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arXiv:1711.10247 · Communications Physics

Observing the transition from quantum to classical energy correlations with photon pairs

Stefan Lerch, André Stefanov

Published Jan 1, 2017• 10 citations
arXivHTMLPDF

Abstract

Abstract The exact role of entanglement in various quantum metrology schemes is still subject to debates. This is why it would be interesting to be able to experimentally control the relative amount of quantum and classical correlations. Here, we demonstrate a method to tune energy correlations between two photons from a pair emitted by spontaneous parametric downconversion. Decoherence in the energy basis is achieved by applying random spectral phases on the photons. As a consequence, a diverging temporal second-order correlation function is observed and is explained by a mixture between an energy entangled pure state and a fully classically correlated mixed state. Such source of tunable energy entangled photon pairs could be used to demonstrate quantum advantages in future energy-time sensing schemes.

DOI
10.1038/s42005-018-0027-2

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Abstract The exact role of entanglement in various quantum metrology schemes is still subject to debates. This is why it would be interesting to be able to experimentally control the relative amount of quantum and classical correlations. Here, we demonstrate a method to tune energy correlations between two photons from a pair emitted by spontaneous parametric downconversion. Decoherence in the energy basis is achieved by applying random spectral phases on the photons. As a consequence, a diverging temporal second-order correlation function is observed and is explained by a mixture between an energy entangled pure state and a fully classically correlated mixed state. Such source of tunable energy entangled photon pairs could be used to demonstrate quantum advantages in future energy-time sensing schemes.
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The thorough characterization of entangled-photon sources is vital for their optimal use in quantum communication. However, this task is not trivial at telecommunication wavelengths. While cameras and spectrometers are well developed for visible and near-infrared spectra, this does not apply in the mid-infrared range. Here, we present a spatial and spectral characterization of photon pairs emitted in a type-0 phase-matched spontaneous parametric downconverted source. We experimentally show how these photon properties are modified by the crystal temperature. This parameter allows easy modification of photon-pair properties to fit multiplexing schemes based on only one entanglement photon source. Our results pave the way for the optimal design and use of spatial and spectral properties of quantum-correlated photon pairs at telecommunication wavelengths.
Spectrally correlated photon pairs can be used to improve the performance of long-range fiber-based quantum communication protocols. We present a source based on spontaneous parametric downconversion, which allows one to control spectral correlations within the entangled photon pair without spectral filtering by changing the pump-pulse duration or the characteristics of the coupled spatial modes. The spectral correlations and polarization entanglement are characterized. We find that the generated photon pairs can feature both positive spectral correlations, decorrelation, or negative correlations at the same time as polarization entanglement with a high fidelity of 0.97 (no background subtraction) with the expected Bell state.
Franson interferometry is a well-known quantum measurement technique for probing photon-pair frequency correlations that is often used to certify time-energy entanglement. We demonstrate, for the first time, the complementary technique in the time basis called conjugate-Franson interferometry. It measures photon-pair arrival-time correlations, thus providing a valuable addition to the quantum toolbox. We obtain a conjugate-Franson interference visibility of 96±1% without background subtraction for entangled photon pairs generated by spontaneous parametric down-conversion. Our measured result surpasses the quantum-classical threshold by 25 standard deviations and validates the conjugate-Franson interferometer (CFI) as an alternative method for certifying time-energy entanglement. Moreover, the CFI visibility is a function of the biphoton's joint temporal intensity, and is therefore sensitive to that state's spectral phase variation: something that is not the case for Franson interferometry or Hong-Ou-Mandel interferometry. We highlight the CFI's utility by measuring its visibilities for two different biphoton states: one without and the other with spectral phase variation, observing a 21% reduction in the CFI visibility for the latter. The CFI is potentially useful for applications in areas of photonic entanglement, quantum communications, and quantum networking.
The ability to manipulate and measure the time-frequency structure of quantum light is useful for information processing and metrology. Measuring this structure is also important when developing quantum light sources with high modal purity that can interfere with other independent sources. Here, we present and experimentally demonstrate a scheme based on intensity interferometry to measure the joint spectral mode of photon pairs produced by spontaneous parametric down-conversion. We observe correlations in the spectral phase of the photons due to chirp in the pump. We show that our scheme can be combined with stimulated emission tomography to quickly measure their mode using bright classical light. Our scheme does not require phase stability, nonlinearities, or spectral shaping and thus is an experimentally simple way of measuring the modal structure of quantum light.