A standard beam splitter consists of four ports: two input ports and two output ports.

A typical lossless beam splitter has two input ports where light enters and two output ports where the split beams exit, resulting in a total of four ports (input 1, input 2, output 3, output 4) in classical and quantum optics contexts . The input light can be split into transmitted and reflected components according to the beam splitter's reflection/transmission ratio, and the outputs carry the resulting beams . Types of beam splitters include cube, plate, pellicle, and polka dot designs, but regardless of type, the fundamental port structure remains the same for standard two-way splitting . Cube beam splitters, for example, are made from two right-angle prisms cemented together with a semi-reflective coating at the interface, while plate beam splitters use a coated flat glass plate to achieve the splitting effect . In quantum optics, the four-port model is essential for describing photon behavior, where a photon entering one input port can emerge in a superposition across the two output ports, and the transformation is represented by a unitary matrix . This four-port configuration allows the beam splitter to function both as a splitter and, in reverse, as a combiner of beams . Thus, whether in classical or quantum applications, a beam splitter fundamentally consists of four ports, enabling controlled division and recombination of light.