Daylight free-space quantum key distribution in three papers

Short answer: Sunlight adds noise that has historically limited free-space quantum key distribution (QKD) to night time. Three papers report daylight operation: a 53 km link at 1550 nm in 2017, a silicon-photonics chip transmitter over 145 m in 2021, and a continuous-variable system in 2025 that works in daytime and rain. These are laboratory and field-test results from the authors, not services.

Why is daylight a problem?

The 2017 Nature Photonics abstract says that long-distance free-space quantum communication experiments could previously only be done at night, because bright background sunlight prohibits transmission over long distances under high channel loss. The 2025 npj Quantum Information abstract adds that conventional QKD requires strict filtering at the receiver, and any ambient light entering the system can increase the bit error rate significantly and even make the system inoperable.

What did the 53 km experiment show?

The 2017 authors chose a working wavelength of 1,550 nm and developed free-space single-mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors. With these they report free-space QKD over 53 km during the day. The total channel loss was about 48 dB, which they say is greater than the 40 dB loss between a satellite and the ground and between low-Earth-orbit satellites. They conclude it demonstrates the feasibility of satellite-based quantum communication in daylight, and that the telecom-band wavelength is compatible with ground fibre networks. These are the authors' conclusions about feasibility.

What did the chip-based prototype show?

The 2021 paper presents a full prototype for daylight QKD at 1550 nm that uses an integrated silicon-photonics chip as the state encoder. It was tested in the urban area of Padua, Italy, over a 145 m free-space link. The authors report a quantum bit error rate around 0.5 percent and an averaged secret key rate of 30 kbps over a whole sunny day, from 11:00 to 20:00. They describe the chip as a cost-effective solution for portable free-space transmitters and a promising resource for future satellite payload designs, which is their outlook.

What did the continuous-variable system show?

The 2025 paper presents a Gaussian-modulated coherent-state continuous-variable QKD system that the authors say works effectively during the day or even on rainy days without complex filtering operations. It maintains a secure key rate of more than 100 kbps at a frequency of 10 MHz, which the authors say lays a foundation for low-cost, robust free-space quantum communication. The abstract does not give the link distance.

What do the three have in common?

All three papers work at or near telecom wavelengths or with receivers that tolerate background light, and each frames its result as a step towards free-space links that do not stop at sunset. The 2017 and 2021 papers both mention satellite use as a motivation, while the 2025 paper stresses low cost and robustness. None of the abstracts states a security proof, a commercial plan or an operating network, so those questions are left open here.

How to read this

The three links differ in distance, protocol and conditions, so their key rates do not compare directly. Security analyses are not covered by these abstracts. This post is based on abstracts and visible introductions, and full texts were not checked. It makes no claim about deployed networks or products.

FAQ

Why use 1550 nm?

It is a telecom-band wavelength, and the 2017 authors say it makes the system compatible with ground fibre networks.

How long was the longest link?

53 km, in the 2017 daylight experiment.

What key rate was reported for the chip transmitter?

An averaged 30 kbps with a quantum bit error rate around 0.5 percent over 145 m.

Does continuous-variable QKD avoid filtering?

The 2025 authors say their system works without complex filtering operations.