Interference (communication)

Quantum mechanics

Quantum key distribution

Engineering

Quantum channel

Computer security

Operating system

Distribution (mathematics)

Telecommunications

Optical communication

Computer science

Acoustics

Statistical physics

Free space

Photon

Physics

Free-space optical communication

Mathematics

Optics

Electronic engineering

Measurement device

Synchronization (alternating current)

Mathematical analysis

Quantum

Quantum information

Channel (broadcasting)

Space (punctuation)

Key (lock)

Physical Review Letters

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.Received 7 June 2020Accepted 11 November 2020DOI:https://doi.org/10.1103/PhysRevLett.125.260503© 2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasOptical quantum information processingQuantum communicationQuantum cryptographyQuantum opticsQuantum InformationAtomic, Molecular & Optical

Long-Distance Free-Space Measurement-Device-Independent Quantum Key Distribution