The possibility of investigating macroscopic coherent quantum states in polariton condensates and of engineering polariton landscapes in semiconductors has triggered interest in using polaritonic systems to simulate complex many-body phenomena. However, advanced experiments require superior trapping techniques that allow for the engineering of periodic and arbitrary potentials with strong on-site localization, clean condensate formation, and nearest-neighbor coupling. Here we establish a technology that meets these demands and enables strong, potentially tunable trapping without affecting the favorable polariton characteristics. The traps are based on a locally elongated microcavity which can be formed by standard lithography. We observe polariton condensation with non-resonant pumping in single traps and photonic crystal square lattice arrays. In the latter structures, we observe pronounced energy bands, complete band gaps, and spontaneous condensation at the M-point of the Brillouin zone.Exciton polaritons are an ideal system for studying the collective behavior of macroscopic coherent quantum states in a solid-state environment [1]. The possibility of engineering polariton trapping potentials [2] has triggered interest in using polaritonic systems to simulate complex many-body phenomena, such as the physics of high-temperature superconductors, graphene, and frustrated spin lattices [3][4][5]. Quantum simulators are envisaged as a highly desirable tool for understanding complex many-body properties of novel solid-state, chemical, and biological systems, which are otherwise difficult to access. Quantum simulations rely on the emulation of Hamiltonians via potential landscape engineering in a highly controllable quantum system [6]. Ultracold atoms are superb candidates for quantum simulation schemes [7] since modern techniques allow for arranging them in optical lattices with high precision, leading to spectacular observations such as simulating the physics of a quantum phase transition in a Bose-Hubbard system [8]. However, a system based on cold atoms needs to operate at very low temperatures in the nK-μK range, it can hardly ever be fully scalable, and its integration is difficult due to the requirement of careful isolation from the environment. Polariton gases in microcavities have been identified as promising candidates for solid-state quantum simulators, as they fulfill a range of important prerequisites. First of all, they can form bosonic condensates [9,10], which implies a macroscopic occupation of a single energy state close to thermal equilibrium [9]. Furthermore, they can enter a superfluid phase [1,11,12], possess internal (pseudo-spin) degrees of freedom [13], can be localized by lithographic [2] or optical techniques [14] possibly down to the single-polariton level [15], and their interaction constants are tunable [13]. An ideal trapping technique for the implementation of polariton quantum emulation should combine the following features: (i) the confinement depth should be tunable in a wide range;...
Parametric downconversion (PDC) in semiconductor Braggreflection waveguides (BRW) is routinely exploited for photon-pair generation in the telecommunication range. Contrary to many conventional PDC sources, BRWs offer possibilities to create spectrally broadband but nevertheless indistinguishable photon pairs in orthogonal polarizations that simultaneously incorporate high frequency entanglement. We explore the characteristics of copropagating twin beams created in a type-II ridge BRW. Our PDC source is bright and efficient, which serves as a benchmark of its performance and justifies its exploitation for further use in quantum photonics. We then examine the coalescence of the twin beams and investigate the effect of their inevitable multiphoton contributions on the observed photon bunching. Our results show that BRWs have a great potential for producing broadband indistinguishable photon pairs as well as multi-photon states.
We study the polarization properties of light emitted by quantum dots that are embedded in chiral photonic crystal structures made of achiral planar GaAs waveguides. A modification of the electromagnetic mode structure due to the chiral grating fabricated by partial etching of the waveguide layer has been shown to result in a high circular polarization degree ρ c of the quantum dot emission in the absence of external magnetic field. The physical nature of the phenomenon can be understood in terms of the reciprocity principle taking into account the structural symmetry. At the resonance wavelength, the magnitude of |ρ c | is predicted to exceed 98%. The experimentally achieved value of |ρ c | = 81% is smaller, which is due to the contribution of unpolarized light scattered by grating defects, thus breaking its periodicity. The achieved polarization degree estimated removing the unpolarized nonresonant background from the emission spectra can be estimated to be as high as 96%, close to the theoretical prediction.
Based on the interaction between different spatial modes, semiconductor Bragg-reflection waveguides (BRWs) provide a highly functional platform for non-linear optics. For achieving any desired quantum optical functionality, we must control and engineer the properties of each spatial mode. To reach this purpose we extend the Fabry-Perot technique and achieve a detailed linear optical characterization of dispersive multimode semiconductor waveguides. With this efficient broadband spectral method we gain direct experimental access to the relevant modes of our BRWs and determine their group velocities. Furthermore, we show that our waveguides have lower than expected loss coefficients. This renders them suitable for integrated quantum optics applications.
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