Broadband-laser-diode pumped periodically poled potassium titanyl phosphate-Sagnac polarization-entangled photon source

Abstract

We experimentally demonstrate a polarization-entangled photon source at 810 nm using a type-II phase-matched periodically poled potassium titanyl phosphate (PPKTP) crystal pumped by a low-cost, broadband laser diode with a central wavelength of 405 nm and a typical bandwidth of 0.53 nm. The PPKTP crystal is placed in a Sagnac loop to realize the compact size and high stability. The downconverted biphotons, the signal and the idler, have typical bandwidths of 5.57 and 7.32 nm. We prepare two Bell states | Ψ + ⟩ and | Ψ − ⟩ with the fidelities of 0.948 ± 0.004 and 0.963 ± 0.002 . In the polarization correlation measurement, the visibilities are all higher than 96.2%, and in the Bell inequality test, the S value can achieve 2.78 ± 0.01 . This high-quality and low-cost entangled photon source may have many practical applications in quantum information processing.

Controllable transitions among phase-matching conditions in a single nonlinear crystal

“…The first laser is a narrowband laser (Kunteng QTechnics) with tunable center wavelengths of 401-409 nm and a linewidth of 10 MHz. The second laser is a broadband laser [29] , which has a strong-power portion at 405 nm and a weak-power portion at around 315 nm. During our experiment, we control the pump power using a half-wave plate (HWP) and a polarizing beam splitter (PBS).…”

“…This wavelength range, particularly around 405 nm, holds significant importance for entangled photon sources, being widely utilized not only in the laboratory [33][34][35] but also in satellite-based endeavors [36] . The popularity of this wavelength range can be attributed to the maturity of blue-violet laser technology, which allows the production of high-power blue lasers at relatively affordable costs [37][38][39][40][41][42] . The identification of three phase-matching conditions in this study holds the potential to advance the quantum applications of entangled photon sources utilizing blue lasers and PPKTP crystals.…”