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High-speed measurement-device-independent quantum key distribution with integrated silicon photonics
Kejin Wei, Wei Li, Hao Tan, Yang Li, Hao Min, Wei-Jun Zhang, Hao Li, Lixing You, Zhen Wang, Xiao Jiang, Teng-Yun Chen, Sheng-Kai Liao, Cheng-Zhi Peng, Feihu Xu, Jian-Wei Pan
TL;DR
MDI-QKD needs practical low-cost transmitters while placing complex measurement equipment at an untrusted relay. This work demonstrates a 1.25 GHz polarization-encoding system using silicon photonic chips with random state and decoy modulation, achieving 31 bps over 36 dB loss and supporting future low-cost quantum networks.
Problem
MDI-QKD requires low-cost user transmitters while the complicated and expensive measurement devices remain at a central untrusted relay.
Method
The experiment uses silicon photonic transmitters integrating intensity modulation, polarization modulation, and variable optical attenuation for polarization-encoded MDI-QKD.
Results
31 bps finite-key secret rate is achieved over 36 dB channel loss, corresponding to 180 km standard fibre.
Takeaways & Limitations
The demonstrated chip-based system is presented as a promising solution for low-cost, scalable QKD networks with an untrusted relay.
Takeaways & Limitations
The finite-key analysis assumes a failure probability of 10^-10, and the chip's carrier-depletion-modulator bandwidth must be carefully estimated because it affects performance.
Abstract
from arXiv · showhide
Measurement-device-independent quantum key distribution (MDI-QKD) removes all detector side channels and enables secure QKD with an untrusted relay. It is suitable for building a star-type quantum access network, where the complicated and expensive measurement devices are placed in the central untrusted relay and each user requires only a low-cost transmitter, such as an integrated photonic chip. Here, we experimentally demonstrate a 1.25 GHz silicon photonic chip-based MDI-QKD system using polarization encoding. The photonic chip transmitters integrate the necessary encoding components for a standard QKD source. We implement random modulations of polarization states and decoy intensities, and demonstrate a finite-key secret rate of 31 bps over 36 dB channel loss (or 180 km standard fiber). This key rate is higher than state-of-the-art MDI-QKD experiments. The results show that silicon photonic chip-based MDI-QKD, benefiting from miniaturization, low-cost manufacture and compatibility with CMOS microelectronics, is a promising solution for future quantum secure networks.
source.
The work combines silicon photonic transmitter chips with MDI-QKD to build a compact, high-speed system in which users transmit encoded states while an untrusted relay performs detection. The experiment demonstrates random polarization and decoy-state modulation, stable operation, and secret-key generation over substantial channel loss.
- source.: 1.25 GHz polarization-encoding MDI-QKD is experimentally demonstrated with silicon photonic chip transmitters.The system uses two chip transmitters and an untrusted relay for Bell-state measurements.
- source.: The experiment reports higher key rates than previous MDI-QKD experiments and supports low-cost, scalable networks with untrusted relays.The authors describe the system as a step toward wafer-scale manufactured MDI-QKD systems and quantum networks.
- source.: Each transmitter integrates intensity modulation, polarization modulation, and variable optical attenuation for QKD state preparation.The integrated components support decoy intensities, four BB84 polarization states, and attenuation to single-photon levels.
- source.: The packaged chip provides stable polarization encoding and decoy-state modulation, while QBER remains low over several hours.The device has a 4.8 × 3 mm^2 chip footprint and is assembled in a compact packaged system designed for low-cost production.
Methods
Each user generates synchronized, phase-randomized optical pulses at 1.25 GHz using gain-switched lasers, enabling high-visibility two-photon interference.
- 1.25 GHz pulses are generated by individually driven master and slave gain-switched lasers.The lasers use 500-ps and 200-ps square-wave pulses, with a 1-ps electrical delay for temporal overlap.
- Seeding photons produce low-jitter, phase-randomized pulses approximately 100 ps wide.The generated pulses pass through a 10-GHz filter to reduce frequency chirp.
- The two-photon interference test attenuates each detector count rate to approximately 3.5 MHz and records data for 100 s.The coincidence time window is 600 ps.
- 48.4% visibility is obtained in the two-photon interference experiment.
Si transmitter chip
The silicon photonic transmitter integrates intensity and polarization modulation with variable attenuation, preparing BB84 states and randomly selected decoy intensities for MDI-QKD.
- The chip integrates an intensity modulator, polarization modulator, and variable optical attenuator.The intensity modulator uses a Mach-Zehnder interferometer, while the VOA attenuates pulses to single-photon levels.
- Four intensity choices, {µ, ν, ω, 0}, are randomly generated for decoy-state operation.The integrated intensity modulator has approximately 30 dB static and 20 dB dynamic extinction ratios; an external LiNbO3 modulator improves vacuum extinction.
- The polarization modulator prepares the four required states by controlling arm amplitudes and imposing phase shifts θ ∈ {0, π/2, π, 3π/2}.The polarization rotator combiner converts and recombines the two polarization components.
- The prepared states are characterized in Charlie using polarization controllers and polarization beam splitters aligned to rectilinear and diagonal bases.
- An average polarization extinction ratio of approximately 23 dB is achieved, sufficient for low-error MDI-QKD operation.
Detection
Charlie performs Bell-state measurements with synchronized timing and four superconducting nanowire detectors configured for high-rate operation.
- Charlie electrically distributes the synchronization clock with tunable 1-ps delays to compensate temporal drifts among Alice, Bob, and Charlie.
- A 50-ohm shunt resistor prevents detector latching at the system’s GHz repetition rate without sacrificing detection efficiency.The system requires SNSPDs to tolerate peak counting rates above 5 MHz.
- The high-speed time tagger supports a maximum data transfer rate of 65 MHz.
- A 600-ps coincidence window provides a trade-off between detection efficiency and X-basis error rate.
SUPPLEMENTARY MATERIALS
The experiment implements four-intensity decoy-state MDI-QKD with polarization-encoded bases, optimized intensity probabilities, and joint statistical constraints for finite-key extraction.
- Four-intensity decoy-state MDI-QKD uses three decoy intensities in the X basis and one signal intensity in the Z basis.Alice and Bob randomly prepare weak coherent pulses in the Z or X basis.
- The Z basis generates secret key, while the X basis supports decoy-state analysis.
- Alice and Bob use the same six optimized parameters [s, µ, ν, P_s, P_µ, P_ν] for intensities and probabilities.The setup performs a full optimization of these parameters.
- Joint constraints combine common observables and can produce a higher key rate than independent constraints.
- The secret key is extracted using a formula based on observed gains and QBER together with bounded single-photon yield and phase-error terms.The error-correction efficiency is set to 1.16.
- 48.4% visibility is measured in the Hong-Ou-Mandel interference experiment.The photon count rate is approximately 3.5 MHz per detector, with 100 s of data collection and a 600-ps coincidence window.
Si chip transmitter.
The silicon photonic transmitter integrates phase, intensity, and polarization modulation for high-speed QKD state and decoy preparation. Characterization shows GHz-rate operation, substantial attenuation, and well-defined polarization states.
- Intensity modulation: 19.8 dB dynamic extinction ratio is obtained at a 1.25-GHz repetition rate.The extinction ratio decreases from 21.5 dB at 625 MHz to 19.8 dB at 1.25 GHz.
- Intensity modulation: 1.25 GHz operation supports random intensity modulation for one signal state and three decoy states.Four RF voltage levels generate the intensities s, µ, ν, and ω.
- Intensity modulation: ∼110 dB maximum attenuation is achieved using three cascaded PIN-diode variable optical attenuators.A single attenuator provides 38.0 dB, while differential biasing across the cascade raises the maximum attenuation.
- Polarization modulation: Four polarization states in conjugate bases are prepared with ∼26 dB extinction ratio and ∼0.993 degree of polarization.The states are characterized using a polarimeter after applying appropriate RF signals to the polarization modulator.
- Polarization modulation: ∼23 dB average polarization extinction ratio is obtained after aligning the measurement device to rectilinear and diagonal bases.The alignment uses EPC adjustments and RF voltages between 0 and 7.5 V.
Polarization alignment.
The experiment requires Alice, Bob, and Charlie to share a common polarization reference because quantum-channel polarization drifts must be compensated. An automatic alignment procedure calibrates their references through sequential EPC adjustments.
- Motivation: A shared polarization reference is required among Alice, Bob, and Charlie for the experiment.The alignment procedure addresses polarization drift in the quantum channel.
- Alignment procedure: The automatic alignment method rapidly calibrates the polarization reference through three sequential adjustment steps.The procedure is represented by a dedicated alignment-system schematic and flowchart.
- Alignment procedure: Bob and Charlie first establish a common reference by adjusting EPC-1 and EPC-2.This is the first step of the flowchart-based procedure.
- Alignment procedure: Alice then aligns her bases by adjusting EPC-A.This follows the second step of the alignment flowchart.
- Alignment procedure: Charlie finally aligns one PBS to the Z-basis by adjusting EPC-2.This completes the three-step polarization alignment procedure.
Electronic control board.
The electronics board combines FPGA control, high-speed serial-to-parallel conversion, programmable delays, analog amplification, thermal control, and synchronization. These modules generate precisely timed drive signals for the chip-based QKD experiment.
- Board architecture: The FPGA board integrates memory, serial/parallel conversion, delay, thermoelectric-control, synchronization, and analog-output modules.All modules except analog output are implemented on a Xilinx Kintex-7 FPGA.
- High-speed control: 12.5 Gbps data transmission is achieved by converting low-speed serial data into high-speed parallel streams.The conversion supports simultaneous transmission across parallel channels.
- High-speed control: 1 ps delay resolution enables temporal overlap of laser pulses and accurate intensity and polarization modulation.The delay module independently adjusts the timing of each channel.
- Synchronization: Four delayed pulse sequences can be synchronized to internal or external clocks, allowing all experimental stations to operate synchronously.The synchronization module generates up to four precisely delayed output sequences.
- Laser synchronization: A 48.4% Hong-Ou-Mandel interference visibility is measured for the gain-switch lasers.The result is associated with the laser-seeding characterization used in the experiment.