By pulsed s-shell resonant excitation of a single quantum dot-micropillar system, we generate long streams of 1000 near-transform-limited single photons with high mutual indistinguishability. The HongOu-Mandel interference of two photons is measured as a function of their emission time separation varying from 13 ns to 14.7 μs, where the visibility slightly drops from 95.9(2)% to a plateau of 92.1(5)% through a slow dephasing process occurring at a time scale of 0.7 μs. A temporal and spectral analysis reveals the pulsed resonance fluorescence single photons are close to the transform limit, which are readily useful for multiphoton entanglement and interferometry experiments. DOI: 10.1103/PhysRevLett.116.213601 Self-assembled InGaAs quantum dots (QDs) are promising single-photon emitters with a high quantum efficiency and a fast decay rate [1]. In the past decades, extensive efforts have been devoted to producing single photons with high purity (that is, a vanishing two-photon emission probability), near-unity indistinguishability, and high extraction efficiency [2][3][4][5][6][7][8][9][10]. These key properties have been compatibly combined simultaneously on the same QD micropillar very recently [11][12][13].An important next challenge is to extend the singlephoton sources to multiple photonic quantum bits [14], as required by various quantum information protocols such as boson sampling [15], quantum teleportation [16], quantum computation [17], and quantum metrology [18]. To this aim, one approach is to use many independent QDs [19] that are tuned into an identical emission wavelength [20] and efficiently emit single photons stringently at the transform limit, that is, T 2 ¼ 2T 1 , where T 2 and T 1 are the photon's coherence time and lifetime, respectively. Another-probably less demanding-solution is based on only one perfect QD emitting single-photon pulse trains with high efficiency [11,12], which are then either demultiplexed into N spatial modes or dynamically controlled using time-bin encoding in a loop-based architecture [21]. Implementing N-photon quantum circuits in this configuration demands streams of N mutually indistinguishable single photons far apart in emission time.However, previous Hong-Ou-Mandel (HOM) type interference experiments [7][8][9][10][11][12][13] were performed with a time separation of only a few nanoseconds between two photons emitted consecutively from a QD. Spectral diffusions [22] with a time scale much slower than nanoseconds were speculated-yet without a conclusive study-to account for the mismatch between the observed near-unity transient indistinguishability and the nonunity time-averaged T 2 =2T 1 ratio [7][8][9][10]13]. Thus, it is highly desirable to study the two-photon interference as a function of their emission time separation and test how far apart the high indistinguishability persists. The ultimate goal is to generate efficient and truly transform-limited single photons, with which perfect interference can be achieved regardless of their time separation, and ...

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