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[交流] 《自然》：量子计算机研制取得新进展

荷兰科学家最近成功利用两个量子比特（quantum bit，简称qubit），实现了逻辑运算过程。这一研究成果表明，科学家向着量子计算机的方向又迈进了重要的一步。相关论文发表在6月14日的《自然》杂志上。
荷兰代夫特工业大学（Delft University of Technology）的科学家利用现有的芯片制造工艺，分别研究了利用微超导环（tiny superconducting ring）和量子点（quantum dots）产生的两种类型的量子比特。最终，量子传输（Quantum Transport）研究小组成功地利用超导环，首次实现了两个量子比特的“控制非”（controlled-NOT）运算，这也为其他的量子运算奠定了基础。
量子计算机主要基于量子系统的独特性质（纠缠态等），而量子比特正是未来量子计算机的基本构建单位。一个量子比特能够以两种状态同时存在，而且量子比特携带的信息之间能够以特殊的方式相互“纠缠”。
与电子计算机相比，量子计算机的性能更加优越，它们有望能够完成任何普通计算机都无法完成的工作。

Nature 447, 836-839 (14 June 2007) | doi:10.1038/nature05896; Received 21 February 2007; Accepted 26 April 2007

Demonstration of controlled-NOT quantum gates on a pair of superconducting quantum bits
J. H. Plantenberg1, P. C. de Groot1, C. J. P. M. Harmans1 & J. E. Mooij1

Quantum computation requires quantum logic gates that use the interaction within pairs of quantum bits (qubits) to perform conditional operations1. Superconducting qubits may offer an attractive route towards scalable quantum computing. In previous experiments on coupled superconducting qubits, conditional gate behaviour2 and entanglement3 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.

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