We perform electromagnetically induced transparency (EIT) experiments in cesium vapor with pulses on the single-photon level for the first time. This was made possible by an extremely large total suppression of the EIT coupling beam by 118 dB mainly due to a newly developed triple-pass planar Fabry-Pérot etalon filter. Slowing and shaping of single-photon light pulses as well as the generation of pulses suitable for quantum key distribution applications and testing of approaches for single photon storage is demonstrated. Our results extend single-photon EIT to the particularly interesting wavelength of the Cs D1 line.

We present a full triple-coincidence analysis of photon-pair states generated by spontaneous parametric down-conversion. By increasing the coherence time of the source with the help of an intracavity setup, our measurements are not spoiled by detection time jitter. Signal-idler, but also thermal signal-signal, correlations are clearly resolved in this regime. Via introduction of an artificial coincidence window, we discuss in detail the transition to the previously studied cases where typically no single-arm correlation is observed. We investigate the heralded antibunching characteristics to show that in our system further studies of continuously generated photon states, possibly higher-photon-number entangled states, can be performed with respect to their (non)applicability in quantum information tasks.

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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We describe a combined ultranarrow bandpass filtering setup for single-photon experiments in quantum optics. The filter is particularly suitable for single-photon electromagnetically induced transparency (EIT) experiments, but can also be used in several similar applications. A multipass planar Fabry-Perot etalon together with polarization filters and spatial filtering allows 114 dB pump beam suppression, while the signal beam is attenuated by just 4 dB, although both wavelengths are only separated by 0.025 nm (9.2 GHz). The multipass etalon alone accounts for 46 dB suppression while it has a peak transmission of 65%. We demonstrate EIT experiments in Cs vapor at room temperature with probe power in the femtowatt regime using this filter.