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]

Spatially entangled twin photons allow the study of high-dimensional entanglement, and the Laguerre-Gauss modes are the most commonly used basis to discretize the single-photon mode spaces. In this basis, to date only the azimuthal degree of freedom has been investigated experimentally due to its fundamental and experimental simplicity. We show that the full spatial entanglement is indeed accessible experimentally; i.e., we have found practicable radial detection modes with negligible cross correlations. This allows us to demonstrate hybrid azimuthal-radial quantum correlations in a Hilbert space with more than 100 dimensions per photon.

We explore experimentally if the light's orbital angular momentum (OAM) interacts with chiral nematic polymer films. Specifically, we measure the circular dichroism of such a material using light beams with different OAM. We investigate the case of strongly focussed, non-paraxial light beams, where the spatial and polarization degrees of freedom are coupled. Within the experimental accuracy, we cannot find any influence of the OAM on the circular dichroism of the cholesteric polymer.PACS numbers:

We demonstrate the detection of bipartite bound entanglement as predicted by the Horodecki's in 1998. Bound entangled states, being heavily mixed entangled quantum states, can be produced by incoherent addition of pure entangled states. Until 1998 it was thought that such mixing could always be reversed by entanglement distillation; however, this turned out to be impossible for bound entangled states. The purest form of bound entanglement is that of only two particles, which requires higher-dimensional (d > 2) quantum systems. We realize this using photon qutrit (d = 3) pairs produced by spontaneous parametric downconversion, that are entangled in the orbital angular momentum degrees of freedom, which is scalable to high dimensions. Entanglement of the photons is confirmed via a 'maximum complementarity protocol'. This conceptually simple protocol requires only maximized complementary of measurement bases; we show that it can also detect bound entanglement. We explore the bipartite qutrit space and find that, also experimentally, a significant portion of the entangled states are actually bound entangled.

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01Å. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

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...

Higher-order optical vortices are inherently unstable in the sense that they tend to split up in a series of vortices with unity charge. We demonstrate this vortex-splitting phenomenon in beams produced with holograms and spatial light modulators and discuss its generic and practically unavoidable nature. To analyze the splitting phenomena in detail, we use a multi-pinhole interferometer to map the combined amplitude and phase profile of the optical field. This technique, which is based on the analysis of the far-field interference pattern observed behind an opaque screen perforated with multiple pinholes, turns out to be very robust and can among others be used to study very 'dark' regions of electromagnetic fields. Furthermore, the vortex splitting provides an ultra-sensitive measurement method of unwanted scattering from holograms and other phase-changing optical elements.

We demonstrate genuine multipartite quantum entanglement of four photons in their orbital angular momentum degrees of freedom, where a high-dimensional discrete Hilbert space is attached to each photon. This can encode more quantum information compared to the qubit case, but it is a long-standing problem to entangle more than two such photons. In our experiment we use pulsed spontaneous parametric down-conversion to produce the photon quadruplets, which allows us to detect about one four-photon event per second. By means of quantum state reconstruction and a suitable witness operator we find that the photon quadruplets form a genuine multipartite entangled symmetric Dicke state. This opens a new tool for addressing foundational questions in quantum mechanics, and for exploration of novel high-dimensional multiparty quantum information applications such as secret sharing. DOI: 10.1103/PhysRevLett.116.073601 Experimental control over systems where more than two particles are entangled is of interest for the study of foundational questions in quantum mechanics, and for multiparty quantum information schemes. So far, up to 14 particles have been entangled [1,2], but in each case the single-particle Hilbert space was strictly two-dimensional, i.e., qubits. For photons, the spatial degrees of freedom enable high-dimensional single particle spaces, which can be discretized in the photon orbital angular momentum (OAM). This enables implementation of novel quantum information protocols [3][4][5], and the study of fundamentally new quantum states [6,7]. To date, only two such multidimensional particles have been entangled [8,9] albeit with ever increasing dimensionality [10][11][12]; only in continuous variables, a first study goes beyond this [13]. Here, we use pulsed spontaneous parametric downconversion (SPDC) [14] to produce photon quadruplets that are entangled in their OAM, or transverse-mode, degrees of freedom [8,15], and witness genuine multipartite Dicke-type entanglement [16][17][18]. Apart from addressing foundational questions [19][20][21], this could find applications in quantum metrology, imaging, and secret sharing [22,23].Photons that are generated by near-collinear SPDC are correlated in several degrees of freedom and exhibit quantum entanglement. Apart from the well-known polarization degrees, the photons can also be correlated in their spatial degrees; this manifests itself in continuous wave vector or (the Fourier-related) position entanglement. This can be discretized using transverse optical modes, and a particular useful choice for experiments is the LaguerreGauss (LG) modes. Their azimuthal part factorizes and describes phase vortices [24] exp ðilϕÞ, where ϕ is the azimuth and l ¼ −∞, …, ∞ determines the twisting number of the wave front, corresponding to an orbital angular momentum of lℏ per photon [25] (in addition to the spin angular momentum). The LG and the related Hermite-Gauss modes have well-known propagation dynamics; thus, they are suitable for the long-distance distribution of high-d...