Abstract
We demonstrate a compact, robust, and highly efficient source of polarization-entangled photons, based on linear bi-directional down-conversion in a novel 'folded sandwich' configuration. Bi-directionally pumping a single periodically poled KTiOPO(4) (ppKTP) crystal with a 405-nm laser diode, we generate entangled photon pairs at the non-degenerate wavelengths 784 nm (signal) and 839 nm (idler), and achieve an unprecedented detection rate of 11.8 kcps for 10.4 μW of pump power (1.1 million pairs / mW), in a 2.9-nm bandwidth, while maintaining a very high two-photon entanglement quality, with a Bell-state fidelity of 99.3 ± 0.3%.
Highlights
Entangled photons are essential for many fundamental quantum optics experiments, as well as a key resource in quantum communication [1] and the emerging field of quantum technologies [2]
The quality of an entangled photon source is commonly characterized by its brightness, that is, the number of generated pairs per mW of pump power and per nm of generated bandwidth, as well as the purity of the entangled state, or visibility
With experiments incorporating entangled photons expanding to the fields of biology and telecom engineering, other essential criteria are the ease of operation and long-term stability, as well as the number, cost and complexity of optical components required to build the source
Summary
Entangled photons are essential for many fundamental quantum optics experiments, as well as a key resource in quantum communication [1] and the emerging field of quantum technologies [2]. In the case of collinear SPDC, two commonly implemented configurations are based on crossed crystals and non-degenerate wavelengths [11], or a bi-directionally pumped crystal in a Sagnac-loop [9,10] These schemes share the advantage that no active interferometric stabilization is required, due to the common path configuration for down-converted and pump photons. The Sagnac scheme is not prone to such issues as it makes use of only a single down-converting crystal, but requires overlapping the SPDC emission in both arms of the interferometer in order to achieve high brightness and visibility This involves an intricate alignment procedure, time-consuming for less experienced users, which becomes even more daunting with the small beam waists required to achieve high pair-generation efficiencies. The source can be further integrated and is likely to become compliant with the severe requirements of space flight and operation
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