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arXiv:1202.3195 · Physical Review A

Cancellation of atmospheric turbulence effects in entangled two-photon beams

M. V. da Cunha Pereira, Luísa. A. P. Filpi, Carlos Henrique Monken

Published Nov 22, 2013• 28 citations
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Abstract

We introduce a scheme for odd-order aberration-free transmission of two-photon beams through the turbulent atmosphere. We show that a two-photon beam with a Gaussian fourth-order correlation profile can be transmitted through a turbulence cell with very low signal loss. Our scheme relies on the spatial correlations present in two-photon states generated by spontaneous parametric down conversion.

DOI
10.1103/physreva.88.053836

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We introduce a scheme for odd-order aberration-free transmission of two-photon beams through the turbulent atmosphere. We show that a two-photon beam with a Gaussian fourth-order correlation profile can be transmitted through a turbulence cell with very low signal loss. Our scheme relies on the spatial correlations present in two-photon states generated by spontaneous parametric down conversion.
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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.
Due to the transfer of the angular spectrum of the pump beam to the two-photon state in spontaneous parametric downconversion, the generated twin photons are entangled in their spatial degrees of freedom. This spatial entanglement can be observed through correlation measurements in any set of modes in which one may choose to perform measurements. Choosing, e.g., a Hermite-Gaussian (HG) set of spatial modes as a basis, one can observe correlations present in their spatial degrees of freedom. In addition, these modes can be used as alphabets for quantum communication. For global quantum communication purposes, we derive an analytic expression for two-photon detection probability in terms of HG modes, taking into account the effects of the turbulent atmosphere. Our result is more general as it accounts for the propagation of both signal and idler photons through the atmosphere, as opposed to other works considering one photon's propagation in vacuum. We show that while the restrictions on both the parity and order of the downconverted HG fields no longer hold, due to the crosstalk between modes when propagating in the atmosphere, the crosstalk is not uniform: there are more robust modes that tend to keep the photons in them. These modes can be employed in order to increase the fidelity of quantum communication.
The turbulent mixing of air at different temperatures turns the refractive index of air in the atmosphere into a randomly varying quantity. Together with scattering by fog, this fluctuation figures among the greatest obstacles in information transmission through free-space optics. In this thesis, a passive method is introduced that attenuates the distortions created by the atmosphere through the use of correlation beams: entities which describe photon correlations and propagate formally as regular beams. The correct choice of correlation mitigates some of the most severe aberrations and allows for information coding immune to the fluctuations up to good approximation. The models are demonstrated via Monte Carlo simulations and through an experiment in which turbulence is generated in the lab with a turbulence emulating chamber and the correlation beam is created using spontaneous parametric down-conversion. One of the correlation beam's spatial properties, parity, is shown to be particularly robust against the fluctuations and an experimental proposal is presented that exploits this capability. Investigations are done concerning other implications of a robust correlation beam, which indicate at least one drawback: a reduction in the overall signal collected by a receiver.
Correlated photon pairs produced by a spontaneous parametric down conversion (SPDC) process can be used for secure quantum communication over long distances including free space transmission over a link through turbulent atmosphere. We experimentally investigate the possibility to utilize the intrinsic strong correlation between the pump and output photon spatial modes to mitigate the negative targeting effects of atmospheric beam wander. Our approach is based on a demonstration observing the deflection of the beam on a spatially resolved array of single photon avalanche diodes (SPAD-array).