A Tolerance-Enhanced Spontaneous ParametricDownconversion Source of Bright Entangled Photons

Abstract

Spontaneous parametric down-conversion (SPDC), a primary resource ofphotonic quantum entangled states, strongly depends on the intrinsic phasematching condition. This makes it susceptible to changes in factors such asthe pump wavelength, crystal temperature, and crystal axes orientation. Suchintolerance to changing environmental factors prohibits deployment ofSPDC-based sources in non-ideal environments outside controlledlaboratories. Here, a novel system architecture based on a hybrid linear andnon-linear solution that is shown to make the source tolerance-enhancedwithout sacrificing brightness. This linear solution is a lens-axicon pair,judiciously placed, which is tested together with two common non-linearcrystals, quasi-phase-matched periodically-poled KTiOPO4 , andbirefringent-phase-matched BiB 3 O6 . This approach has the benefit ofsimultaneous tolerance to the environment and high brightness, which isdemonstrated by using the proposed architecture as a stable entangledphoton source and a spectral brightness as high as 22.58 ± 0.15 kHz mW−1with a state fidelity of 0.95 ± 0.02, yet requiring a crystal temperature stabilityof only ±0.8 ◦C, a 5× enhanced tolerance as compared to the conventionalhigh brightness SPDC configurations is reported. This solution offers a newapproach to deployable high-brightness quantum sources that are robust totheir environment, for instance, in satellite-based quantum applications.