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OSA Advanced Photonics Congress

SKR improvement for an entanglement assisted BB84 system using adaptive optics on an FSO link

John Gariano, Ivan B. Djordjević

Published Jan 1, 2019• 1 citation
Publisher

Abstract

Implementing QKD systems over FSO channels can suffer due to time-varying channel degradations. To reduce the channel degradations use of adaptive optics to compensate for atmospheric turbulence is studied in a 30 km maritime channel.
DOI
10.1364/sppcom.2019.qtw2e.4

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•Oct 5, 2017•2 citations
Transmitting a beam through the atmosphere over long distances can be a challenging problem. Factors from the channel, such as atmospheric turbulence causing beam wander and distortions to the wavefront, or the optical design can change the loss induced in the channel. This work simulates a quantum key distribution system implementing the BB84 protocol with entangled photons operating over a 30 km maritime channel using one, four, and nine spatial modes. For the cases of multiple spatial modes, atmospheric turbulence increases the channel cross talk, resulting in an increase in the noise for each channel. To compensate for distortions in the wavefront, adaptive optics at the transmitter or receiver are used to pre- or post-correct the wavefront.
Entanglement is key quantum information processing feature enabling quantum communication, networking, and sensing [1]. Unfortunately, long-distance quantum communication is affected by the photon loss, given that quantum signals cannot be amplified. As an alternative, free-space optical (FSO) links can be used instead for entanglement distribution. However, the FSO links are severely affected by atmospheric turbulence effects, in addition to scattering effects. In this paper, we demonstrate that adaptive optics [2] is a useful tool to improve fidelity of entanglement when entangled states are distributed over turbulent FSO links. We present results from our FSO communication experiment where we generated entangled photons pairs, launched them into free-space, and later collected them for coincidence count measurement. We demonstrate that adaptive optics can be used to make improvements in coincidence counts.
When using quantum key distribution (QKD), one of the trade-offs for security is that the generation rate of a secret key is typically very low. Recent works have shown that using a weak coherent source allows for higher secret key generation rates compared to an entangled photon source, when a channel with low loss is considered. In most cases, the system that is being studied is over a fiber-optic communication channel. Here a theoretical QKD system using the BB92 protocol and entangled photons over a free-space maritime channel with multiple spatial modes is presented. The entangled photons are generated from a spontaneous parametric down conversion (SPDC) source of type II. To employ multiple spatial modes, the transmit apparatus will contain multiple SPDC sources, all driven by the pump lasers assumed to have the same intensity. The receive apparatuses will contain avalanche photo diodes (APD), modeled based on the NuCrypt CPDS-1000 detector, and located at the focal point of the receive aperture lens. The transmitter is assumed to be located at Alice and Bob will be located 30 km away, implying no channel crosstalk will be introduced in the measurements at Alice’s side due to turbulence. To help mitigate the effects of atmospheric turbulence, adaptive optics will be considered at the transmitter and the receiver. An eavesdropper, Eve, is located 15 km from Alice and has no control over the devices at Alice or Bob. Eve is performing the intercept resend attack and listening to the communication over the public channel. Additionally, it is assumed that Eve can correct any aberrations caused by the atmospheric turbulence to determine which source the photon was transmitted from. One, four and nine spatial modes are considered with and without applying adaptive optics and compared to one another.