First Free-Space Entanglement Demonstration

A significant breakthrough in 2004 [1] unveiled the successful free-space distribution of entangled photon pairs across an intricate and turbulent ground atmosphere spanning 13 kilometers. This study demonstrated that the desired entanglement remains intact despite the entangled photons traversing the arduous ground atmosphere, surpassing the effective thickness of the aerosphere.

Experimental setup. Credit: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.94.150501

Shortcomings of previous experiments —

• Before this experiment, strides in free-space quantum communication included the distribution of attenuated laser pulses over 23.4 kilometers and entangled photon pairs over 600 meters. However, these advances came with specific challenges.
• The quantum cryptography experiment utilizing attenuated laser pulses faced the issue of significant photon loss in the transmission channel, providing an eavesdropping loophole.
• The limited distance achieved in the entanglement distribution experiment fell far below the effective thickness of the aerosphere, and the low transmission efficiency fell short of efficient link establishment over extensive distances, a crucial requirement for satellite-based free-space quantum communication.

In this study, the researchers employed a laser-pulse-assisted synchronization method and custom-designed telescope systems to drive free-space technology further. They successfully distributed entangled photon pairs across a 13-kilometer distance through a noisy ground atmosphere, confirming the retention of the desired entanglement beyond the effective aerosphere thickness. Additionally, they demonstrated the Bennett-Brassard 1984 quantum cryptography scheme without succumbing to eavesdropping vulnerabilities. This achievement was a significant stride towards realizing satellite-based global quantum communication.

This study marked a modest yet significant advancement. It demonstrated that entanglement remains intact after penetrating the aerosphere’s effective thickness, evident in a strong violation of the Bell inequality. This violation ensured absolute security in quantum cryptography by closing the eavesdropping loophole. The established, highly stable transmission channel and synchronization methods paved the way for global quantum cryptography and teleportation experiments in free space.

References —

1. Experimental Free-Space Distribution of Entangled Photon Pairs Over 13 km: Towards Satellite-Based Global Quantum Communication

Note: This article is a part of my Womanium Online Quantum Media Project. Find out about it here .

#WomaniumQuantum #Quantum30 #QCI