Realization of the Cirac–Zoller controlled-NOT quantum gate

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Abstract
Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac–Zoller proposal1 for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac–Zoller proposal1. In our experiment, two 40Ca+ ions are held in a linear Paul trap and are individually addressed using focused laser beams2; the qubits3 are represented by superpositions of two long-lived electronic states. Our work relies on recently developed precise control of atomic phases4 and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques5,6.