  The orbital angular momentum (OAM) of photons presents a degree of freedom
for enhancing the secure key rate of free-space quantum key distribution (QKD)
through mode-division multiplexing (MDM). However, atmospheric turbulence can
lead to substantial modal crosstalk, which is a long-standing challenge to MDM
for free-space QKD. Here, we show that the digital generation of time-reversed
wavefronts for multiple probe beams is an effective method for mitigating
atmospheric turbulence. We experimentally characterize seven OAM modes after
propagation through a 340-m outdoor free-space link and observe an average
modal crosstalk as low as 13.2% by implementing real-time time reversal. The
crosstalk can be further reduced to 3.4% when adopting a mode spacing $\Delta
\ell$ of 2. We implement a classical MDM system as a proof-of-principle
demonstration, and the bit error rate is reduced from $3.6\times 10^{-3}$ to be
less than $1.3\times 10^{-7}$ through the use of time reversal. We also propose
a practical and scalable scheme for high-speed, mode-multiplexed QKD through a
turbulent link. The modal crosstalk can be further reduced by using faster
equipment. Our method can be useful to various free-space applications that
require crosstalk suppression.
