  Transmittance fluctuations in turbulent atmospheric channels result in
quadrature excess noise which limits applicability of continuous-variable
quantum communication. Such fluctuations are commonly caused by beam wandering
around the receiving aperture. We study the possibility to stabilize the
fluctuations by expanding the beam, and test this channel stabilization in
regard of continuous-variable entanglement sharing and quantum key
distribution. We perform transmittance measurements of a real free-space
atmospheric channel for different beam widths and show that the beam expansion
reduces the fluctuations of the channel transmittance by the cost of an
increased overall loss. We also theoretically study the possibility to share an
entangled state or to establish secure quantum key distribution over the
turbulent atmospheric channels with varying beam widths. We show the positive
effect of channel stabilization by beam expansion on continuous-variable
quantum communication as well as the necessity to optimize the method in order
to maximize the secret key rate or the amount of shared entanglement. Being
autonomous and not requiring adaptive control of the source and detectors based
on characterization of beam wandering, the method of beam expansion can be also
combined with other methods aiming at stabilizing the fluctuating free-space
atmospheric channels.
