Photons are ideal in many aspects for quantum technologies. However, the absence of direct photon-photon interaction and the problem of wave-packet distortion inherent in nonlinear media-based interactions have long posed a grand challenge in constructing two-qubit logic gates, which are indispensable for universal quantum information processing. Here we report on the theory and design to overcome the challenge, leading to a deterministic, passive, and near-unity-fidelity (exceeding 99%) controlled-$\pi$-phase gate for photonic qubits. The proposed gate comprises only a single two-level emitter and a few cavities and operates under the key concept of time-reversal-symmetric photon transport. It ensures a nonlinear $\pi$ phase shift and circumvents the long-standing difficulty of wave- packet distortion inherent in photonic phase gates using nonlinear media. We develop an analytical solution to reveal the dynamics of two-photon wave-packet transport and to obtain the optimal design. The proposed gate architecture is compatible with the platforms of integrated photonics.

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• Cavity quantum electrodynamics
• Integrated optics
• Optical quantum information processing
• Quantum gates
• Spontaneous emission
• Photonic qubits
• T-symmetry