Plantenberg J H, de Groot P C, Harmans C J P M, Mooij J E
Kavli Institute of NanoScience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands.
Nature. 2007 Jun 14;447(7146):836-9. doi: 10.1038/nature05896.
Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour and entanglement were demonstrated. Here we demonstrate selective execution of the complete set of four different controlled-NOT (CNOT) quantum logic gates, by applying microwave pulses of appropriate frequency to a single pair of coupled flux qubits. All two-qubit computational basis states and their superpositions are used as input, while two independent single-shot SQUID detectors measure the output state, including qubit-qubit correlations. We determined the gate's truth table by directly measuring the state transfer amplitudes and by acquiring the relevant quantum phase shift using a Ramsey-like interference experiment. The four conditional gates result from the symmetry of the qubits in the pair: either qubit can assume the role of control or target, and the gate action can be conditioned on either the 0-state or the 1-state. These gates are now sufficiently characterized to be used in quantum algorithms, and together form an efficient set of versatile building blocks.
量子计算需要量子逻辑门，这些逻辑门利用量子比特（qubit）对之间的相互作用来执行条件操作。超导量子比特可能为实现可扩展的量子计算提供一条有吸引力的途径。在先前关于耦合超导量子比特的实验中，已经证明了条件门行为和纠缠现象。在此，我们通过向一对耦合的磁通量子比特施加适当频率的微波脉冲，展示了对完整的四种不同控制非门（CNOT）量子逻辑门的选择性执行。所有两比特计算基态及其叠加态都用作输入，同时两个独立的单次测量超导量子干涉器件（SQUID）探测器测量输出态，包括量子比特间的相关性。我们通过直接测量态转移幅度以及使用类似拉姆齐干涉实验获取相关量子相移来确定门的真值表。这四个条件门源于量子比特对中的对称性：任意一个量子比特都可以承担控制或目标的角色，并且门操作可以基于0态或1态。这些门现在已经得到充分表征，可用于量子算法，并且共同构成了一组高效的通用构建模块。
