Stable, low-error and calibration-free polarization encoder for free-space quantum communication

M. Avesani¹, C. Agnesi¹, A. Stanco¹, G. Vallone¹, P. Villoresi¹
Opt. Lett. 45, 4706-4709 (2020)
DOI:10.1364/OL.396412

¹Department of Information Engineering, University of Padova, via Gradenigo 6/B, 35131 Padova, Italy

Polarization-encoded free-space Quantum Communication requires a quantum state source featuring fast polarization modulation, long-term stability and a low intrinsic error rate. Here we present a source based on a Sagnac interferometer and composed of polarization maintaining fibers, a fiber polarization beam splitter and an electro-optic phase modulator. The system generates predetermined polarization states with a fixed reference frame in free-space that does not require calibration neither at the transmitter nor at the receiver. In this way we achieve long-term stability and low error rates. A proof-of-concept experiment is also reported, demonstrating a Quantum Bit Error Rate lower than 0.2% for several hours without any active recalibration of the devices.