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SPIE Quantum Technologies

Accidental coincidences in camera-based high-dimensional entanglement certification

Raphaël Guitter, Baptiste Courme, Chloé Vernière, P. Švihra, ANDREI NOMEROTSKI, Hugo Defienne

Published Nov 5, 2025• 1 citation
Publisher

Abstract

High-dimensional entangled states, such as spatially entangled photon pairs produced by spontaneous parametric down-conversion (SPDC), are a key resource for quantum technologies. In recent years, camera-based coincidence counting approaches have considerably improved the ability to characterize them in terms of speed and dimensionality. However, these methods have limitations, including in most of them the necessity to subtract accidental coincidences. Here, we study the role of these accidentals in entanglement certification for a single-photon avalanche diode (SPAD) array and an intensified time-stamping (Tpx3Cam) camera. Using both Einstein-Podolsky-Rosen (EPR) and entropy-based criteria, we show that the level of accidental coincidences-determined by the temporal characteristics of the camera-and whether they are subtracted critically impact entanglement certification. In particular, we demonstrate that current single-photon camera technologies enable entanglement certification without accidental subtraction only if a Gaussian approximation is applied to the measured two-photon state. Our work is important for developing quantum-optics applications in adversarial scenarios, such as high-dimensional quantum key distribution (HD-QKD) and also for loophole-free experimental testing of quantum foundations.

DOI
10.1117/12.3112235

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High-dimensional entangled states, such as spatially entangled photon pairs produced by spontaneous parametric down-conversion (SPDC), are a key resource for quantum technologies. In recent years, camera-based coincidence counting approaches have considerably improved the ability to characterize them in terms of speed and dimensionality. However, these methods have limitations, including in most of them the necessity to subtract accidental coincidences. Here, we study the role of these accidentals in entanglement certification for a single-photon avalanche diode (SPAD) array and an intensified time-stamping (Tpx3Cam) camera. Using both Einstein-Podolsky-Rosen (EPR) and entropy-based criteria, we show that the level of accidental coincidences-determined by the temporal characteristics of the camera-and whether they are subtracted critically impact entanglement certification. In particular, we demonstrate that current single-photon camera technologies enable entanglement certification without accidental subtraction only if a Gaussian approximation is applied to the measured two-photon state. Our work is important for developing quantum-optics applications in adversarial scenarios, such as high-dimensional quantum key distribution (HD-QKD) and also for loophole-free experimental testing of quantum foundations.
High-dimensional entangled states, such as spatially-entangled photon pairs produced by Spontaneous Parametric Down-Conversion (SPDC), are a key resource for quantum technologies. In recent years, camera-based coincidence counting approaches have considerably improved the ability to characterize them in terms of speed and dimensionality. However, these methods have limitations, including in most of them the necessity to subtract accidental coincidences. Here, we study the role of these accidentals in entanglement certification for a single-photon avalanche diode (SPAD) array and an intensified time-stamping (Tpx3Cam) camera. Using both Einstein-Podolsky-Rosen (EPR) and entropy-based criteria, we show that the level of accidental coincidences - determined by the temporal characteristics of the camera - and whether they are subtracted critically impact entanglement certification. In particular, we demonstrate that current single-photon camera technologies enable entanglement certification without accidental subtraction only if a Gaussian approximation is applied to the measured two-photon state. Our work is important for developing quantum-optics application in adversarial scenarios, such as high-dimensional quantum key distribution (HD-QKD), and also for loophole-free experimental testing of quantum foundations.
High-dimensional entanglement is a promising resource for quantum technologies [1], [2]. Being able to certify it for any quantum state is essential. However, to date, experimental entanglement certification methods are imperfect and leave some loopholes open, due to the presence of accidental coincidences and the hypotheses necessary for their subtraction in post-processing [3], [4]. We present here an experimental study of the role of these accidentals depending on the temporal resolution of the coincidence detection apparatus. Our results show that high-dimensional entanglement certification can be achieved without accidental subtraction using current single-photon camera technologies. Using the recently developed time-stamping Tpx3Cam camera [5], we certify high-dimensional entanglement without accidental subtraction in under 16 seconds. In our experiment, we measure position-momentum Einstein-Podolsky-Rosen (EPR) [6] correlations in pairs of photons generated by Spontaneous Parametric Down Conversion (SPDC) by collecting all spatial output modes simultaneously on the camera. We achieved an EPR factor of0.13<1/20.13 < 1/20.13<1/2, which allows us to certify entanglement.
High-dimensional entanglement is a promising resource for quantum technologies. Being able to certify it for any quantum state is essential. However, to date, experimental entanglement certification methods are imperfect and leave some loopholes open. Using a single-photon-sensitive time-stamping camera, we quantify high-dimensional spatial entanglement by collecting all output modes and without background subtraction, two critical steps on the route toward assumptions-free entanglement certification. We show position-momentum Einstein-Podolsky-Rosen (EPR) correlations and quantify the entanglement of formation of our source to be larger than 2.8 along both transverse spatial axes, indicating a dimension higher than 14. Our work overcomes important challenges in photonic entanglement quantification and paves the way toward the development of practical quantum information processing protocols based on high-dimensional entanglement.
The characterization of high-dimensional quantum entanglement is crucial for advanced quantum computing and quantum information algorithms. Traditional methods require extensive data acquisition and suffer from limited visibility due to experimental noise. Here, we introduce a sparsity-driven framework to enhance the detection and certification of high-dimensional entanglement in spatially entangled photon pairs. By applying ℓ1\ell_1ℓ1​-regularized reconstruction to sample covariance matrices obtained from measurements on photons produced via spontaneous parametric down-conversion (SPDC) measurements, we enhance the visibility of the correlation signal while suppressing noise. We demonstrate, using a position-momentum Einstein-Podolsky-Rosen (EPR) entanglement criterion, that this approach enables certification of an entanglement dimensionality that cannot be achieved without regularization. Our method is scalable, simple to use and compatible with existing quantum-optics platforms, thus paves the way for efficient, real-time analysis of high-dimensional quantum states.
Abstract Spatial correlations between two photons are the key resource in realising many quantum imaging schemes. Measurement of the bi-photon correlation map is typically performed using single-point scanning detectors or single-photon cameras based on charged coupled device (CCD) technology. However, both approaches are limited in speed due to the slow scanning and the low frame rate of CCD-based cameras, resulting in data acquisition times on the order of many hours. Here, we employ a high frame rate, single-photon avalanche diode (SPAD) camera, to measure the spatial joint probability distribution of a bi-photon state produced by spontaneous parametric down-conversion, with statistics taken over 10 7 frames. Through violation of an Einstein–Podolsky–Rosen criterion by 227 sigmas, we confirm the presence of spatial entanglement between our photon pairs. Furthermore, we certify, in just 140 s, an entanglement dimensionality of 48. Our work demonstrates the potential of SPAD cameras in the rapid characterisation of photonic entanglement, leading the way towards real-time quantum imaging and quantum information processing.
Entanglement is a central resource in quantum technologies. In this respect, high-dimensional entangled states are very promising for developing robust quantum communication schemes and enhanced imaging protocols. Being able to quantify high-dimensional entanglement quickly and accurately is therefore essential. However, to date, all experimental methods use assumptions about the detected state or imperfect measurement techniques, such as single-outcomes and accidental subtractions. Entanglement certification processes are therefore not only slow, but they also contain loopholes, which is not acceptable in adversarial scenarios such as quantum key distribution (QKD). In our work, we demonstrate the certification of high-dimensional entanglement using an event-based single-photon sensitive camera by performing measurements in two mutually unbiased bases. Our work provides an experimental method allowing for the efficient characterization of high-dimensional quantum states, paving the way for the development of high-dimensional QKD protocols and practical photon-pairs-based quantum imaging approaches.