些由态 |GHZ>= (|000+ |111>)/ sqrt [2] 和 |W>= (|001> + |010> + |100>)/ sqrt [3] 典型代表。在这里，我们演示了三个耦合超导相位量子比特的操作，并使用它们来创建和测量 |GHZ>和 |W>态。使用量子态层析成像对这些态进行了全面表征，并证明它们满足纠缠见证，这证实了它们确实是三量子比特纠缠的例子，而不是两量子比特纠缠的混合物。
