High-Visibility Franson Interference Enabled by Passive Photonic Integrated Interferometers at Telecom Wavelengths

High-visibility Franson interference at telecom C-band wavelengths is achieved using a cascaded periodically poled lithium niobate (PPLN) waveguide photon-pair source combined with fully passive, path-imbalanced Mach-Zehnder interferometers implemented on photonic integrated circuits (PICs). The interferometers require neither on-chip phase shifters nor active stabilization; instead, the phase is scanned via thermal tuning of the chip. By employing a narrow-linewidth continuous-wave (CW) pump and dense wavelength-division multiplexing (DWDM) filtering, energy-time entangled photon pairs with high spectral indistinguishability are generated. We achieve a 4.8% heralding efficiency and a two-photon interference visibility of 97.1% from sinusoidal fringe fitting (raw visibility 95.2% and background-corrected visibility 95.6%), alongside a coincidence-to-accidental ratio (CAR) exceeding 1000 at only 1.7 mW of pump power. These results represent one of the highest Franson-interference visibilities reported for a PIC-based analyzer within a compact, fiber-integrated platform.

💡 Research Summary

This paper presents the achievement of high-visibility Franson interference at telecom C-band wavelengths using a fully passive, photonic integrated circuit (PIC)-based analyzer combined with a cascaded periodically poled lithium niobate (PPLN) waveguide photon-pair source.

The experimental setup begins with a narrow-linewidth continuous-wave (CW) laser at 1560 nm. This pump light is first converted to 780 nm via second-harmonic generation (SHG) in a PPLN waveguide. The resulting 780 nm light then pumps a second PPLN waveguide to generate photon pairs via spontaneous parametric down-conversion (SPDC) around the 1560 nm band. A key feature of this “cascaded” architecture is that it produces a narrowband pump for the SPDC process, which enhances the spectral indistinguishability of the generated photon pairs—a critical factor for high-interference visibility.

The broadband SPDC output is then filtered using a dense wavelength-division multiplexing (DWDM) demultiplexer, selecting specific ITU-grid channels (CH20 and CH22) for the signal and idler photons. This filtering defines the photon wavelengths and, more importantly, drastically reduces the single-photon coherence time to a few picoseconds, which suppresses any single-photon interference in the subsequent analyzers.

Each photon is directed into a separate unbalanced Mach-Zehnder interferometer (uMZI) fabricated on a photonic integrated circuit. These interferometers have a path-length difference corresponding to a temporal delay of approximately 0.8 ns. A significant innovation is that these PIC-based uMZIs are entirely passive; they contain no on-chip phase shifters, heaters, or active stabilization elements. Instead, the relative interferometric phase is controlled by uniformly changing the temperature of the entire chip, leveraging the thermo-optic effect. This monolithic, common-mode design provides excellent passive stability against environmental perturbations.

The system performance was characterized by several key metrics. At a low pump power of only 1.7 mW, the setup achieved a heralding efficiency of 4.8%. The two-photon interference visibility, which is the central result, reached 97.1% as derived from a sinusoidal fit to the fringe data (with raw and background-corrected visibilities of 95.2% and 95.6%, respectively). Furthermore, the coincidence-to-accidental ratio (CAR) exceeded 1000, indicating a very high signal-to-noise ratio and low multi-pair generation noise.

In summary, this work demonstrates a compact, fiber-pigtailed platform that integrates a high-quality entangled photon-pair source with a stable, passive PIC-based analyzer. By eliminating the need for active phase stabilization and utilizing telecom-compatible components and wavelengths, the research represents a major step towards practical, deployable quantum technologies for applications such as quantum networking and distributed quantum sensing. The achieved performance metrics rank among the highest reported for a PIC-based Franson interferometer, highlighting the potential of integrated photonics for scalable quantum information processing.