Quantum Light for Satellites express
A light beam sent from Earth to a satellite scatters in the atmosphere, like a flashlight in fog. To keep the signal intact, you need short-wavelength light that spreads less. Such light is born in special crystals, but shrinking them to chip size is tough.
A research team made a chip from silicon nitride — a transparent material similar to glass but stronger. Inside the chip, light loops around like on a racetrack, occasionally splitting into a pair of particles. Previously, this material didn't work for short wavelengths: light 'slowed down' in it in the wrong way. The scientists routed the light along a different path inside the chip, and outsmarted the limitation. Now the chip produces 48.7 million pairs per second per milliwatt of power.
This chip has already survived radiation exposure like in space without breaking. That means it can be mounted on satellites. Secure quantum communication could then work even during the day, when sunlight interferes with conventional detectors.
Original abstract
as written by the authors arXiv:2601.13617
Quantum state distribution over vast distances is essential for global-scale quantum networks and fundamental test of quantum physics at space scale. While satellite platforms have demonstrated thousand-kilometer entanglement distribution, quantum key distribution and quantum teleportation with ground, future constellations and deep-space missions demand photon sources that are robust, compact, and power-efficient. Integrated photonics offers a scalable solution, yet a critical spectral gap persists. Although telecom-band integrated photon-pair sources are well established, near-visible photons offer distinct advantages for satellite-to-ground links by mitigating diffraction loss and maximizing the collection efficiency of optical telescopes. Scalable integrated sources in this regime have remained elusive due to the fundamental challenge of achieving anomalous dispersion in materials transparent at visible wavelengths. Here we bridge this gap by demonstrating an integrated near-visible photon-pair source based on a wide-bandgap, ultralow-loss, silicon nitride (Si$$_3$$N$$_4$$) microresonator. By engineering the dispersion of higher-order waveguide modes, we overcome the intrinsic normal dispersion limit to achieve efficient phase matching. The device exhibits a spectral brightness of 4.87$$\times$$10$$^7$$ pairs/s/mW$$^2$$/GHz and a narrow photon linewidth of 357 MHz. We report high-purity heralded single-photon generation with a heralding rate up to 2.3 MHz and a second-order correlation function as low as 0.0041. Furthermore, we observe energy-time entanglement with 98.4% interference visibility, violating the CHSH limit even at flux exceeding 40.6 million pairs/s. Combined with the proven radiation hardness of Si$$_3$$N$$_4$$, this source constitutes a flight-ready hardware foundation for daylight quantum communications and protocols requiring on-orbit multiphoton interference.
The light particles from the chip are so synchronized that they could serve as perfect hourglasses: if one 'flips,' the other does the same in the same instant.
In the series 'The Expanse,' quantum communication works through relays, but the chip from this article could fit inside every spacesuit.