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Photonics North (PN)

Double-heralded single photons by cascaded downconversion

Patrick F. Poitras, Deny R. Hamel, Evan Meyer-Scott

Published Jun 1, 2017• 0 citations
Publisher

Abstract

Heralded single photon sources are the subject of intense research owing to their usefulness in metrology and quantum information. These are often created using a pair of photons produced by spontaneous parametric downconversion (SPDC), from which one photon is used to certify the presence of the other. However, multiple-pair production and detector dark counts limit the quality of such sources. Here, we investigate whether cascading two downconversion events to precertify the presence of a photon pair would allow us to reduce these effects. Using analytical calculations and simulations, we find that higher purity photons, as quantified by the heralded second order correlation function, can be obtained for some detector performance ranges.

DOI
10.1109/pn.2017.8090567

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Heralded single photon sources are the subject of intense research owing to their usefulness in metrology and quantum information. These are often created using a pair of photons produced by spontaneous parametric downconversion (SPDC), from which one photon is used to certify the presence of the other. However, multiple-pair production and detector dark counts limit the quality of such sources. Here, we investigate whether cascading two downconversion events to precertify the presence of a photon pair would allow us to reduce these effects. Using analytical calculations and simulations, we find that higher purity photons, as quantified by the heralded second order correlation function, can be obtained for some detector performance ranges.
Various quantum technology applications require high-purity single photons with high generation rate. Although different methods are employed to generate such photons, heralded single photons from spontaneous parametric down-conversion (SPDC) is the most commonly used approach. Photon generation rate from the heralded single-photon sources are limited by the efficiency of the detectors to record coincidence detection of the photon pairs which are lower than the single-photon counts recorded separately on each detector. In this paper we present a revised expression to calculate second-order temporal correlation function, g(2)g^{(2)}g(2) for any fixed time window (bin) and report the experimental characterization of purity of unheralded and heralded single photons from the SPDC process. With an appropriate choice of time bin for a given pump power, without heralding we show that higher rate of single photons with g(2)(0)=0g^{(2)}(0) = 0g(2)(0)=0 can be generated with very high probability.
Heralded single photon sources (HSPS) using spontaneous parametric down conversion (SPDC) are widely used in photonic quantum information. However, the excess-photon pair generation in SPDC process is a problem, which results in the events where more than two photons exist in a pulse. Such excess photons degrade the visibility (V) of two-photon Hong-Ou-Mandel interference, and are one of the main error causes in the advanced linear optics photonic circuits. Recently, we have developed a novel HSPS which combines two methods to suppress such excess photon numbers. The first method (A) is to use multiple SPDC sources in one HSPS, where upon the detection of the idler photon from one of the source, the corresponding signal photon is output. The second method (B) is to use cascaded detectors to monitor the photon-pair number generated from the source, where the detector distinguish the single-pair generation from multi-pair generation and thus the excess-photon generation can be suppressed. In this presentation, we report the theoretical calculations how such a source can improve the visibility for a two photon interference. For example, when the average number of pairs from SPDC in a pulse is 0.1, and the detection efficiency is 70%, V= 83.2% for a conventional HSPS. V is improved to 90.3%, 87.1% and 92.7% by using method A, B and our hybrid approach, respectively.
A double-heralding technique is presented for producing heralded entangled photon pairs from spontaneous parametric down-conversion (SPDC). Compared to the swap-heralded schemes studied in previous cascaded SPDC and zero-added-loss multiplexing (ZALM) proposals, this double-heralding technique is found to yield the most resource-efficient implementation in terms of minimizing the total number of sources and detectors required to achieve a specified rate and fidelity. This is achieved by reducing the number of modes and mode-sorting optics needed on the heralding path. Specifically, by immediately detecting any two signal photons from an array of down-converters, the corresponding idler photons can be projected onto an anti-correlated pair state which is shown to be unitarily equivalent to the state produced by swap-heralded sources, and hence can be used directly for long-range entanglement distribution in a ZALM architecture. Quasi-deterministic operation through two distinct multiplexing techniques is analyzed. The analysis derives expressions for the heralded pair probability and fidelity assuming realistic detectors with losses, dark counts, and partial photon number resolution (PNR), providing a framework for implementation of the source on a photonic integrated circuit (PIC).
Single photons exhibit nonclassical, counterintuitive behavior that can be exploited in the developing field of quantum technology. They are needed for 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. This drives the current intensive research into the realization of true deterministic sources of single photons on demand. Lacking such a source, many researchers default to the well-established workhorse: spontaneous parametric down-conversion that generates 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-photon-resolving detectors can only be used to characterize the signal-idler correlations rather than the nature of the signal-photon state alone.
The generation of high purity single photons and entangled photon pairs is a crucial first step in many quantum optics experiments, including those that require quantum teleportation [1], entanglement swapping [2], and the generation of multi-photon entanglement [3]. The most widely used and established approach for creating photon pairs is spontaneous parametric down conversion (SPDC) - where pump photons undergo a non-linear optical process to create a down-converted photon pair [4]. Traditionally, the generation of high-purity heralded single photons required severe spectral filtering, introducing additional loss. However, through careful engineering of spectral correlations of the down-converted photon pairs one can circumvent the need for filtering and avoid the accompanying reduction in heralding efficiency.
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.