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Full daylight quantum-key-distribution at 1550 nm enabled by integrated silicon photonics
M. Avesani, L. Calderaro, M. Schiavon, A. Stanco, C. Agnesi, A. Santamato, M. Zahidy, A. Scriminich, G. Foletto, G. Contestabile, M. Chiesa, D. Rotta, M. Artiglia, A. Montanaro, M. Romagnoli, V. Sorianello, F. Vedovato, G. Vallone, P. Villoresi
TL;DR
Daylight free-space QKD still needs to combine daytime operation, telecom-fiber compatibility, stable fiber coupling, and compact hardware for future networks. The paper presents QCoSOne, a silicon-photonic chip-based 1550 nm QKD prototype tested over a 145 m urban link. It operated from 11:00 to 20:00 with about 0.5% QBER and secret-key rates reaching 65 kbps.
Problem
Free-space QKD remains limited relative to fiber-based systems and must meet daytime, telecom-wavelength, and stable single-mode-fiber-coupling requirements.
Method
QCoSOne uses an integrated silicon-photonic chip for decoy and polarization modulation, packaged for field use and combined with active turbulence correction, filtering, and SNSPD detection.
Results
QBER was around 0.5%, with secret-key rates up to 65 kbps during daylight operation from 11:00 to 20:00 over a 145 m free-space link.
Takeaways & Limitations
The prototype demonstrates chip-based daylight free-space QKD at 1550 nm and supports its use as a resource for portable transmitters and satellite optical payloads.
Takeaways & Limitations
Further performance improvements would require higher clock rates and adaptive optics to increase SMF coupling, tolerable losses, and link distance.
Abstract
from arXiv · showhide
The future envisaged global-scale quantum communication network will comprise various nodes interconnected via optical fibers or free-space channels, depending on the link distance. The free-space segment of such a network should guarantee certain key requirements, such as daytime operation and the compatibility with the complementary telecom-based fiber infrastructure. In addition, space-to-ground links will require the capability of designing light and compact quantum devices to be placed in orbit. For these reasons, investigating available solutions matching all the above requirements is still necessary. Here we present a full prototype for daylight quantum key distribution at 1550 nm exploiting an integrated silicon-photonics chip as state encoder. We tested our prototype in the urban area of Padua (Italy) over a 145m-long free-space link, obtaining a quantum bit error rate around 0.5% and an averaged secret key rate of 30 kbps during a whole sunny day (from 11:00 to 20:00). The developed chip represents a cost-effective solution for portable free-space transmitters and a promising resource to design quantum optical payloads for future satellite missions.
INTRODUCTION
The paper addresses daylight free-space QKD requirements by combining telecom-band operation with integrated silicon photonics. QCoSOne demonstrates daylight QKD at 1550 nm over a 145 m urban link, achieving low QBER and substantial secret-key rates.
- Free-space QKD must support fullday functionality, telecom-wavelength fiber compatibility, and stable coupling into a single-mode fiber.
- Sunlight-induced background noise has limited most free-space QKD demonstrations to nighttime operation.
- 1550 nm operation supports standard fiber-based quantum communication and compatibility with compact, low-power integrated silicon-photonic devices.
- Atmospheric turbulence requires active correction to maintain stable single-mode-fiber coupling, enabling use of superconducting nanowire detectors.
- QCoSOne combines a packaged silicon-photonic state encoder with decoy and polarization modulation for 3-state 1-decoy QKD.
- QBER was around 0.5%, with secret-key rates up to 65 kbps during continuous daylight operation from 11:00 to 20:00.
Description of QCoSOne prototype
QCoSOne integrates source encoding, filtering, free-space optics, turbulence correction, and fiber-based detection into a field-deployable prototype. Its silicon-photonic chip prepares decoy-state polarization qubits while active pointing maintains coupling to Bob’s receiver.
- Alice and Bob use 12 cm-aperture refractor and 31.5 cm-aperture Cassegrain telescopes linked by a free-space channel.
- A fast-steering mirror uses beacon-centroid feedback to correct tip-tilt turbulence and achieves approximately 4% average SMF coupling efficiency.
- A silicon photonic integrated circuit uses interferometric components, thermo-optic modulators, and carrier-depletion modulators for compact state preparation.
- The packaged chip occupies 1.2 cm × 1.5 cm × 1.2 cm and is designed for rugged, portable field use.
- Amplitude modulation prepares two intensity levels, while polarization modulation generates the protocol’s key-generation and control-basis states.
- Each produced polarization state reaches an extinction ratio of up to 30 dB.
- A 100 GHz-spaced WDM filter, attenuator, splitter, SPAD monitor, and telescope form Alice’s output chain.
- Bob’s analyzer uses a matched WDM filter, basis-selecting beam splitter, polarization optics, and four SNSPDs.
The field trial
The field trial demonstrated daylight QKD over multiple sunny days, including eight continuous hours and operation near maximum solar elevation. The system maintained low QBER and produced secret keys at tens-of-kilobits-per-second rates.
- The field trial: Eight hours of continuous daylight QKD were performed on April 18th, with detection rates averaging about 100 kHz.The experiment ran from daylight conditions while tracking detection and noise performance.
- The field trial: Less than 0.75% QBER was measured throughout the eight-hour run, reaching approximately 0.45% in Z and 0.25% in X.These values were obtained without active polarization stabilization.
- The field trial: 50–150 kbps sifted-bit rates and 20–70 kbps asymptotic SKRs were measured across three sunny days.Finite-size SKR reached 65.8 kbps in the final acquisition on April 17th.
- The field trial: 33 kbps mean finite-size SKR was obtained during approximately 50-minute runs on April 18th.This outperformed comparable 1550 nm free-space QKD systems by two orders of magnitude.
DISCUSSION
The prototype demonstrated chip-based free-space quantum communication in daylight at a telecom-band wavelength, with low QBER and secret-key rates of several tens of kilobits per second. Its compact resource requirements support consideration for portable and satellite quantum-communication payloads, while higher clock rates and adaptive optics remain improvement paths.
- DISCUSSION: The chip-based prototype operated in daylight at a telecom C-band wavelength and achieved approximately 0.5% QBER with secret-key rates of several tens of kbps.The demonstration also operated with the Sun at maximum elevation.
- DISCUSSION: Single-chip intensity and polarization modulation was demonstrated for decoy-state QKD in a real urban free-space trial.The prototype used dedicated packaging for the field experiment.
- DISCUSSION: Low power, weight, and space requirements make the solution attractive for portable terminals and satellite quantum-communication payloads.The stated application scope concerns optical payload design.
- DISCUSSION: Increasing the clock rate and using adaptive optics could improve coupling efficiency, tolerable losses, and achievable link distance.The passage gives 1 GHz as an example of a higher system clock rate.
Description of the QKD protocol
The protocol uses a three-state, one-decoy efficient BB84 scheme with polarization and intensity choices, followed by basis sifting, error correction, and privacy amplification.
- The implementation uses the 3-state 1-decoy version of efficient BB84 with polarization encoding.
- Alice randomly selects the Z or X basis, while the X basis is mutually unbiased with respect to Z.
- Three polarization states are generated: |0⟩ and |1⟩ for Z, and |+⟩ for X.
- Each pulse randomly uses one of two intensity levels, µ1 or µ2, with basis-dependent values permitted for yield analysis.
- After basis announcement, Z-basis detections form the raw sifted key, while X-basis detections estimate potential eavesdropper information.
- Error correction and finite-key privacy amplification produce a secure secret key of l bits for each privacy-amplification block.
- The security calculation uses vacuum and single-photon detection bounds, phase-error estimation, binary entropy, and error-correction leakage with fEC ≈1.06.
Additional details on the PAT system
The PAT system combines coarse telescope alignment with closed-loop fine alignment for single-mode-fiber coupling, improving coupling in the 145-m link despite turbulence.
- The PAT system uses counter-propagating 635-nm and 850-nm beacons with CMOS cameras for coarse telescope alignment.
- Fine alignment controls a fast-steering mirror in closed loop to maintain single-mode-fiber coupling.
- 4% mean coupling efficiency was achieved with tip-tilt correction versus approximately 1% without control in the 145-m link.Peak coupling values reached 10%.
- The corrected-link coupling remained below the 20% laboratory value because of turbulence in the optical link.