Narrowband tunable source of polarization-entangled biphotons

In this work, we study a Type-II spontaneous down conversion-based source for generation of narrowband polarization entangled photons for │ψ〉⁺ and │ψ〉^– bell states for various applications in quantum technology. For long-distance quantum communication, in addition to high spectral brightness, a narrow bandwidth of entangled photons is required to compensate for chromatic dispersion in the fibres. Also, in order to match the bandwidth requirements of telecom quantum repeaters based on the storage of photons in the MHz-GHz range, spectral bandwidths in at least the GHz range are highly desirable while maintaining high brightness. This also offers advantages in quantum teleportation between end-users for the efficient transfer of quantum information. Also, such sources can be used as nodal points in quantum networks. The generated polarization entangled state is first checked for entanglement using two-photon interference in mutually orthogonal basis. The fringe visibility observed is >94%. We achieved a high fidelity ~>95% and high concurrence ~>90% based on tomographic measurements. A very low QBER ~<3% was observed using the setup. This source can be potentially used in practical quantum communication systems owing to its compact, narrowband and reliable nature.