A quantum gate between a flying optical photon and a single trapped atom

Quantum gates — in which stationary quantum bits are combined with ‘flying’ quantum bits, that is, photons — will be essential in quantum networks; such a gate, between a laser-trapped atomic quantum bit and a single photon, is now reported. Through the gate to quantum networks The development of a quantum gate between a flying optical photonic qubit (polarization) and a single trapped atomic qubit (spin) has been a long-standing goal in quantum information science. Such gates are required both for quantum computation to be scaled to a large number of qubits and for quantum communication to be scaled to long distances. Now two groups, working independently, report the successful implementation of such gates. Gerhard Rempe and colleagues demonstrate a quantum gate between a laser-trapped atomic qubit and a single photon, where the polarization of the photon is flipped depending exactly on the spin state of the atom. Mikhail Lukin and co-workers describe a similar achievement — a quantum gate effect between a single atom trapped near a photonic crystal and a single photon. 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 1 but which still has to be scaled to high rates over large distances, and quantum computation, which will fundamentally enhance computability 2 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 3 could solve the scalability problem of each field—that of communication by use of quantum repeaters 4 , and that of computation by use of an optical interconnect between smaller quantum processors 5 , 6 . To this end, the development of a robust two-qubit gate that allows the linking of distant computational nodes is “a pressing challenge” 6 . 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 7 , 8 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,