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Unlike conventional laser, the topological laser is able to emit coherent light robustly against disorders and defects because of its nontrivial band topology. As a promising platform for low-power consumption, exciton polariton topological lasers require no population inversion, a unique property that can be attributed to the part-light-part-matter bosonic nature and strong nonlinearity of exciton polaritons. Recently, the discovery of higher-order topology has shifted the paradigm of topological physics to topological states at boundaries of boundaries, such as corners. However, such topological corner states have never been realized in the exciton polariton system yet. Here, on the basis of an extended two-dimensional Su-Schrieffer-Heeger lattice model, we experimentally demonstrate the topological corner states of perovskite polaritons and achieved polariton corner state lasing with a low threshold (approximately microjoule per square centimeter) at room temperature. The realization of such polariton corner states also provides a mechanism of polariton localization under topological protection, paving the way toward on-chip active polaritonics using higher-order topology.

The topological spin-Hall effect causes different spins to propagate in opposite directions based on Hermitian physics. The non-Hermitian skin effect causes the localization of a large number of modes of a system at its edges. Here we propose a system based on exciton-polariton elliptical micropillars hosting both the effects. The polarization splitting of the elliptical micropillars gives rise to the topological spin-Hall effect in a one dimensional lattice. When a circularly polarized external incoherent laser is used to imbalance effective decay rates of the different spin polarizations, the system transits to a non-Hermitian regime showing the skin effect. These effects have implications for robust polariton transport as well as the deterministic formation of multiply charged vortices and persistent currents., Close to the published version in ACS Photonics

The detection of entanglement provides a definitive proof of quantumness. Its ascertainment might be challenging for hot or macroscopic objects, where entanglement is typically weak, but nevertheless present. Here we propose a platform for measuring entanglement by connecting the objects of interest to an uncontrolled quantum network, whose emission (readout) is trained to learn and sense the entanglement of the former. First, we demonstrate the platform and its features with generic quantum systems. As the network effectively learns to recognise quantum states, it is possible to sense the amount of entanglement after training with only non-entangled states. Furthermore, by taking into account measurement errors, we demonstrate entanglement sensing with precision that scales beyond the standard quantum limit and outperforms measurements performed directly on the objects. Finally, we utilise our platform for sensing gravity-induced entanglement between two masses and predict an improvement of two orders of magnitude in the precision of entanglement estimation compared to existing techniques., Main text: 6 pages with 4 figures. Appendix: 10 pages with 5 figures

The minimum time required for a quantum system to evolve to a distinguishable state is set by the quantum speed limit, and consequently influences the change of quantum correlations and other physical properties. Here we study the time required to maximally entangle two principal systems interacting either directly or via a mediating ancillary system, under the same energy constraints. The direct interactions are proved to provide the fastest way to entangle the principal systems, but it turns out that there exist mediated dynamics that are just as fast. We show that this can only happen if the mediator is initially correlated with the principal systems. These correlations can be fully classical and can remain classical during the entangling process. The final message is that correlations save energy: one has to supply extra energy if maximal entanglement across the principal systems is to be obtained as fast as with an initially correlated mediator., 8 pages, 3 figures

Quantized vortices appearing in topological excitations of quantum phase transition play a pivotal role in strongly correlated physics involving the underlying confluence of superfluids, Bose–Einstein condensates and superconductors. Exciton polaritons as bosonic quasiparticles have enabled studies of non-equilibrium quantum gases and superfluidity. Exciton–polariton condensates in artificial lattices intuitively emulate energy-band structures and quantum many-body effects of condensed matter, underpinning constructing vortex lattices and controlling quantum fluidic circuits. Here, we harness exciton–polariton quantum fluids of light in a frustrated kagome lattice based on robust metal–halide perovskite microcavities, to demonstrate vortex lasing arrays and modulate their configurations at room temperature. Tomographic energy–momentum spectra unambiguously reveal massless Dirac bands and quenched kinetic-energy flat bands coexisting in kagome lattices, where polariton condensates exhibit prototypical honeycomb and kagome spatial patterns. Spatial coherence investigations illustrate two types of phase textures of polariton condensates carrying ordered quantized-vortex arrays and π-phase shifts, which could be selected when needed using lasing emission energy. Our findings offer a promising platform on which it is possible to study quantum-fluid correlations in complex polaritonic lattices and highlight feasible applications of structured light.

Multiquantum-well (MQW) perovskite is one of the forerunners in high-efficiency perovskite LED (PeLEDs) research. Despite the rapid inroads, PeLEDs suffer from the pertinent issue of efficiency decrease with increasing brightness, commonly known as "efficiency roll-off". The underlying mechanisms are presently an open question. Herein, we explicate the E-field effects on the exciton states in the archetypal MQW perovskite (C6H5C2H4NH3)2PbI4, or PEPI, in a device-like architecture using field-assisted transient spectroscopy and theoretical modeling. The applied E-field results in a complex interplay of spectral blueshifts and enhancement/quenching of the different exciton modes. The former originates from the DC Stark shift, while the latter is attributed to the E-field modulation of the transfer rates between bright/dark exciton modes. Importantly, our findings uncover crucial insights into the photophysical processes under E-field modulation contributing to efficiency roll-off in MQW PeLEDs. Electrical modulation of exciton properties presents exciting possibilities for signal processing devices. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version Q.Z. is supported by the Singapore Ministry of Education under its AcRF Tier 2 Grant MOE2019-T2-2-2-066. T.K., K.D., and T.C.H.L. are supported by the AcRF Tier 2 Grant MOE2019-T2-1-004. T.C.S., S.Y., and M.F. acknowledge the support from the Singapore National Research Foundation NRF-NRFI-2018-04 and AcRF Tier 2 Grants MOE2019-T2-1-006 and MOE-T2EP50120-0004.

We consider theoretically a system of exciton-polariton micropillars arranged in a honeycomb lattice. The naturally present TE-TM splitting and an alternating Zeeman splitting, where the different sublattices experience opposite Zeeman splitting, shifts the Dirac points in energy, giving rise to antichiral behavior. In a strip geometry having zigzag edges, two pairs of edge states exist and propagate in the same direction (including the states at the opposite edges). The edge modes localized at the opposite edges have opposite spins (circular polarizations), which leads to co-propagating "+-" spin channels. The antichiral edge states are protected by non-zero winding numbers and can propagate around a 60 degree bend without being reflected. We further compare the transport properties of these edge states with chiral edge modes and propose a scheme to realize them experimentally. Ministry of Education (MOE) Published version The work was supported by the Ministry of Education, Singapore (Grant No. MOE2019-T2-1-004).

Parametric nonlinear optical processes are at the heart of nonlinear optics underpinning the central role in the generation of entangled photons as well as the realization of coherent optical sources. Exciton-polaritons are capable to sustain parametric scattering at extremely low threshold, offering a readily accessible platform to study bosonic fluids. Recently, two-dimensional transition-metal dichalcogenides (TMDs) have attracted great attention in strong light–matter interactions due to robust excitonic transitions and unique spin-valley degrees of freedom. However, further progress is hindered by the lack of realizations of strong nonlinear effects in TMD polaritons. Here, we demonstrate a realization of nonlinear optical parametric polaritons in a WS2 monolayer microcavity pumped at the inflection point and triggered in the ground state. We observed the formation of a phase-matched idler state and nonlinear amplification that preserves the valley population and survives up to room temperature. Our results open a new door towards the realization of the future for all-optical valley polariton nonlinear devices. Ministry of Education (MOE) Submitted/Accepted version We acknowledge the following funding sources: National Natural Science Foundation of China under grant no. 12020101003 and funding support from State Key Laboratory of Low-Dimensional Quantum Physics of Tsinghua University (Q.X.); Singapore Ministry of Education via the AcRF Tier 3 Programme ‘Geometrical Quantum Materials’ and AcRF Tier 2 project under grant nos. MOE2018-T3-1-002 and MOE2018-T2-2-068 (T.C.H.L.). Ministry of Education, University and Research of Italy via the PRIN project ‘Interacting Photons in Polariton Circuits—INPhoPOL’, FISR-COVID project ‘Wavesense’ and Joint Bilateral Agreement CNR-RFBR (Russian Foundation for Basic Research) project `Hardware implementation of a polariton neural network for neuromorphic computing`, Triennal Program 2021–2023, under grant nos. 2017P9FJBS_001 and FISR2020IP_04324 (D.S.).

Unconventional photon blockade refers to the suppression of multi-photon states in weakly nonlinear optical resonators via the destructive interference of different excitation pathways. It has been studied in a pair of coupled nonlinear resonators and other few-mode systems. Here, we show that unconventional photon blockade can be greatly enhanced in a chain of coupled resonators. Specifically, the strength of the nonlinearity in each resonator needed to achieve unconventional photon blockade is suppressed exponentially with lattice size. The analytic derivation, based on a weak drive approximation, is validated by wavefunction Monte Carlo simulations. These findings show that customized lattices of coupled resonators can be powerful tools for controlling multi-photon quantum states., Comment: 17 pages, 6 figures

The emerging technology of quantum neural networks (QNNs) offers a quantum advantage over classical artificial neural networks (ANNs) in terms of speed or efficiency of information processing tasks. It is well established that nonlinear mapping between input and output is an indispensable feature of classical ANNs, while in a QNN the roles of nonlinearity are not yet fully understood. As one tends to think of QNNs as physical systems, it is natural to think of nonlinear mapping originating from a physical nonlinearity of the system, such as Kerr nonlinearity. Here we investigate the effect of Kerr nonlinearity on a bosonic QNN in the context of both classical (simulating an XOR gate) and quantum (generating Schrödinger cat states) tasks. Aside offering a mechanism of nonlinear input-output mapping, Kerr nonlinearity reduces the effect of noise or losses, which are particularly important to consider in the quantum setting. We note that nonlinear mapping may also be introduced through a nonlinear input-output encoding rather than a physical nonlinearity: for example, an output intensity is already a nonlinear function of input amplitude. While in such cases Kerr nonlinearity is not strictly necessary, it still increases the performance in the face of noise or losses. Ministry of Education (MOE) Published version This work was supported by the Singapore Ministry of Education under its AcRF Tier 2 Grant T2EP50121-0006.

International audience; Excitons in nitride quantum wells (QWs) are naturally indirect due to the strong internal electric field: electron and hole within such excitons are spatially separated, leading to strong dipole moments and long radiative lifetimes. The physics of indirect excitons (IXs) has been extensively studied in GaAs-based heterostructures: they can propagate over large distances, be trapped in gate-controlled electrostatic traps, and form a cold and dense gas of interacting bosons. Compared to traditional IXs in arsenide heterostructures, IXs in GaN QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature, and over a much larger density range. The length scale of the excitonic transport is an important figure of merit of the indirect excitons. The excitonic transport was first observed at low temperature in the case of hetero-epitaxy of the GaN QWs on sapphire substrates [1]. The improvement of the sample quality through homo-epitaxy on GaN substrates allowed observing an efficient transport up to room temperature [2]. Under a focused non-resonant excitation of the quantum well, we observe a spatial decay of the exciton density (deduced from the local blue shift of the QW transition), with a length scale of 100µm at 10K and 30µm at T=300K in our most recent homo-epitaxial samples. Such long propagation distances are a pre-requisite to the development of IX-based optoelectronic devices operating on a large temperature range, and to the exploration of exciton collective states. The realization of an excitonic trap is a first step forward, and we recently investigated the exciton transport in electrostatic traps formed when a semi-transparent gate is deposited on top of the QW sample. Trapping of the excitons is observed, with a typical trap depth of 60-90 meV. The excitons can also be generated outside the trap and relax into the trap, so to spatially separate the trap from the generation spot, which is expected to be a source of heat and decoherence for the excitonic cloud. These results are promising for the investigation of trapped collective states of IXs, and their cooling into exotic phases.