We observe the unconventional photon blockade effect in quantum dot cavity QED, which, in contrast to the conventional photon blockade, operates in the weak coupling regime. A single quantum dot transition is simultaneously coupled to two orthogonally polarized optical cavity modes, and by careful tuning of the input and output state of polarization, the unconventional photon blockade effect is observed. We find a minimum second-order correlation g ð2Þ ð0Þ ≈ 0.37, which corresponds to g ð2Þ ð0Þ ≈ 0.005 when corrected for detector jitter, and observe the expected polarization dependency and photon bunching and antibunching; close by in parameter space, which indicates the abrupt change from phase to amplitude squeezing. DOI: 10.1103/PhysRevLett.121.043601 A two-level system strongly coupled to a cavity results in polaritonic dressed states with a photon-number dependent energy. This dressing gives rise to the photon blockade effect [1,2] resulting in photon-number dependent transmission and reflection, enabling the transformation of incident coherent light into specific photon number states such as single photons. Single photon sources are a crucial ingredient for various photonic quantum technologies ranging from quantum key distribution to optical quantum computing. Such sources are characterized by a vanishing second-order autocorrelation g ð2Þ ð0Þ ≈ 0 [3]. In the strong coupling regime, where the coupling between the two-level system and the cavity is larger than the cavity decay rate ðg > κÞ [4], photon blockade has been demonstrated in atomic systems [5], quantum dots in photonic crystal cavities [6], and circuit QED [7,8]. At the onset of the weak coupling regime (g ≈ κ), it has been shown that by detuning the dipole transition frequency with respect to the cavity resonance, photon blockade can still be observed [9]. However, moving further into the weak coupling regime (g < κ), which is much easier to achieve [10,11] (in particular if one aims for a small polarization mode splitting), the conventional photon blockade is no longer possible because the energy gap between the polariton states vanishes. Nevertheless, also in the weak coupling regime, the two-level system enables photon number sensitivity, which has recently enabled high-quality single photon sources using polarization postselection [12][13][14] or optimized cavity in-coupling [15,16].In 2010, Liew and Savona introduced the concept of the unconventional photon blockade (UPB) [17,18]

We present a fully fiber-coupled source of high-fidelity single photons. An (In,Ga)As semiconductor quantum dot is embedded in an optical Fabry-Perot microcavity with a robust design and rigidly attached single-mode fibers, which enables through-fiber cross-polarized resonant laser excitation and photon extraction. Even without spectral filtering, we observe that the incident coherent light pulses are transformed into a stream of single photons with high purity (97%) and indistinguishability (90%), which is measured at an in-fiber brightness of 5% with an excellent cavity-mode-to-fiber coupling efficiency of 85%. Our results pave the way for fully fiber-integrated photonic quantum networks. Furthermore, our method is equally applicable to fiber-coupled solid-state cavity-QED-based photonic quantum gates. DOI: 10.1103/PhysRevApplied.9.031002 Every isolated two-level quantum system-for example, an atom, an ion, a color center, or a quantum dot-can, in principle, be turned into a bright single-photon source [1,2]. Ideally, such a source produces a stream of single photons, with never more or less than one photon per time bin, and with all having the same Fourier limited spectrum and timing. Such a source would be essential for the exploration of numerous quantum technologies, among them optical quantum computing [3-6] and simulation [7]. Furthermore, the reduced fluctuations of such single-photon light would enable exciting opportunities if noise is a limiting factor, in fields ranging from metrology to microscopy.However, only very recently have high-fidelity singlephoton sources been demonstrated [8][9][10][11][12][13] that simultaneously fulfill the key requirements: near-unity single-photon purity and indistinguishability of consecutively emitted photons, and high brightness. For a single-photon source, high brightness and on-demand availability is crucial for the efficient implementation of quantum photonic protocols. Additionally, to exploit the power of quantum interference, consecutively produced photons need to be indistinguishable, meaning that their wave functions must overlap well. Until recently, heralded spontaneous parametric down-conversion sources [14] were the state of the art for single-photon sources [15], with which most quantum communication and optical quantum computing protocols have been demonstrated [16]. The main problem with these sources is that the Poissonian statistics of the generated twin photons will always result in a trade-off between single-photon purity (the absence of N > 1 photon number states) and brightness (the probability of obtaining a photon per time slot).One way to deterministically produce single photons is to use trapped atoms [17], where single-photon rates up to around 100 kHz have recently been obtained [18]. In order to enable integration and an increase of the photon rate, solid-state systems have been investigated: of particular promise are semiconductor quantum dots (QDs) [1,19,20]. QDs have nanosecond-lifetime transitions that enable gigahertz-rate produc...

Single photon nonlinearities based on a semiconductor quantum dot in an optical microcavity are a promising candidate for integrated optical quantum information processing nodes. In practice, however, the finite quantum dot lifetime and cavity-quantum dot coupling lead to reduced fidelity. Here we show that, with a nearly polarization degenerate microcavity in the weak coupling regime, polarization pre- and postselection can be used to restore high fidelity. The two orthogonally polarized transmission amplitudes interfere at the output polarizer; for special polarization angles, which depend only on the device cooperativity, this enables cancellation of light that did not interact with the quantum dot. With this, we can transform incident coherent light into a stream of strongly correlated photons with a second-order correlation value up to 40, larger than previous experimental results, even in the strong-coupling regime. This purification technique might also be useful to improve the fidelity of quantum dot based logic gates.