Experimental demonstration of Continuous-Variable Quantum Key Distribution with a silicon photonics integrated receiver

Quantum Key Distribution (QKD) is a prominent application in the field of quantum cryptography providing information-theoretic security for secret key exchange. The implementation of QKD systems on photonic integrated circuits (PICs) can reduce the size and cost of such systems and facilitate their deployment in practical infrastructures. To this end, continuous-variable (CV) QKD systems are particularly well-suited as they do not require single-photon detectors, whose integration is presently challenging. Here we present a CV-QKD receiver based on a silicon PIC capable of performing balanced detection. We characterize its performance in a laboratory QKD setup using a frequency multiplexed pilot scheme with specifically designed data processing allowing for high modulation and secret key rates. The obtained excess noise values are compatible with asymptotic secret key rates of 2.4 Mbit/s and 220 kbit/s at an emulated distance of 10 km and 23 km, respectively. These results demonstrate the potential of this technology towards fully integrated devices suitable for high-speed, metropolitan-distance secure communication.

💡 Research Summary

This paper presents a comprehensive experimental demonstration of a continuous‑variable quantum key distribution (CV‑QKD) receiver that is fully implemented on a silicon photonic integrated circuit (Si‑PIC). The authors aim to address the size, cost, and stability limitations of conventional bulk‑optics CV‑QKD setups by leveraging the maturity of silicon photonics, which allows the use of standard room‑temperature photodiodes instead of cryogenic single‑photon detectors.

Device Architecture
The receiver consists of a single balanced homodyne detector (BHD) realized on the PIC. Light from the quantum channel and the local oscillator (LO) are coupled into the chip via a pair of grating couplers, combined on a 50/50 beam splitter, and directed to two germanium photodiodes placed in series after variable optical attenuators (VOAs). The VOAs enable fine balancing of the optical powers in the two arms, which is essential for common‑mode suppression and linear operation. Each photodiode is reverse‑biased at –0.5 V (or +0.5 V for the opposite arm) to keep the dark current low while preserving a high responsivity (≈1.25 A/W at 1550 nm).

Electronic Front‑End
Because the differential photocurrent generated by the BHD is weak, a low‑noise amplification chain is required. The authors designed a two‑stage circuit: a transimpedance amplifier (TIA) based on the OPA818 RF amplifier, followed by a non‑inverting voltage amplifier. The TIA was optimized for a closed‑loop bandwidth of at least 100 MHz and a shot‑to‑electronic‑noise clearance of ≥10 dB. Careful layout, wire‑bonding of the PIC, and minimization of parasitic capacitance (effective input capacitance ≈1.8–2 pF) yielded a substantial improvement over a reference bulk BHD built with Hamamatsu photodiodes. Noise power spectral density (PSD) measurements show that the PIC‑based receiver maintains a clearance of 26 dB at 1 MHz, 14 dB at 10 MHz, and still above 9 dB at 100 MHz, defining an operational bandwidth of roughly 150 MHz for quantum‑signal detection.

Linear Range and Efficiency
The detector’s linearity was verified up to an optical input power of ~8 mW (corresponding to ≈1.1 mW incident on each photodiode before saturation). The overall detection efficiency, from fiber array input to electronic output, was measured at about 26 %, which is competitive for integrated CV‑QKD receivers.

CV‑QKD Protocol Implementation
A variant of the GG02 protocol is employed, using Gaussian‑modulated coherent states generated by optical single‑sideband (OSSB) modulation. The authors introduce a frequency‑multiplexed pilot scheme: low‑frequency pilot tones are transmitted alongside the quantum symbols to enable real‑time recovery of the LO phase, frequency offset, and clock. Custom digital signal processing (DSP) algorithms perform carrier recovery, phase estimation, and symbol demodulation, allowing the system to operate close to the Shannon limit for the given bandwidth.

Experimental Results
The integrated receiver was tested in a laboratory QKD setup that emulated channel losses corresponding to 10 km (≈2 dB) and 23 km (≈5 dB) of standard single‑mode fiber. Measured excess noise, together with the calibrated detection efficiency and clearance, were used to compute asymptotic secret‑key rates under the assumption of collective Gaussian attacks. The results are:

• 10 km equivalent: 2.4 Mbit s⁻¹ secret key rate.
• 23 km equivalent: 220 kbit s⁻¹ secret key rate.

These rates surpass previously reported silicon‑based CV‑QKD demonstrations, which typically achieved only a few hundred kilobits per second, and demonstrate that a single‑chip balanced detector can support metropolitan‑scale secure communications.

Significance and Outlook
The work validates that silicon photonics can meet the stringent noise, bandwidth, and linearity requirements of high‑speed CV‑QKD. By integrating the detector, VOAs, and passive routing on a single chip, the authors reduce alignment complexity and improve mechanical stability, paving the way toward fully integrated transmitters and receivers. Remaining challenges include further increasing detection efficiency (e.g., through better grating couplers or low‑loss waveguides), scaling to multi‑channel (wavelength‑division multiplexed) operation, and implementing measurement‑device‑independent or composable security analyses in a fully integrated platform. Nonetheless, this demonstration constitutes a crucial step toward practical, low‑cost quantum‑secure networks for urban environments.