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Physical Review A

Direct measurement of heralded single-photon statistics from a parametric down-conversion source

David Höckel, Lars Koch, Oliver J. Benson

Published Jan 4, 2011• 47 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.83.013802

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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.
Heralded single-photon sources are the most commonly used sources for optical quantum technology applications. There is strong demand for accurate prediction of their spectral features and temporal correlations with ever-increasing precision. This is particularly important in connection with the intrinsically stochastic photon-pair generation process in heralded sources. Here we present a complete theoretical description of the temporal correlation of a signal-idler, signal-signal, and signal-signal-idler coincidences of photons generated by continuous-wave pumped cavity-enhanced spontaneous parametric down-conversion. The theory excellently predicts the measurements, which has been experimentally confirmed in our setup utilizing single-photon detectors with high temporal resolution. This enables us to resolve and analyze the multiphoton correlation functions in great detail.
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Parametric down-conversion is a widely used source of nonclassical light in quantum optics and photonic quantum technologies. While stimulated parametric down-conversion with strong classical seeds is well studied, the regime in which stimulation occurs at the single-photon level has hitherto remained largely unexplored experimentally. Here, we study continuous-wave, low-gain down-conversion seeded by a weak coherent field with an average photon number well below one per coherence time. By measuring third-order temporal correlations, we observe a clear enhancement that cannot be accounted for by spontaneous processes or accidental coincidences alone, and is consistent with stimulation involving the seed and the generated photon pair. These results provide time-domain evidence of seed-induced three-photon correlations and suggest new ways to engineer and probe multi-photon states for quantum imaging, sensing, and information processing.
Low-photon-number sources can exhibit non-classical, counterintuitive behavior that can be exploited in the developing field of quantum technology. Single photons play a special role in this arena since they represent the ultimate lowphoton- number source. They are considered an important element in various applications such as quantum key distribution, optical quantum information processing, quantum computing, intensity measurement standards, and others yet to be discovered in this developing field. True deterministic sources of single photons on demand are currently an area of intensive research, but have not been demonstrated in a practical setting. As a result, researchers commonly default to the well-established workhorse: spontaneous parametric down-conversion generating entangled signal-idler pairs. Since this source is thermal-statistical in nature, it is common to use a detected idler photon to herald the production of a signal photon. The need exists to determine the quality of the single photons generated in the heralded signal beam. Quite often, the literature reports a "heralded second-order coherence function" of the signal photons conditioned on the idler photons using readily available single-photon detectors. In this work, we examine the applicability of this technique to single-photon characterization and the consequences of the fact that the most commonly used single-photon detectors are not photon-number resolving. Our results show that this method using non-photonresolving detectors can only be used to characterize the signal-idler correlations rather than the nature of the signalphoton state alone.
Direct measurement of photon bunching (g(2){g}_{(2)}g(2)​ correlation function) in one output arm of a spontaneous-parametric-down-conversion source operated with a continuous pump laser in the single-photon regime is demonstrated. The result is in agreement with the statistics of a thermal field of the same coherence length and shows the feasibility of investigating photon statistics with compact continuous-wave-pumped sources. Implications for entanglement-based quantum cryptography are discussed.
We demonstrate a technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time-resolved single-photon detection by femtosecond up-conversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anticorrelation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth to produce a nearly unentangled two-photon state that is expected to yield a heralded single-photon state with a purity of 0.88. The time-domain correlation technique complements existing frequency-domain measurement methods for a more complete characterization of photonic entanglement.