Abstract
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.
Highlights
Quantum communication refers to the distribution of quantum states between two parties via a quantum channel
The excess noise of 0.01 shot-noise units of the signal states compared to vacuum is mainly of technical origin, and is already introduced in Alices signal generation step. This is due to the stronger fluctuating voltage pulses applied to the electro-optical modulators (EOMs) compared to the unmodulated vacuum slots
We experimentally demonstrated the preservation of quantum properties through the verification of EE for an atmospheric point-to-point link for CV quantum communication in an urban environment
Summary
Quantum communication refers to the distribution of quantum states between two parties via a quantum channel. It is crucial that this quantum channel preserves the quantum properties of the distributed states. The most common channel implementations are optical fibers and free space. The latter offers great flexibility in terms of infrastructure establishment and links to moving objects are feasible, see e.g. Quantum key distribution (QKD) [3, 4] is probably the most practical branch of quantum communication and concerns the establishment of a secret key jointly between two legitimate parties, Alice and Bob. As the security is based on the laws of quantum mechanics, in principle information theoretic-security can be achieved [4]
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