Chip-integrated visible-telecom photon pair sources for quantum communication (1805.04011v1)

Abstract: Photon pair sources are fundamental building blocks for quantum entanglement and quantum communication. Recent studies in silicon photonics have documented promising characteristics for photon pair sources within the telecommunications band, including sub-milliwatt optical pump power, high spectral brightness, and high photon purity. However, most quantum systems suitable for local operations, such as storage and computation, support optical transitions in the visible or short near-infrared bands. In comparison to telecommunications wavelengths, the significantly higher optical attenuation in silica at such wavelengths limits the length scale over which optical-fiber-based quantum communication between such local nodes can take place. One approach to connect such systems over fiber is through a photon pair source that can bridge the visible and telecom bands, but an appropriate source, which should produce narrow-band photon pairs with a high signal-to-noise ratio, has not yet been developed in an integrated platform. Here, we demonstrate a nanophotonic visible-telecom photon pair source for the first time, using high quality factor silicon nitride resonators to generate bright photon pairs with an unprecedented coincidence-to-accidental ratio (CAR) up to (3800 +/- 200). We further demonstrate dispersion engineering of the microresonators to enable the connection of different species of trapped atoms/ions, defect centers, and quantum dots to the telecommunications bands for future quantum communication systems.

Chip-Integrated Visible-Telecom Photon Pair Sources for Quantum Communication

This paper introduces a novel approach to photon pair generation aimed at enhancing quantum communication networks, particularly in connecting quantum systems operating at different wavelengths. The research focused on developing chip-integrated nanophotonic sources that generate entangled photon pairs bridging visible and telecommunications (telecom) bands. Such an innovation addresses the limitation encountered in interfacing local quantum systems, often operating at visible wavelengths, with long-distance communication networks optimized for telecom wavelengths around 1550 nm.

The authors implemented high-quality silicon nitride ($\text{Si}_3\text{N}_4$) microresonators to demonstrate such photon pair sources for the first time. The microresonators utilized in this paper offer key advantages including high spectral brightness, high photon purity, and operation at sub-milliwatt optical pump power. The use of $\text{Si}_3\text{N}_4$ is particularly beneficial due to its wide bandgap, enabling efficiency across a broad range of wavelengths from near-UV to mid-infrared, thus supporting the necessary visible-to-telecom transition.

A defining feature of the device is its ability to produce bright photon pairs with a high coincidence-to-accidental ratio (CAR), achieving a CAR of $3800 \pm 200$ at the lowest generating pair flux of $1.2 \times 10^3$ counts per second. This value is significantly higher than previously reported for similar photon pair sources, which have typically recorded CARs below 20. The high CAR indicates the superior signal-to-noise performance of this new source, a crucial factor for practical quantum communication applications.

The device’s design leverages spontaneous four-wave mixing (SFWM) within microresonators to generate photon pairs, requiring delicate phase-matching and frequency-matching of the pump, signal, and idler modes. Innovations in dispersion engineering allowed for these matches over a spectral separation exceeding 250 THz, a challenging task met by adjusting fabricated microring parameters like width and thickness along with precise electromagnetic simulations. The successful matching underscores the feasibility of using integrated photonics for demanding optical quantum states.

In practical terms, the tuning capability of this photonic source is notable. By modulating both the pump wavelength and microring geometry, the authors successfully shifted the visible photons’ wavelengths from 630 nm to 810 nm, thereby bridging a broad array of quantum systems, including defect centers and quantum dots to existing telecom infrastructure.

From a theoretical perspective, this research advances our understanding and ability to craft integrated photonic systems that couple distinct quantum memories through entanglement swapping. The resulting potential to link local quantum processing units with global networks could be transformative, opening the door to scalable quantum networks. Moreover, these implementations position nanophotonics as a cornerstone of future quantum technologies, alongside developments in materials science and optical engineering.

In summary, this paper presents a significant technological advancement in quantum communication systems via the development of chip-integrated photonic devices for photon pair generation. The effectiveness of this approach lies in its high CAR values, tunability, and integration capabilities, setting a promising direction for further exploration in quantum state engineering and the implementation of practical, large-scale quantum networks. The implications and future directions include further device optimization, exploration of broader wavelength tunability, and initiation of real-world quantum networking trials.