The advancement of integrated quantum photonic technologies has sparked notable interest in high-speed, cost-effective, miniature and readily manufacturable devices for quantum communications, with the specific emphasis on practical QKD applications30. Previously, protocols of prepare-and-measure QKD31,32,33,34,35,36,37, MDI-QKD38,39,40 and entanglement-based QKD13,41,42,43 have been implemented with integrated-photonics devices but primarily in the point-to-point scenario. Notably, chip-based point-to-point TF-QKD has been demonstrated using partially integrated InP photonic chips44 and recent work shows the potential of microcombs in MDI-QKD networks45.
Integrated microcombs at the server node
Our integrated microcomb that emits tens of ultralow-noise comb lines is realized through the hybrid integration of an InP distributed feedback (DFB) laser with an ultrahigh-Q Si 3 N 4 microresonator (see Fig. 2a,b). This is enabled by the self-injection-locking process, which allows the DFB laser to lock onto the resonant mode and narrows laser linewidths through direct backscattering of the intracavity field26,27. The microresonator was fabricated on 100-nm-thick Si 3 N 4 waveguides (see the scanning electron microscope image in Fig. 2b) having an ultralow loss of 1.7 dB m−1. Figure 2c reports the measured intrinsic Q factor for the modes around λ 0 at 1,549.78 nm, with a mean Q of 20 million. Its free spectral range was measured to be 30.03 GHz. Fabrication and characterization details for the wafer-scale microresonators are provided in Supplementary Information Section 1, exhibiting consistently high Q factors and well-controlled dispersion. By adjusting the air gap between the DFB laser and the microresonator, the dynamics of nonlinear phase could be tailored, which then resulted in linewidth narrowing and dark-pulse comb generation. Figure 2d reports the measured spectrum for the generated dark-pulse microcombs (see simulation data in Supplementary Fig. 4). It exhibits a flat-top multichannel spectrum with a total output power of around 10 dBm. Coloured comb lines (partly from λ −5 to λ +8 ) were selected for TF-QKD implementations. To assess the spectral stability, which is crucial for practical implementations, we conducted continuous system operation over 12 h. The measured flat temporal output intensity profile of the microcomb in Fig. 2e demonstrated high system-level stability, which can be further improved through co-packaging hybrid integration of the DFB laser and microresonator26. Figure 2f presents measured frequency noises for the comb lines46. In comparison with the free-running DFB laser, the comb lines exhibit more than 25 dB noise suppression and yield a white-noise floor around 13 Hz2 Hz−1, indicating short-term linewidths around 40 Hz, which is essential for TF-QKD.
Fig. 2: Integrated-photonics microcomb and QKD transmitter chips. a, Schematic diagram of chips and networks. Server-chip microcomb with Hz-level linewidths is used to drive and phase-lock slave lasers on QKD client chips separated at metropolitan areas A and B corresponding to group A and group B chips. Client chips locally regenerate low-noise light field for phase reference (at λ 0 , yellow pulses) and weak coherent states for quantum key coding (at λ i , coloured pulses) using phase-locked slave lasers. Paired client chips of {A i , B i } send quantum states through the λ i channel in long-haul upstream fibres (in which all wavelength channels are multiplexed), with single-photon interference measurements at the server node returning secure keys. A complete set-up diagram is provided in Supplementary Fig. 5. b, Device and experimental set-up of integrated microcombs by self-injection-locking a DFB laser to a Si 3 N 4 ultrahigh-Q microresonator. Scale bar, 100 nm. c, Measured intrinsic Q values for the resonate modes of the Si 3 N 4 microresonator with a mean value of 20.0 ± 2.8 million. Mode number specifies the ordinal position relative to the central mode. d, Measured spectrum of dark-pulse microcombs. Coloured channels indicate those used in the present network. e, Normalized intensity fluctuation of the microcomb output under the spectral stability characterization. The spectrum of the microcomb was collected every minute with an optical spectrum analyser over consecutive 12 h. The normalized intensity fluctuation shows excellent system stability, maintaining a mean relative intensity of 0.994 ± 0.006. f, Frequency noise characterization of microcomb lasers at the server. Power spectral densities were measured using the correlated self-heterodyne method46. g, Summary of intrinsic linewidths for the microcomb and 20 phase-locked slave lasers on the client chips denoted as {A i , B i }. Linewidths are derived from the white-noise floor of measured power spectral densities in f and i. Points are experimental data and dashed lines are their mean values. h, Device and experimental set-up of InP-based QKD client chips, featuring monolithic integration of all components as shown in a. i, Frequency noise characterization of phase-locked slave lasers on the QKD client chips, showing comparable performance with the microcomb (see summary in g). In f and i, free-running lasers (black) are plotted for comparison. j, Spectra of locked DBR lasers on QKD client chips. The top plot shows the continuous tuning spectrum of a single client laser, aligning and phase-locking to specific comb lines. The middle and bottom plots show spectra of injection-locked slave lasers on different client chips. k, Characterization of 120 EOPMs on 20 independent InP client chips (each with six EOPMs, forming IMs). Points are experimental data and lines are their mean values for the measured V π and interference visibility of IMs. Data points of zero V π or visibility indicate three hardware failures of modulators. Error bars denote ±1 standard deviation of measured data. Full size image
Integrated transmitters at the user node