Distributing entanglement and single photons through an intra-city, free-space quantum channel

Area of Science:

• Quantum Information Science
• Free-Space Quantum Communication
• Experimental Quantum Physics

Background:

• Entanglement distribution is crucial for quantum communication networks.
• Previous free-space quantum communication experiments faced challenges in urban environments and over significant distances.
• Urban areas present unique obstacles for maintaining quantum channel integrity.

Purpose of the Study:

• To demonstrate the feasibility of distributing entangled photons through the atmosphere over a city.
• To establish a robust quantum channel for free-space quantum communication in an urban setting.
• To test the performance of entanglement-based quantum communication over a 7.8 km link.

Main Methods:

• Distribution of entangled photons through the atmosphere between two stations in Vienna.
• Utilizing triggered single photon sources at the sender.
• Calculating cross-correlation of locally-recorded timestamps shared over the internet to establish coincidence counts.
• Measuring polarization correlations to determine the Bell parameter (S).

Main Results:

• A quantum channel was maintained for 40 minutes, yielding approximately 60,000 coincident detection events.
• The measured Bell parameter S=2.27+/-0.019 significantly violated the CHSH-Bell inequality by 14 standard deviations.
• Successful entanglement distribution over a 7.8 km free-space link in an urban environment at night.

Conclusions:

• The experiment shows promise for entanglement-based free-space quantum communication in high-density urban areas.
• The achieved link length exceeds atmospheric equivalents, encouraging ground-to-satellite quantum communication.
• This work validates the potential of urban free-space quantum links for future quantum networks.