Atmospheric continuous-variable quantum communication

• Published in: New journal of physics Vol. 16; no. 11; pp. 113018 - 13
• Main Authors: Heim, B, Peuntinger, C, Killoran, N, Khan, I, Wittmann, C, Marquardt, Ch, Leuchs, G
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 07-11-2014
• Subjects: 03.67.Hk; 42.68.Bz; 42.68.Mj; atmospheric turbulence; Atmospherics; Channels; Continuity (mathematics); Entanglement; Links; Multiplexing; Optimization; Oscillators; Physics; Polarization; polarization in atmospheric optics; quantum communication; Quantum cryptography; Quantum entanglement; Signal encoding; Variation; Visibility
• ISSN: 1367-2630; 1367-2630
• DOI: 10.1088/1367-2630/16/11/113018

We present a quantum communication experiment conducted over a point-to-point free-space link of 1.6 km in urban conditions. We study atmospheric influences on the capability of the link to act as a continuous-variable (CV) quantum channel. Continuous polarization states (that contain the signal encoding as well as a local oscillator (LO) in the same spatial mode) are prepared and sent over the link in a polarization multiplexed setting. Both signal and LO undergo the same atmospheric fluctuations. These are intrinsically auto-compensated which removes detrimental influences on the interferometric visibility. At the receiver, we measure the Q-function and interpret the data using the framework of effective entanglement (EE). We compare different state amplitudes and alphabets (two-state and four-state) and determine their optimal working points with respect to the distributed EE. Based on the high entanglement transmission rates achieved, our system indicates the high potential of atmospheric links in the field of CV quantum key distribution.