A light approach to quantum advantage Quantum computational advantage or supremacy is a long-anticipated milestone toward practical quantum computers. Recent work claimed to have reached this point, but subsequent work managed to speed up the classical simulation and pointed toward a sample size–dependent loophole. Quantum computational advantage, rather than being a one-shot experimental proof, will be the result of a long-term competition between quantum devices and classical simulation. Zhong et al. sent 50 indistinguishable single-mode squeezed states into a 100-mode ultralow-loss interferometer and sampled the output using 100 high-efficiency single-photon detectors. By obtaining up to 76-photon coincidence, yielding a state space dimension of about 10 30 , they measured a sampling rate that is about 10 14 -fold faster than using state-of-the-art classical simulation strategies and supercomputers. Science , this issue p. 1460

Quantum teleportation provides a 'disembodied' way to transfer quantum states from one object to another at a distant location, assisted by previously shared entangled states and a classical communication channel. As well as being of fundamental interest, teleportation has been recognized as an important element in long-distance quantum communication, distributed quantum networks and measurement-based quantum computation. There have been numerous demonstrations of teleportation in different physical systems such as photons, atoms, ions, electrons and superconducting circuits. All the previous experiments were limited to the teleportation of one degree of freedom only. However, a single quantum particle can naturally possess various degrees of freedom--internal and external--and with coherent coupling among them. A fundamental open challenge is to teleport multiple degrees of freedom simultaneously, which is necessary to describe a quantum particle fully and, therefore, to teleport it intact. Here we demonstrate quantum teleportation of the composite quantum states of a single photon encoded in both spin and orbital angular momentum. We use photon pairs entangled in both degrees of freedom (that is, hyper-entangled) as the quantum channel for teleportation, and develop a method to project and discriminate hyper-entangled Bell states by exploiting probabilistic quantum non-demolition measurement, which can be extended to more degrees of freedom. We verify the teleportation for both spin-orbit product states and hybrid entangled states, and achieve a teleportation fidelity ranging from 0.57 to 0.68, above the classical limit. Our work is a step towards the teleportation of more complex quantum systems, and demonstrates an increase in our technical control of scalable quantum technologies.

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...