  Quantum Key Distribution (QKD) has become an essential technology in the
realm of secure communication, with applications ranging from secure data
transmission to quantum networks. This paper presents a simple, compact, and
cost-effective setup for undergraduate tutorial demonstrations of QKD. It
relies on using weak coherent pulses, which can be readily produced using an
attenuated laser. The system employs the simplified three-state BB84 protocol
in free space, the states are encoded using linear polarization. Polarization
encoding can be done passively or actively, depending on the budget available.
Time multiplexing is implemented at the receiver to reduce the number of
required detectors. Only two detectors are used to implement measurements on
two bases, with a total of four outcomes. The result demonstrates the
practicality of the system for free-space quantum communication, and its
compact and portable nature makes it particularly suitable for pedagogical
demonstrations. This work paves the way for engaging undergraduate students in
the field of quantum communication through hands-on laboratory projects.
