| 摘要: | 量子通訊是量子科技的重要分支之一。在建構大規模量子網路時，需要具備
傳輸、儲存與編碼能力的量子資訊載體，其中光子因其高速傳輸能力與低環境敏
感性，成為最具潛力的載體選擇。常見的光子編碼方式包括偏振及時間-頻率自
由度，相較於已發展成熟的偏振編碼，時間-頻率自由度因具備高維度編碼潛力，
近年來逐漸受到重視。
本研究透過分析光子的頻譜與量子干涉效應，深入探討時間-頻率自由度的資
訊特性。我們使用中心波長 784 nm、脈衝頻率 79.8 MHz 的雷射，經由非線性晶
體 MCLN 實現 Type-II 自發參量下轉換 (Spontaneous parametric down-donversion,
SPDC），產生具正交偏振的糾纏光子對。因單光子頻譜極窄，傳統頻譜儀難以
解析其頻率分布，因此採用飛行時間光譜儀（Time-of-flight-spectrometer, TOFs），
以雷射脈衝作為時間基準，量測光子對通過色散介質後的飛行時間差，進而重建
其聯合頻譜強度（Joint spectral intensity, JSI）。
過往研究指出，洪-歐-孟德爾 (Hong-Ou-Mandel, HOM）干涉源自兩個不可區
分的光子同時離開分光鏡同一端的傾向。本研究藉調控光子對的時間延遲與晶體
溫度，成功觀測到即使光子對的時間差明顯超過單光子波包寬度，頻域中仍呈現
清晰且連續的干涉條紋，展現出頻率自由度下的高干涉可見度。本研究透過自然
色散提高頻譜解析能力，進一步揭示即便光子在某一自由度（如時間）可區分，
仍可能於另一自由度（如頻率）呈現明顯的干涉現象，補足了對 HOM 干涉中
「不可區分性」的理解，並為未來高維頻率編碼的量子通訊系統提供實驗依據。
;Quantum communication is one of the key branches of quantum technology. In constructing large-scale quantum networks, it is essential to have quantum information carriers capable of transmission, storage, and encoding. Among the candidates, photons stand
out as the most promising carriers due to their high-speed transmission capabilities and
low sensitivity to environmental disturbances. Common encoding schemes for photons
include polarization and time-frequency degrees of freedom. Compared to polarization
encoding, which is already well-developed, time-frequency encoding has recently attracted increasing attention for its potential in high-dimensional quantum information
encoding.
This study investigates the informational characteristics of the time-frequency degree
of freedom by analyzing photon spectra and quantum interference effects. A pulsed laser
with a central wavelength of 784 nm and a repetition rate of 79.8 MHz is used, and Type-II
Spontaneous Parametric Down-Conversion (SPDC) is performed in a nonlinear MCLN
crystal to generate entangled photon pairs with orthogonal polarizations. Since singlephoton spectra are extremely narrow and cannot be effectively resolved using traditional
spectrometers, we employ a Time-of-Flight Spectrometer (TOFs). Using the laser pulse
rate as a time reference, we measure the arrival time difference of photon pairs after
passing through a dispersive medium, and reconstruct the Joint Spectral Intensity (JSI).
Previous studies have shown that Hong-Ou-Mandel (HOM) interference arises when
two indistinguishable photons tend to exit the same output port of a beam splitter simultaneously. In this work, by tuning the temporal delay of the photon pairs and the crystal
temperature, we successfully observe clear and continuous interference fringes in the
frequency domain—even when the temporal delay between the photon pairs exceeds the
coherence time of the individual photons. This demonstrates high interference visibility in the frequency domain. Through the use of natural dispersion to enhance spectral
resolution, our study further reveals that even when photons are distinguishable in one
degree of freedom (e.g., time), they may still exhibit interference in another (e.g., frequency). These results enrich our understanding of photon indistinguishability in HOM
interference and provide experimental evidence supporting future development of highdimensional frequency-encoded quantum communication systems. |
