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An elementary experiment in optics consists of a light source and a detector. Yet, if the source generates nonclassical correlations such an experiment is capable of unambiguously demonstrating the quantum nature of light. We realized such an experiment with a defect center in diamond and a superconducting detector. Previous experiments relied on more complex setups, such as the Hanbury Brown and Twiss configuration, where a beam splitter directs light to two photodetectors, creating the false impression that the beam splitter is a fundamentally required element. As an additional benefit, our results provide a simplification of the widely used photon-correlation techniques. Loudon [4] that a much simpler experiment in which the light is arranged to fall on a single phototube would be sufficient. Here, we perform such an experiment and show single-photon statistics from a quantum emitter with only one detector. The superconducting detector we fabricated has a dead time shorter than the coherence time of the emitter. No beam splitter is employed, yet anticorrelations are observed. Our work simplifies a widely used photon-correlation technique [5,6].A single-photon Fock state is a single excitation of a mode k of the electromagnetic field a † k |0 . A more general single-photon state appropriate to describe the final wave packet generated by a single-photon source in an experiment is a superposition of different spatio-temporal modes containing in total one excitation. The probability P (n) of finding exactly n excitations in the modes may distinguish different states of light. Figures 1(a) and 1(b) show a schematic representation of a coherent state where P (n) is a Poissonian distribution together with a number (or Fock) state with exactly 1 photon per mode, respectively. In the case of a single-photon state (n = 1) detection of a single excitation projects the measured mode to the vacuum state; i.e., the probability of detecting another photon in the very same mode is zero. Since the temporal mode profile is associated with a characteristic coherence time τ c , coincidence events within the time interval τ c are absent; antibunching is observed. On the contrary, * steudle@physik.hu-berlin.de † http://www.physik.hu-berlin.de/nano for a coherent state the probability of detecting a photon is independent of any previous detection event. Antibunching is thus not only a consequence of photons being indivisible particles but requires a specific quantum statistical distribution of discrete excitations. The latter requirement is overlooked in a simple explanation of antibunching in a HBT experiment [ Fig. 1(c)]. There a photon is regarded as a classical indivisible particle and necessarily has to decide which path to take when impinging on a beam splitter. Such an interpretation is certainly naïve. It even led to paradoxical conclusions, such as in some implementations of Wheeler's delayed choice paradox [7].Today, many different sources have been realized that generate antibunched light such as single-photon sources ...

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Abstract:Reaching the Sustainable Development Goals requires a fundamental socio-economic transformation accompanied by substantial investment in low-carbon infrastructure. Such a sustainability transition represents a non-marginal change, driven by behavioral factors and systemic interactions. However, typical economic models used to assess a sustainability transition focus on marginal changes around a local optimum, which-by construction-lead to negative effects. Thus, these models do not allow evaluating a sustainability transition that might have substantial positive effects. This paper examines which mechanisms need to be included in a standard computable general equilibrium model to overcome these limitations and to give a more comprehensive view of the effects of climate change mitigation. Simulation results show that, given an ambitious greenhouse gas emission constraint and a price of carbon, positive economic effects are possible if (1) technical progress results (partly) endogenously from the model and (2) a policy intervention triggering an increase of investment is introduced. Additionally, if (3) the investment behavior of firms is influenced by their sales expectations, the effects are amplified. The results provide suggestions for policy-makers, because the outcome indicates that investment-oriented climate policies can lead to more desirable outcomes in economic, social and environmental terms.