Fiber optic beam splitters are used to divide light from one fiber into two or more fibers. Key Laboratory of Ultra-Weak Magnetic Field Measurement Technology, Ministry of Education, School of Instrumentation and Optoelectronic Engineering, Beihang University, Beijing, China 2. Beamsplitters are often classified according to their construction: cube or plate. Abstract Beam splitters form very important components of quantum photonic devices and this chapter presents a quantum description of the beam splitter. Output states from beam splitters under different inputs such as single photons entering through one port, two photons entering through the two. For a standard beam splitter with two independent interfering modes $a,b$, one can write the interaction Hamiltonian as $$H=frac {i} {2} (e^ {-iphi}a^dagger b + e^ {iphi}b^dagger a)=-frac {1} {2} (a^dagger b+b^dagger a) $$ where I have assumed a $frac {pi} {2}$ phase added upon. on non-absorbing beam splitters.
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