Abstract
Counter-propagating parametric conversion processes in non-linear bulk crystals have been shown to feature unique properties for efficient narrowband frequency conversion. In quantum optics, the generation of photon pairs with a counter-propagating parametric down-conversion process (PDC) in a waveguide, where signal and idler photons propagate in opposite directions, offers unique material-independent engineering capabilities. However, realizing counter-propagating PDC necessitates quasi-phase-matching (QPM) with extremely short poling periods. Here, we report on the generation of counter-propagating single-photon pairs in a self-made periodically poled lithium niobate waveguide with a poling period on the same order of magnitude as the generated wavelength. The single photons of the biphoton state bridge GHz and THz bandwidths with a separable joint temporal-spectral behavior. Furthermore, they allow the direct observation of the temporal envelope of heralded single photons with state-of-the art photon counters.
Highlights
Nonlinear optical processes exploiting quasi-phasematching (QPM) [1, 2] in periodically poled χ(2) materials provide a versatile means for numerous frequency conversion devices, e.g. optical parametric oscillators (OPOs) [3]
We fabricate the waveguide with short poling periods to achieve counter-propagating phase matching
Field-assisted domain inversion is a mature technique for periodic poling of LiNbO3 waveguides
Summary
Nonlinear optical processes exploiting quasi-phasematching (QPM) [1, 2] in periodically poled χ(2) materials provide a versatile means for numerous frequency conversion devices, e.g. optical parametric oscillators (OPOs) [3]. Efficient mirrorless OPOs [19] in bulk KTP crystals with ultra-short periods have been demonstrated [20,21,22] Different from such classical devices, harnessing counter-propagation for quantum applications is still in its infancy. Neither efficient counter-propagating separable quantum state generation nor a detailed study of temporal and spectral properties of such two-photon states have been demonstrated which is mostly due to practical limitations due to low efficiency To overcome these limitations waveguide devices promise many benefits to achieve the required efficiencies by exploiting the strong confinement of the fields over a long interaction length. We demonstrate that with on optimization of the underlying technologies periodically poled waveguides with periods of 1.7 μm can be fabricated With these devices counter-propagating photon pair generation exploiting 5-th order QPM could be demonstrated and a detailed quantum spectral and temporal characterization. Apart from gaining inside into the fundamental process of counter-propagating pair generation, the devices with the ultra-short poling periods complement the toolbox of lithium niobate components [33] for future advanced quantum circuits, and the substantially improved poling technology paves the way towards integrated quantum photonic circuits based on lithium niobate thin films [34]
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