在一个飞行的光学光子量子位(偏振)和一个被束缚的原子量子位(自旋)之间形成一个量子门，是量子信息学领域一个长期未能实现的目标。这种“门”对于将量子计算升级到大量量子位和将量子通信升级到长距离来说都是所必需的。现在，独立工作的两个小组报告了这种“门”的成功实现。Gerhard Rempe及同事演示了在被激光束缚的一个原子量子位和单独一个光子之间的一个量子门，在其中该光子的偏振正是根据该原子的自旋状态被翻转。Mikhail Lukin及同事描述了一个类似的成果——在被束缚在一个光子晶体附近的单独一个原子与单独一个光子之间的一个量子门效应。
The steady increase in control over individual quantum systems supports the promotion of a quantum technology that could provide functionalities beyond those of any classical device. Two particularly promising applications have been explored during the past decade: photon-based quantum communication, which guarantees unbreakable encryption but which still has to be scaled to high rates over large distances, and quantum computation, which will fundamentally enhance computability if it can be scaled to a large number of quantum bits (qubits). It was realized early on that a hybrid system of light qubits and matter qubits could solve the scalability problem of each field—that of communication by use of quantum repeaters, and that of computation by use of an optical interconnect between smaller quantum processors. To this end, the development of a robust two-qubit gate that allows the linking of distant computational nodes is “a pressing challenge”. Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The gate mechanism presented is deterministic and robust, and is expected to be applicable to almost any matter qubit. It is based on reflection of the photonic qubit from a cavity that provides strong light–matter coupling. To demonstrate its versatility, we use the quantum gate to create atom–photon, atom–photon–photon and photon–photon entangled states from separable input states. We expect our experiment to enable various applications, including the generation of atomic and photonic cluster states and Schrödinger-cat states, deterministic photonic Bell-state measurements, scalable quantum computation and quantum communication using a redundant quantum parity code.