O波段矽光子馬赫任德與微環元件實現脈衝位元差分相移量子密鑰城域傳輸

Abstract

為順應矽光子時代的來臨，應用於需要大頻寬的資料傳輸以及高穩定的訊號調變的量子加密通訊系統中的元件逐漸由傳統的塊材轉向整合多功能的積體化矽晶片。其中，光強度調變器的調變響應直接反映光量子位元的訊號雜訊比，並且顯著地影響量子誤碼率以及資訊安全性。

第二章探討差分相移鍵控量子金鑰分發(DPSK-QKD)的實現受限於單光子源與光強度調變器(IM)的調變性能，這些元件的穩定性及其操作條件的選擇是一大挑戰。在本研究中，以高穩定性的溫度控制器與低雜訊的驅動電流源來穩定分佈式回饋雷射二極體(DFBLD)的輸出功率及波長。此外，採用被動絕熱封裝系統使DFBLD和延遲線干涉儀(DLI)的功率擾動(dP/P)由±0.015%降低至±0.0025%以及由±6.25%降低至±0.1%，此封裝方式有效地抑制了回授驅動電路的阻尼振盪與熱對流。用一千兆赫(1-GHz)的歸零開閉鍵控(RZ-OOK)調製訊號來分析用於將連續波(CW)脈衝化的鈮酸鋰(LiNbO3, LNO)和矽光子(SiPh)強度調變器的傳遞函數的最佳操作條件。以30%佔空比的電調變訊號操作LNO-IM進行脈衝調變時，解調成功率相對較高；相較之下，對於SiPh-IM而言，潛在的非線性調變效應需要以10%佔空比的電調變訊號在具有較低半波電壓(Vπ)的高偏壓區域內操作以達到最佳的消光比(ER)。在平均光子數為每脈衝含有0.25顆的條件下，透過以LNO-IM和SiPh-IM調變的光脈衝來進行DPSK-QKD，其量子誤碼率(QBER)、篩選密鑰率(RSifted)和安全密鑰率(RSecure)在背靠背(Back-to-Back, BtB)傳輸中分別為(1.91%, 28.75 kbit/s, 5.78 kbit/s)以及(1.67%, 33.19 kbit/s, 7.72 kbit/s)，並且在平均光子數衰減至每脈衝含有0.016顆的狀況下逐漸升高至(3.32%, 1.79 kbit/s, 0.06 kbit/s)以及(2.96%, 1.86 kbit/s, 0.14 kbit/s)。

第三章描述近年來隨著高密度光子積體電路(PIC)中光互連的突破，對緊湊的設備空間、低工作偏壓和寬頻寬的需求推動了從馬赫任德調變器(MZM)到微環調變器(MRM)的轉變。與實驗室實驗不同，基於時序的DPSK-QKD在現場實驗中不可避免地會受到各種自然現象的影響。在本研究中，採用DFBLD作為單光子源來攜帶量子位元(Qubits)，其具有約100 kHz的窄時域光譜拓展線寬和相對較低的瞬態響應D因子(D-Factor) 1.21 GHz/√mA。在接收端採用絕熱及防震式封裝的光纖式解調器可有效抑制因溫度擾動和機械振動等環境因素所造成的隨機相位波動。光損耗因光纖固有的吸收和散射效應而隨傳播距離呈指數增加，嚴重限制了長距離傳輸並加劇相干性的退化。以單光子偵測器進行光子偵測來分析經模擬修改的光子出現機率公式，得出了當準單光子干涉可見度具有最佳值95.86%時，對應的平均光子數與QBER為每脈衝含有0.25顆以及3.17%。採用自製的微環脈衝調變器作為量子位元產生器，在與中華電信(CHT)合作部署的13公里商用城域光纖鏈路上實現DPSK-QKD。透過具有編碼/解碼、低密度奇偶檢查碼(LDPC)和明文加密/解密的圖形化使用者介面(GUI)程式的輔助下，得出QBER以及篩選/安全密鑰率(Si/SeKRs)分別為3.51%, 24.22 kbit/s以及0.39 kbit/s。

With the silicon photonics era advancing, the components employed for quantum crypto-communication systems demanding wide-band data throughput and high modulation stability have gradually evolved from conventional bulk materials to multifunctional and integrated silicon photonic chips. The modulation response of the optical intensity modulator directly reflects the signal-to-noise ratio of the optical Qubits and significantly influences the QBER and information security.

In chapter 2, the realization of differential phase shift keying quantum key distribution (DPSK-QKD) is constrained by the performance of the single-photon source and the optical intensity modulator (IM). Notably, the stability of these components and operating conditions presents considerable challenges. The steady output of the distributed feedback laser diode (DFBLD) is managed by a high-stability temperature controller and a low-noise driving current source. Additionally, the power perturbations (dP/P) of the DFBLD and delayed line interferometer (DLI) are mitigated from ±0.015% to ±0.0025% and from ±6.25% to ±0.1% with the passively adiabatic packaging system, effectively reducing the damping oscillations induced by feedback driving circuits and thermal convection. By analyzing the transfer functions of the lithium niobate (LNO) and silicon photonics (SiPh) IMs, the optimal operating conditions for pulsating the continuous wave (CW) are investigated by applying 1-GHz return-to-zero on-off-keying (RZ-OOK) modulation signals. For the LNO-IM, the success rate of demodulation is comparably elevated when the electrical modulating signal functions at a 30% duty cycle. Conversely, for the SiPh-IM, the potential non-linear modulation effects necessitate establishing the operating point within the higher bias region with a 10% duty cycle modulating signal, characterized by a lower half-wave voltage (Vπ) for attaining an optimal extinction ratio (ER). The DPSK-QKD is executed with an initial mean photon number (μ) of 0.25 #/pulse. The quantum bit error rate (QBER), the sifted key rate (RSifted) and the secure key rate (RSecure) with the optical pulse streams modulated by the LNO-IM and the SiPh-IM result in (1.91%, 28.75 kbit/s, 5.78 kbit/s) and (1.67%, 33.19 kbit/s, 7.72 kbit/s) in back-to-back (BtB) transmission, gradually escalate into (3.32%, 1.79 kbit/s, 0.06 kbit/s) and (2.96%, 1.86 kbit/s, 0.14 kbit/s) with the mean photon number being attenuated to 0.016 #/pulse.

In chapter 3, the transition from Mach-Zehnder modulators (MZM) to micro-ring modulators (MRM) has been driven by the requirements in recent years for compact device dimensions, reduced operating bias, and broad bandwidth, with the breakthrough of photonic interconnections in high-density photonic integrated circuits (PICs). Unlike laboratory experiments, the time-bin-based DPSK-QKD is inevitably impacted by various naturally-occurring phenomena in the field experiments. In this work, a DFBLD with a reduced temporal spectral broadening linewidth of ~100 kHz and a relatively low transient response D-factor of 1.21 GHz/√mA is employed as a single-photon source to carry the quantum bits (Qubits). With thermal- and vibration-insulated packaging for a fiberized demodulator, the random phase fluctuations caused by environmental factors such as temperature perturbations and mechanical vibrations can be effectively mitigated at the receiving end. The photon loss, exponentially increasing with propagation distance owing to inherent absorption and scattering in optical fiber, imposes a severe restriction and aggravates the coherence degradation over long distances. After simulating the modified photon emergence probability formula and performing photon detection analysis by single-photon detectors, the optimal quasi-single-photon interferometric visibility of 95.86% and a QBER of 3.17% are yielded for a mean photon number of 0.25 #/pulse. A homemade micro-ring pulsator is utilized as a quantum bits generator for realizing the DPSK-QKD over a 13-km commercially available metropolitan fiber link deployed by Chunghwa Telecom. Assisted by a graphical user interface (GUI) program incorporating the encoding/decoding, the low-density parity-check (LDPC) error correction code, and the text encryption/decryption, the QBER and the sifted/secure key rates (Si/SeKRs) are obtained to be 3.51%, 24.22 kbit/s and 0.39 kbit/s, respectively.