Compared to other types of qubits, photon is one of a kind due to its unparalleled advantages in long-distance quantum information exchange. Therefore, photon is a natural candidate for building a large-scale, modular optical quantum computer operating at room temperature. However, low-fidelity two-photon quantum logic gates and their probabilistic nature result in a large resource overhead for fault tolerant quantum computation. While the probabilistic problem can, in principle, be solved by employing multiplexing and error correction, the fidelity of linear-optical quantum logic gate is limited by the imperfections of single photons. Here, we report the demonstration of a linear-optical quantum logic gate with truth table fidelity of 99.84(3)% and entangling gate fidelity of 99.69(4)% post-selected upon the detection of photons. The achieved high gate fidelities are made possible by our near-optimal Rydberg single-photon source. Our work paves the way for scalable photonic quantum applications based on near-optimal single-photon qubits and photon-photon gates.

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The delayed choice gedanken-experiment initially proposed by J A Wheeler reveals the nontrivial features of wave–particle duality in quantum world, that is, the choice of whether to observe the particle/wave character of a quantum system at a delayed time can affect its evolution history. In this work, the Wheeler’s delayed-choice experiment is conducted and tested in a new regime based on a Rydberg-ground level atomic Mach–Zehnder interferometer with optical coherent manipulation. Our quantum system exhibits an excellent single-excitation of the collective atomic state due to the Rydberg blockade effect, and the delayed choice of the interferometer configuration is realized by a quantum random number generator based on high-quality Rydberg single-photon source. Our experimental results demonstrate the Wheeler’s nontrivial time order inversion in this quantum system and confirm the Copenhagen interpretation of the physical reality of quantum state, which means the particle/wave character can only be ascribed to a quantum system when it was measured. This work is encouraging toward testing the foundations of quantum mechanics using the Rydberg atoms.

Quantum logic gates with fidelity above fault-tolerant threshold are building blocks for scalable quantum technologies[1,2]. Compared to other types of qubits, photon is one of a kind due to its unparalleled advantages in long-distance quantum information exchange[3-5]. As a result, high-fidelity photonic quantum operations are not only indispensable for photonic quantum computation[6-8] but also critical for quantum network[2,9]. However, two-qubit photonic quantum logic gate with fidelity comparable to that of leading physical systems, i.e. 99.7% for superconducting circuits[10] and 99.9% for trapped ions[11], has not been achieved. A major limitation is the imperfection of single photons[12]. Here, we overcome this limitation by using high-quality single photons generated from Rydberg atoms as qubits for the interference-based gate protocol, and achieve a gate fidelity up to 99.84(3)%. Our work paves the way for scalable photonic quantum applications[13-15] based on near-optimal single-photon qubits and photon-photon gates.