  We propose a multiple-input multiple-output (MIMO) quantum key distribution
(QKD) scheme for terahertz (THz) frequency applications operating at room
temperature. Motivated by classical MIMO communications, a transmit-receive
beamforming scheme is proposed that converts the rank-$r$ MIMO channel between
Alice and Bob into $r$ parallel lossy quantum channels. Compared with existing
single-antenna QKD schemes, we demonstrate that the MIMO QKD scheme leads to
performance improvements by increasing the secret key rate and extending the
transmission distance. Our simulation results show that multiple antennas are
necessary to overcome the high free-space path loss at THz frequencies. We
demonstrate a non-monotonic relation between performance and frequency, and
reveal that positive key rates are achievable in the $10-30$ THz frequency
range. The proposed scheme can be used for both indoor and outdoor QKD
applications for beyond fifth-generation ultra-secure wireless communications
systems.
