Interaction- and measurement-free quantum Zeno gates for universal computation with single-atom and single-photon qubits

By extending the concept of interaction-free imaging to the few-atom level, we show that asymptotically on-demand interaction- and measurement-free quantum logic gates can be realized for both single-atom and single-photon qubits. The interaction-free feature suppresses the possibility of qubit decoherence via atomic spontaneous decay, while the elimination of measurements can significantly reduce errors arising from detector inefficiency. We present a general theory of universal quantum Zeno gates, and discuss physical implementations for quantum-information processing with individual atoms and photons. In addition, we propose a loss-tolerant protocol for long-distance quantum communication using quantum Zeno gates incorporated into a Mach-Zehnder interferometer. The efficiency of our Zeno gates is limited primarily by the imprecise control of atom-photon scattering and the finite number of feedback cycles N due to the limited finesse of the optical ring cavity. We find that the success probability scales as 1-O (1/N), and for realistic parameters could be as high as 98.4%. Successful generation of atom-atom entanglement can be heralded by detection of the ancillary photon, upon which the fidelity scales as 1-O (1/ N2), with an achievable fidelity of 99.994%, which comes at the cost of reducing the success probability by the detector efficiency.