Towards on-chip photon-pair bell tests: Spatial pump filtering in a LiNbO3 adiabatic coupler

Abstract

Nonlinear optical waveguides enable the integration of entangled photon sources and quantum logic gates on a quantum photonic chip. One of the major challenges in such systems is separating the generated entangled photons from the pump laser light. In this work, we experimentally characterize double-N-shaped nonlinear optical adiabatic couplers designed for the generation of spatially entangled photon pairs through spontaneous parametric down-conversion, while simultaneously providing spatial pump filtering and keeping photon-pair states pure. We observe that the pump photons at a wavelength of 671 nm mostly remain in the central waveguide, achieving a filtering ratio of over 20 dB at the outer waveguides. We also perform classical characterization at the photon-pair wavelength of 1342 nm and observe that light fully couples from an input central waveguide to the outer waveguides, showing on chip separation of the pump and the photon-pair wavelength.

Integrated Source of Path-Entangled Photon Pairs with Efficient Pump Self-Rejection

“…A common 50/50 beamsplitter at the outcome of our device allows for the remnant of the driving field (which is in the symmetric mode) to be filtered out. A similar spatial-filtering mechanism was recently suggested for pump rejection for the SPDC-based waveguide source of photon pairs [ 16 , 17 ].…”

“…Only recently, a theoretical proposal using photonic crystals has appeared suggesting to exploit spatial features, i.e., suppress coupling of the output mode to the pump by symmetry considerations [ 15 ]. Similar spatial filtering ideas were also suggested for the integrated source of photon pairs based on the three-wave mixing by the spontaneous parametric down-conversion (SPDC) [ 16 , 17 ].…”

“…Such a “built-in” filtering can be further enhanced by exploiting the symmetry properties of the modes (as it was done in recent works with SPDC [ 16 , 17 ]): the output from both waveguides can be interfered on a beamsplitter, in order to filter out the possible remnants of the symmetric mode and put both photons in the same spatial mode.…”

Integrated source of path-entangled photon pairs with efficient pump self-rejection

“…Only recently a theoretical proposal using photonic crystals has appeared suggesting to exploit spatial features, i.e., suppress coupling of the output mode to the pump by symmetry considerations [13]. Similar spatial filtering ideas were suggested also for the integrated source of photon pairs based on the three-wave mixing by the spontaneous down-conversion [14,15].…”

Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers

“…Broadband performance also means that this concept is tolerant to minor fabrication imperfections. Finally, polarization splitting allows using the device for both path-encoded and polarization-encoded state generations, unlike previous implementations not supporting this feature [47]. The core LiNbO 3 chip (integrating a 20-mm long PPLN SPDC source and a 51-mm long IAPMS) in the pump-filtered, broadband tunable Bell-state generator schematically shown in Fig.…”