We develop a production function estimator for the case when firms endogenously select into multiple destination markets where they compete imperfectly, and when output is denominated only in value. The estimator exploits a novel source of variation to identify demand curvature: firm-level export shares. We demonstrate that ignoring the multi-destination dimension (i.e., exporting) yields biased and inconsistent inference. We estimate in French Manufacturing data increasing total returns to scale, decreasing returns to flexible inputs, and demand elasticities between -19.42 and -3.56. These estimates imply 5 to 8 times larger effects of a U.S. tariff increase compared to estimates from alternative estimators.

The frequency or color of photons is an attractive degree of freedom to encode and distribute quantum information over long distances. However, the generation of frequency-encoded photonic qubits has so far relied on probabilistic nonlinear single-photon sources and inefficient gates. Here, we demonstrate the deterministic generation of photonic qubits hyper-encoded in frequency and polarization based on a semiconductor quantum dot in a cavity. We exploit the double dipole structure of a neutral exciton and demonstrate the generation of any quantum superposition in amplitude and phase, controlled by the polarization of the pump laser pulse. The source generates frequency-polarization single-photon qubits at a rate of 4 MHz corresponding to a generation probability at the first lens of 28 +/- 2%, with a photon number purity >98%. The photons show an indistinguishability >91% for each dipole and 88% for a balanced quantum superposition of both. The density matrix of the hyper-encoded photonic state is measured by time-resolved polarization tomography, evidencing a fidelity to the target state of 94 +/- 8% and concurrence of 77 +/- 2%, here limited by frequency overlap in our device. Our approach brings the advantages of quantum dot sources to the field of quantum information processing based on frequency encoding.

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This paper presents a method for estimating treatment effects of regulations when treated and control firms compete on the output market. We develop a GMM estimator that recovers reduced-form parameters consistent with a model of differentiated product markets with multi-plant firms, and use these estimates to evaluate counterfactual revenues and emissions. Our procedure recovers unbiased estimates of treatment effects in Monte Carlo experiments, while difference-in-differences estimators and other popular methods do not. In an application, we find that the European carbon market reduced emissions at regulated plants without undermining revenues of regulated firms, relative to an unregulated counterfactual.

Phenotypic plasticity manifested after acclimatization is a very important source of biological variability among fish species. We hypothesized that hypoxic acclimation, besides potentially generating a temporary hypoxic respiratory phenotype, would also manifest as a continued benefit after reacclimation to normoxia. Hence, we holistically characterized the respiratory phenotype of European sea bass (Dicentrarchus labrax (Linnaeus, 1758)) acclimated to normoxia with or without prior acclimation to hypoxia. Compared with the original normoxic phenotype, prior acclimation to hypoxia and return to normoxia produced a 27% higher absolute aerobic scope (AAS), a 24% higher citrate synthase activity in red muscle, and a 28% lower excess post exercise O2 consumption. Additional testing of hypoxia-acclimated fish under normoxia explored the specific effects of hypoxic acclimation. The hypoxic phenotype, when compared with the original normoxic phenotype, had a lower standard metabolic rate, a better hypoxia performance, and a lower minimum PO2 for supporting 50% AAS. Thus, respiratory plasticity allows sea bass to improve its maximum aerobic capacity after returning to normoxia from hypoxic acclimation, a potential benefit from exploiting a hypoxic habitat. Given this respiratory malleability, general predictions for marine fish exploiting a more hypoxic future should better consider respiratory plasticity and prolonged effects of hypoxic exposures.

One of the physiological mechanisms that can limit the fish's ability to face hypoxia or elevated temperature, is maximal cardiac performance. Yet, few studies have measured how cardiac electrical activity and associated calcium cycling proteins change with acclimation to those environmental stressors. To examine this, we acclimated European sea bass for 6 weeks to three experimental conditions: a seasonal average temperature in normoxia (16 °C; 100% air sat.), an elevated temperature in normoxia (25 °C; 100% air sat.) and a seasonal average temperature in hypoxia (16 °C; 50% air sat.). Following each acclimation, the electrocardiogram was measured to assess how acclimation affected the different phases of cardiac cycle, the maximal heart rate (fHmax) and cardiac thermal performance during an acute increase of temperature. Whereas warm acclimation prolonged especially the diastolic phase of the ventricular contraction, reduced the fHmax and increased the cardiac arrhythmia temperature (TARR), hypoxic acclimation was without effect on these functional indices. We measured the level of two key proteins involved with cellular relaxation of cardiomyocytes, i.e. sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and Na+/Ca2+ exchanger (NCX). Warm acclimation reduced protein level of both NCX and SERCA and hypoxic acclimation reduced SERCA protein levels without affecting NCX. The changes in ventricular NCX level correlated with the observed changes in diastole duration and fHmax as well as TARR. Our results shed new light on mechanisms of cardiac plasticity to environmental stressors and suggest that NCX might be involved with the observed functional changes, yet future studies should also measure its electrophysiological activity.

The excitonic fine structure plays a key role for the quantum light generated by semiconductor quantum dots, both for entangled photon pairs and single photons. Controlling the excitonic fine structure has been demonstrated using electric, magnetic, or strain fields, but not for quantum dots in optical cavities, a key requirement to obtain high source efficiency and near-unity photon indistinguishability. Here, we demonstrate the control of the fine structure splitting for quantum dots embedded in micropillar cavities. We propose and implement a scheme based on remote electrical contacts connected to the pillar cavity through narrow ridges. Numerical simulations show that such a geometry allows for a three-dimensional control of the electrical field. We experimentally demonstrate tuning and reproducible canceling of the fine structure, a crucial step for the reproducibility of quantum light source technology.

This study examines the causal impacts of the Clean Development Mechanism (CDM) on the environmental performance of Indian manufacturing firms, as measured by their energy use, carbon dioxide (CO2) emissions, and intensities of CO2 emissions per sales and per energy use. The impacts of CDM projects are estimated using either two-way fixed-effect regressions or an estimator built for an event study with staggered treatment (Sun and Abraham, 2021) combined with a sample of ever-treated firms only or a sample comparing treated to never-treated control firms using semi-parametric matching. We found that CDM projects significantly increased firms' CO2 emissions and energy use after treatment, but had no effect on CO2 emission intensity per sales and only a small negative effect on the CO2 content of energy use (only for the matched sample). These results reveal that CDM projects led to a positive scale effect (increased sales) after investments were made, and that these investments triggered a limited emission-reducing technique effect (decreased CO2 intensity).

Semiconductor quantum dots in cavities are promising single-photon sources. Here, we present a path to deterministic operation, by harnessing the intrinsic linear dipole in a neutral quantum dot via phonon-assisted excitation. This enables emission of fully polarized single photons, with a measured degree of linear polarization up to 0.994±0.007, and high population inversion-85% as high as resonant excitation. We demonstrate a single-photon source with a polarized first lens brightness of 0.50±0.01, a single-photon purity of 0.954±0.001, and single-photon indistinguishability of 0.909±0.004.

Over the past few decades, wealthy countries have relied increasingly on imports from developing countries, prompting concerns regarding the environmental effects of trade. Increased import demand in wealthy countries certainly increases export flows from developing countries, but emissions need not scale 1 for 1 with exports if domestic sales or emission intensity adjust endogenously to foreign demand. In this paper, we exploit detailed product-line information on production and emissions for Indian manufacturing firms to estimate how firms adjust their production decisions in response to demand shocks in trading partner markets. Using a shift-share instrument, we find that foreign demand growth increased growth in CO2 emissions at the firm level via output growth (scale effect), but that endogenous reductions in emission intensity mitigated roughly 40% of this effect. With output denominated in physical units, both effects are estimated net of price adjustments. The overall effect on CO2 emissions growth is positive, though statistically insignificant at conventional levels. We further document that the scale effect owes to increased growth in both export sales and domestic sales, and that firm-product emission intensity fell when expressed per physical unit of output. The latter result indicates that the firm-level intensity effect owes at least in part to technological adoption.

Hong-Ou-Mandel interference is a cornerstone of optical quantum technologies. We explore both theoretically and experimentally how unwanted multiphoton components of single-photon sources affect the interference visibility, and find that the overlap between the single photons and the noise photons significantly impacts the interference. We apply our approach to quantum dot single-photon sources to access the mean wave packet overlap of the single-photon component. This study provides a consistent platform with which to diagnose the limitations of current single-photon sources on the route towards the ideal device.

Semiconductor quantum dots in cavities are high-performance single-photon sources. Thus far the most efficient sources utilise resonant excitation of an unpolarized quantum emitter coupled to a highly birefringent cavity. However, this demands very high polarization extinction, and challenging experimental operation. Here, we remove these requirements by using off-resonant phonon-assisted excitation of a linear exciton dipole, exploiting the quantum dot's vibrational environment and natural asymmetry. This allows the collection of single photons that are spectrally separated from the excitation laser, and intrinsically present a very high degree of linear polarization up to 0.994 $\pm$ 0.007. This phonon-assisted excitation scheme enables very high single-photon purity and indistinguishability, and only reduces the emitter population by (15 $\pm$ 1) %, as compared to resonant excitation. Overall, we simultaneously demonstrate a polarized first lens brightness of 0.51 $\pm $ 0.01, with a single-photon purity of 0.939 $\pm$ 0.001 and corrected single-photon indistinguishability of 0.915 $\pm$ 0.003.

We present the latest advancements of solid-state based quantum light sources with higher brightness and quantum purity over several devices, demonstrating clear reproducibility of top-performances necessary for their large-scale implementation in optical quantum technologies.

Light states composed of multiple entangled photons-such as cluster states-are essential for developing and scaling-up quantum computing networks. Photonic cluster states can be obtained from single-photon sources and entangling gates, but so far this has only been done with probabilistic sources constrained to intrinsically low efficiencies, and an increasing hardware overhead. Here, we report the resource-efficient generation of polarization-encoded, individually-addressable photons in linear cluster states occupying a single spatial mode. We employ a single entangling-gate in a fiber loop configuration to sequentially entangle an ever-growing stream of photons originating from the currently most efficient single-photon source technology-a semiconductor quantum dot. With this apparatus, we demonstrate the generation of linear cluster states up to four photons in a single-mode fiber. The reported architecture can be programmed for linear-cluster states of any number of photons, that are required for photonic one-way quantum computing schemes.

How do voters' behavioural biases affect political outcomes? We study this question in a model of Downsian electoral competition in which candidates have private information about the benefits of policies, and voters may infer candidates' information from their electoral platforms. If voters are Bayesian, candidates ‘anti-pander’ – they choose platforms that are more extreme than is justified by their private beliefs. However, anti-pandering is ameliorated if voters' inferences are subject to confirmation bias. Voter confirmation bias causes elections to aggregate candidates' information better, and all observers, whether biased or Bayesian, would like the voters in our model to exhibit more confirmation bias than they do themselves.

Single-photon sources based on semiconductor quantum dots have emerged as an excellent platform for high efficiency quantum light generation. However, scalability remains a challenge since quantum dots generally present inhomogeneous characteristics. Here we benchmark the performance of fifteen deterministically fabricated single-photon sources. They display an average indistinguishability of 90.6 +/- 2.8 % with a single-photon purity of 95.4 +/- 1.5 % and high homogeneity in operation wavelength and temporal profile. Each source also has state-of-the-art brightness with an average first lens brightness value of 13.6 +/- 4.4 %. Whilst the highest brightness is obtained with a charged quantum dot, the highest quantum purity is obtained with neutral ones. We also introduce various techniques to identify the nature of the emitting state. Our study sets the groundwork for large-scale fabrication of identical sources by identifying the remaining challenges and outlining solutions.

We report the interfacing of an integrated solid-state single-photon source with an integrated, reconfigurable photonic tritter demonstrating a highly efficient quantum interference of three indistinguishable single photons. © 2019 The Author(s)

Generating light in a pure quantum state is essential for advancing optical quantum technologies. However, controlling its photon number remains elusive. Optical fields with zero and one photon can be produced by single atoms, but, so far, this has been limited to generating incoherent mixtures or coherent superpositions with a very small one-photon term. Here, we report the on-demand generation of quantum superpositions of zero, one and two photons via coherent control of an artificial atom. Driving the system up to full atomic inversion leads to quantum superpositions of vacuum and one photon, with their relative populations controlled by the driving laser intensity. A stronger driving of the system, with 2π pulses, results in a coherent superposition of vacuum, one and two photons, with the two-photon term exceeding the one-photon component, a state allowing phase super-resolving interferometry. Our results open new paths for optical quantum technologies with access to the photon-number degree of freedom.

We have devised an all-fiber scheme for the creation of multipartite entanglement of photons of arbitrary number. A quantum dot single photon source was utilized and entanglement of four photons was demonstrated.

We present a new compact fiber optic system which together with a high brightness single-photon source facilitates the generation of multi-photon entangled states. Two and three entangled photon states have been prepared and measured.