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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Experimental test of the robustness of the non-classicality of single photons

Tobias Huber, Ana Predojević, Miroslav Ježek, Daniel Föger, Glenn S. Solomon, Radim Filip, Gregor Weihs

Published May 1, 2013• 0 citations
Publisher

Abstract

Quantum non-classicality is not only a fundamental feature which shows the solely quantum character of an emitted photon, but is also necessary for modern applications based on the quantum character of light like quantum information processing [1]. The usual way to show that a light source is non-classical is to measure its auto-correlation [2]. In our work instead we used a different criterion developed in [3] which puts a bound on states that cannot be expressed as a mixture of coherent states. With the same measurement technique we have already proven the non-Gaussianity of the photon [4] using a similar criterion [5,6].

DOI
10.1109/cleoe-iqec.2013.6801688

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Quantum non-classicality is not only a fundamental feature which shows the solely quantum character of an emitted photon, but is also necessary for modern applications based on the quantum character of light like quantum information processing [1]. The usual way to show that a light source is non-classical is to measure its auto-correlation [2]. In our work instead we used a different criterion developed in [3] which puts a bound on states that cannot be expressed as a mixture of coherent states. With the same measurement technique we have already proven the non-Gaussianity of the photon [4] using a similar criterion [5,6].
Non-Gaussian states and processes are useful resources in quantum information with continuous variables. An experimentally accessible criterion has been proposed to measure the degree of non-Gaussianity of quantum states based on the conditional entropy of the state with a Gaussian reference. Here we adopt such a criterion to characterize an important class of nonclassical states: single-photon-added coherent states. Our studies demonstrate the reliability and sensitivity of this measure and use it to quantify how detrimental is the role of experimental imperfections in our implementation.
A Quantum Non-Gaussianity Criterion Based on Photon Correlations g(2)g^{(2)}g(2) and g(3)g^{(3)}g(3)
Quantum non-Gaussian states, which cannot be written as mixtures of Gaussian states, are necessary to achieve a quantum advantage in continuous variable systems. They represent an important benchmark for the realization of an advanced quantum light source, as they cannot be made by simple means such as displacement and squeezing. We introduce an attenuation-resistant sufficient criterion for quantum non-Gaussian states based on the second- and third-order correlation functions, g(2)g^{(2)}g(2) and g(3)g^{(3)}g(3). The general non-linear bound for classical mixtures of Gaussian states is g(3)+3g(2)≥2\sqrt{g^{(3)}} + 3 \sqrt{g^{(2)}} \geq 2g(3)​+3g(2)​≥2. Any mixture of Gaussian states must fulfill this inequality, thus, the violation of it represents a direct confirmation of quantum non-Gaussianity. We experimentally show the non-Gaussianity of the state produced by a quantum dot single-photon source, where we obtain g(3)+3g(2)=0.174(13)\sqrt{g^{(3)}} + 3 \sqrt{g^{(2)}} = 0.174 (13)g(3)​+3g(2)​=0.174(13), which represents a statistical significance of more than 100100100 standard deviations.
Quantum attributes of light have been related to non-classicality so far, i. e. to incompatibility with mixtures of coherent states. The progress in quantum optics indicates that this feature does not suffice to witness exotic behavior of light. Contrary, quantum non-Gaussianity is starting to appear as a promising and applicable property reflecting interesting states of light that are suitable for quantum protocols. We investigate a newly introduced hierarchy of criteria of quantum non-Gaussianity and predict this attribute can be observed on light emitted from many single photon emitters, even if the light undergoes realistic optical losses. It contrasts negativity of Wigner function, that is unreachable in experimental platforms with losses above fifty percent.
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Non-Gaussian quantum states of light are essential resources for quantum information processing and precision metrology. Among them, generalized coherent states (GCS), which naturally arise from the evolution of a coherent state with a nonlinear medium, exhibit useful quantum features such as Wigner negativity and metrological advantages [Phys. Rev. Res. 5, 013165 (2023)]. Because these states remain coherent to all orders, their nonclassical character cannot be revealed through standard intensity-intensity correlation measurements. Here, we demonstrate that the intensity-field correlation function alone provides a simple and experimentally accessible witness of nonclassicality. For GCSs, any deviation of this normalized correlation from unity signals nonclassical behavior. We derive analytical results for Kerr-generated states and extend the analysis to statistical mixtures of GCSs. The proposed approach enables real-time, low-complexity detection of quantum signatures in non-Gaussian states, offering a practical tool for experiments across a broad range of nonlinear regimes.