Quantum state preparation of time-bin encoding based on SOI integrated chips

The article focuses on the development of a quantum key distribution (QKD) chip based on silicon-on-insulator (SOI) technology that employs a time-bin encoding scheme with decoy states. This system aims to enhance the performance of QKD by using integrated modulators to achieve high-speed and high-fidelity quantum state encoding.

Key Findings:

1. Chip Design and Integration:

• The QKD chip is designed using a combination of Thermo-optic Phase Modulators (TOPMs) and Carrier-Depletion Modulators (CDMs), allowing for high-speed modulation and precise quantum state preparation. This hybrid design ensures flexibility in adjusting quantum states' relative phase.

• The chip supports the encoding and decoding of four BB84 quantum states at a repetition rate of 100 MHz, a crucial advancement for QKD systems aiming for high-speed, secure communication.

2. Modulator Performance:

• Extinction ratio measurements were conducted for both types of modulators. The TOPM exhibited a high extinction ratio of 26.55 dB, while the CDM had a slightly higher extinction ratio of 32.01 dB, suitable for high-speed modulation.

• The integration of both modulators within the chip ensures stable and efficient quantum state switching, addressing issues like the saturation in phase response seen in CDMs alone.

3. Time-bin and Phase-state Quantum Preparation:

• Time-bin encoding uses temporal pulses, and the system demonstrated high visibility of quantum states, with the phase states |+i and |−i achieving visibility of 93.66% and 92.36%, respectively, confirming high-contrast interference.

• The system also tested the time-bin states |0i and |1i, achieving 19.33 dB and 18.72 dB extinction ratios, respectively.

4. Stability Testing:

• The stability of the quantum states was rigorously tested over a 2-hour period, showing consistent extinction ratios and phase stability for both time- and phase-based states. However, temperature fluctuations in the SOI material impacted phase stability, with a high thermo-optic coefficient of SOI being identified as a potential source of instability.

• The proposed solution to this issue involves integrating Si3N4 for the delay line, as it has a much lower thermo-optic coefficient than SOI, significantly reducing temperature-induced phase shifts.

5. Quantum Key Distribution:

• The test system used a picosecond pulse laser and erbium-doped fiber amplifier to prepare quantum states at Alice's side, which were then transmitted to Bob's side for decoding. The quantum states were decoded using a passive decoding scheme, with the output measured using InGaAs/InP-based single-photon detectors.

• The QKD system demonstrated efficient key distribution over a 2-meter single-mode fiber link, with minimized signal loss, ensuring the integrity of the quantum states during transmission.

Conclusion:

This work demonstrates a successful integration of SOI photonic technology for practical QKD systems. By addressing key challenges such as phase stability and high-speed modulation, the chip design offers significant improvements over previous systems. The introduction of Si3N4 waveguides in the design helps mitigate the effects of temperature fluctuations, enhancing long-term stability. The results from this study contribute to the practical implementation of secure, high-speed quantum key distribution, which could support the future development of quantum communication networks.