Abstract:Single quantum emitters (SQEs) are at the heart of quantum optics 1 We assign this fine structure to two excitonic eigen-modes whose degeneracy is lifted by a large ~0.71 meV coupling, likely due to the electron-hole exchange interaction in presence of anisotropy 8 . Magneto-optical measurements also reveal an exciton g-factor of ~8.7, several times larger than that of delocalized valley excitons 9-12 . In addition to their fundamental importance, establishing new SQEs in 2D quantum materials could give rise to practical advantages in quantum information processing, such as efficient photon extraction and high integratability and scalability. Here, we report the first observation of photon antibunching from localized SQEs in tungsten-diselenide (WSe2) monolayers. WSe2 monolayers are grown on top of a 300 nm SiO2 on silicon substrate by physical vapor transport 26 , a scalable synthesis approach (see Methods).For the optical experiments, the monolayers are held in vacuum inside a cryostat at 4.2 K, where a magnetic field is applied perpendicular to the sample plane (Faraday geometry).Experiments are performed in the reflection geometry where a confocal microscope allows 3 for both laser excitation with a beam focal spot of ~1 µm and collection of the emission (see Methods and Supplementary Fig. S1).The WSe2 monolayer is excited using a continuous-wave (cw) laser at a wavelength of 637 nm. Figure 1a shows the emergence of sharp spectral lines, which are red shifted by ~40-100 meV from the PL of the delocalized valley excitons (see right inset of Fig. 1a). With an excitation power of 6 µW, the peak intensity of the sharp lines are ~500 times stronger than the delocalized valley excitons. The left inset of Fig. 1a shows the fine structure of the highestintensity line (we call SQE1), which is composed of a doublet. The red lines are Lorentzian fits which yield linewidths of ~112 µeV and ~122 µeV (FWHM) and a splitting of 0.68 meV.A statistical histogram on 92 randomly localized emitters from 15 different monolayers is presented in Fig. 1b, yielding linewidths ranging from 58 µeV to 500 µeV, with an average spectral linewidth of 130 µeV, roughly two orders of magnitude smaller than the linewidth of the delocalized exciton PL. The linewidth of these emitters increases dramatically when the temperature is increased (see Supplementary Fig. S2).The sharp lines are highly spatially localized. Figure 1c illustrates a scanning confocal microscope image of the PL from the emission line centered at 1.7186 eV. The isolated bright spots show that the emission is from localized sites, which are likely excitons bound to atomic defects. These sharp lines show strong saturation behavior as a function of laser power. We investigate the power dependence of the SQE1 peak at 1.7156 eV (left inset of Fig. 1a) as an example. Figure 1d shows the integrated counts as a function of excitation power, demonstrating a pronounced saturation behavior similar to an atom-like two-level system.Under excitation with a 3-ps pulsed laser at ...

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Single photon sources based on semiconductor quantum dots offer distinct advantages for quantum information, including a scalable solid-state platform, ultrabrightness, and interconnectivity with matter qubits. A key prerequisite for their use in optical quantum computing and solid-state networks is a high level of efficiency and indistinguishability. Pulsed resonance fluorescence (RF) has been anticipated as the optimum condition for the deterministic generation of high-quality photons with vanishing effects of dephasing. Here, we generate pulsed RF single photons on demand from a single, microcavity-embedded quantum dot under s-shell excitation with 3-ps laser pulses. The π-pulse excited RF photons have less than 0.3% background contributions and a vanishing two-photon emission probability. Non-postselective Hong-Ou-Mandel interference between two successively emitted photons is observed with a visibility of 0.97(2), comparable to trapped atoms and ions. Two single photons are further used to implement a high-fidelity quantum controlled-NOT gate.Single photons have been proposed as promising quantum bits (qubits) for quantum communication [1], linear optical quantum computing [2, 3] and as messengers in quantum networks [4]. These proposals primarily rely upon a high degree of indistinguishability between individual photons to obtain the Hong-Ou-Mandel (HOM) type interference [5] which is at the heart of photonic controlled logic gates and photon-interference-mediated quantum networking [1][2][3][4].Among different types of single-photon emitters [6, 7], quantum dots (QDs) are attractive solid-state devices since they can be embedded in high-quality nanostructure cavities and waveguides to generate ultra-bright sources of single and entangled photons [7][8][9][10]. QDs also provide a light-matter interface [11][12][13] and can in principle be scaled to large quantum networks [14]. Two-photon HOM interference experiments using photons from a single QD [5,15,17], as well as from independent sources [18,19], have not only demonstrated the potential of QDs as single-photon sources, but also revealed the level of dephasing arising from incoherent excitation. The method of incoherent pumping (via above band-gap or p-shell excitation) typically causes reduced photon coherence times due to homogeneous broadening of the excited state [5] and uncontrolled emission time jitter from the nonradiative high-level to s-shell relaxation [6], leading to a decrease of photon indistinguishability.To eliminate these dephasings, an increasing effort has been devoted to s-shell resonant optical excitation of QDs. The Mollow triplet spectra and photon correlations of the resonance fluorescence (RF) have been measured [1][2][3]21]. Under continuous-wave (CW) laser excitation, a high degree of indistinguishability for continuously generated RF photons has been demonstrated through post-selective HOM interference [25]. However, in the CW regime, as the emission time of the RF photons is uncontrolled, the HOM interference relies on th...

Boson sampling is considered as a strong candidate to demonstrate the "quantum computational supremacy" over classical computers. However, previous proof-ofprinciple experiments suffered from small photon number and low sampling rates owing to the inefficiencies of the single-photon sources and multi-port optical interferometers. Here, we develop two central components for high-performance boson sampling: robust multi-photon interferometers with 99% transmission rate, and actively demultiplexed single-photon sources from a quantum-dot-micropillar with simultaneously high efficiency, purity and indistinguishability. We implement and validate 3-, 4-, and 5-photon boson sampling, and achieve sampling rates of 4.96 kHz, 151 Hz, and 4 Hz, respectively, which are over 24,000 times faster than the previous experiments. Our architecture is feasible to be scaled up to larger number of photons and with higher rate to race against classical computers, and might provide experimental evidence against the Extended Church-Turing Thesis.

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An outstanding goal in quantum optics and scalable photonic quantum technology is to develop a source that each time emits one and only one entangled photon pair with simultaneously high entanglement fidelity, extraction efficiency, and photon indistinguishability. By coherent two-photon excitation of a single InGaAs quantum dot coupled to a circular Bragg grating bullseye cavity with broadband high Purcell factor up to 11.3, we generate entangled photon pairs with a state fidelity of 0.90(1), pair generation rate of 0.59(1), pair extraction efficiency of 0.62(6), and photon indistinguishability of 0.90(1) simultaneously. Our work will open up many applications in high-efficiency multi-photon experiments and solid-state quantum repeaters.

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 ...

Single photon sources based on semiconductor quantum dots offer distinct advantages for quantum information, including a scalable solid-state platform, ultrabrightness, and interconnectivity with matter qubits. A key prerequisite for their use in optical quantum computing and solid-state networks is a high level of efficiency and indistinguishability. Pulsed resonance fluorescence (RF) has been anticipated as the optimum condition for the deterministic generation of high-quality photons with vanishing effects of dephasing. Here, we generate pulsed RF single photons on demand from a single, microcavity-embedded quantum dot under s-shell excitation with 3-ps laser pulses. The π-pulse excited RF photons have less than 0.3% background contributions and a vanishing two-photon emission probability. Non-postselective Hong-Ou-Mandel interference between two successively emitted photons is observed with a visibility of 0.97(2), comparable to trapped atoms and ions. Two single photons are further used to implement a high-fidelity quantum controlled-NOT gate.