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arXiv:2006.12911 · Physical Review A

Influence of coincidence detection of a biphoton state through free-space atmospheric turbulence using a partially spatially coherent pump

Samukelisiwe Purity Phehlukwayo, Marie Louise Umuhire, Yaseera Ismail, Stuti Joshi, Francesco Petruccione

Published Sep 30, 2020• 22 citations
arXivHTMLPDF

Abstract

The development of a quantum network relies on the advances of hybrid systems which include ground-to-ground communication. However, the atmospheric turbulence of the environment poses a severe challenge to the optical quantum link. In this paper, we outline a theoretical and experimental investigation of the influence of atmospheric turbulence on the coincidence detection of the entangled photon pairs using a fully and partially spatially coherent pump beam. A spatial light modulator is encoded to mimic the atmospheric turbulence strength based on the Kolmogorov model of turbulence. The results show that the photon pairs generated using a partially spatially coherent pump are more robust towards varying atmospheric turbulence strengths than the photon pairs produced by a fully spatially coherent pump beam.

DOI
10.1103/physreva.102.033732

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The development of a quantum network relies on the advances of hybrid systems which include ground-to-ground communication. However, the atmospheric turbulence of the environment poses a severe challenge to the optical quantum link. In this paper, we outline a theoretical and experimental investigation of the influence of atmospheric turbulence on the coincidence detection of the entangled photon pairs using a fully and partially spatially coherent pump beam. A spatial light modulator is encoded to mimic the atmospheric turbulence strength based on the Kolmogorov model of turbulence. The results show that the photon pairs generated using a partially spatially coherent pump are more robust towards varying atmospheric turbulence strengths than the photon pairs produced by a fully spatially coherent pump beam.
Two photon fields to date, are generated by a fully coherent pump beam. We have experimentally studied the generation of polarisation entangled single photon pairs using a partially coherent pump beam. The spatial coherence effects of the pump beam on the coincidence detection of an entangled single photon source was investigated. The fully coherent coincidence detection was compared to the partial coherence detection. The results show that the partial coherence of the pump beam leads to an increase in coincidence detection. The coherence properties of a beam is significant for freespace optical transmission specifically for free-space quantum communication over long distances.
The underwater quantum channel has been extensively investigated to complete the last piece of the puzzle in building a global quantum communication network. However, the decoherence of the entangled state caused by the underwater turbulence critically destroys the quantum channel and prevents its practical application. Here, we propose a method that uses the partially coherent beam to generate the biphoton state to tackle the problem. We theoretically investigate the impact of the coherence of the pump beam on the coincidence detection of photon pairs through the oceanic turbulence model. Our findings indicate that when the pump beam is partially coherent, the correlation of the generated photon pairs exhibits enhanced stability during propagation through turbulent water. This stability helps mitigate information loss caused by photon scattering, thereby offering a practical approach to improving the efficiency of underwater quantum communication.
Scattering by atmospheric turbulence is one of the main challenges in creating long free-space optical links, and specifically links of entangled photons. Classical compensation methods are hard to apply to entangled photons, due to inherently low signal to noise ratios and the fragility of entanglement. We have recently shown that we can use a bright laser beam that pumps spontaneous parametric down conversion to control the spatial correlations between entangled photons for compensating their scattering. In this work, we apply the pump-shaping technique to compensate for the scrambling of correlations between entangled photons that scatter by emulated atmospheric turbulence. We use a spatial light modulator and Kolmogorov’s turbulence model to emulate atmospheric turbulence in the lab, and enhance the entangled photons’ signal by a factor of fifteen using pump optimization. We show this for both a static and dynamic emulated atmosphere, and also demonstrate the compensation of the scattering of a higher-order mode. Our results can open the door towards realizing free-space quantum links with entangled photons, used in applications such as quantum key distribution.
We demonstrate experimentally how orbital-angular-momentum entanglement of two photons evolves under the influence of atmospheric turbulence. Experimental results are in excellent agreement with our theoretical model, which combines the formalism of two-photon coincidence detection with a Kolmogorov description of atmospheric turbulence. We express the robustness to turbulence in terms of the dimensionality of the measured correlations. This dimensionality is surprisingly robust: scaling up our system to real-life dimensions, a horizontal propagation distance of 2 km seems viable.
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The orbital angular momentum (OAM) state of light can potentially be used to implement higher dimensional entangled systems for quantum communication. Unfortunately, optical fibers in use today support only modes with zero OAM values. Free-space quantum communication is an alternative to traditional way of communicating through optical fibers. However the refractive index fluctuation of the atmosphere gives rise to random phase aberrations on a propagating optical beam. To transmit quantum information successfully through a free-space optical channel, one needs to understand how atmospheric turbulence influences quantum entanglement. Here, we present a numerical study of the evolution of quantum entanglement between a pair of qubits. The qubits consist of photons entangled in the OAM basis. The photons propagate in a turbulent atmosphere modeled by a series of consecutive phase screens based on the Kolmogorov theory of turbulence. Maximally entangled initial states are considered, and the concurrence is used as a measure of entanglement. We show how the evolution of entanglement is influenced by various parameters such as the beam waist, the strength of the turbulence and the wavelength of the beam. We restricted our analysis to the OAM values l = ±1 and we compared our results to previous work.