光子對光源的製備與單光子在原子量子記憶體下的量子儲存

Development of photon-pair source and quantum storage of heralded single photons in the atomic quantum memories

蔡秉儒(Pin-Ju Tsai)

指導教授 ： 林俊達

共同指導教授 ： 陳應誠(Ying-Cheng Chen)

國立臺灣大學/理學院/物理學研究所/博士(2019年)

https://doi.org/10.6342/NTU201901825

摘要

光子對光源在長距離量子通訊中扮演重要角色，一方面是由於光子利用光速進行資訊傳遞；另外，光子對中兩個光子之間強烈的量子關聯也讓光子對光源具備能夠利用這種性質將量子資訊分發到不同的位置同時保有絕對的安全性。然而，在長距離的量子通訊中，由於用於導引光子的光纖存在著有限的耗損，這使得量子通訊距離受到限制。為了解決這個問題，量子中繼站的概念被提出。通過量子中繼站的協定，在光子所攜帶的量子資訊以及量子糾纏變得微弱或是模糊之前，中繼站將量子糾纏轉移到另一組光子對使得量子資訊之傳遞距離得以延伸，通過數個中繼站所建構的網路，量子通訊的距離可以被有效的提升並實現長距離的量子通訊。在量子中繼站中，一個重要的過程是將抵達中繼站的光子暫存至量子記憶體中，用於同步另一組預備執行量子糾纏轉換的光子。正因如此，藉由量子記憶體去執行對單光子的量子儲存會是實現長距離量子通訊的重要里程碑。
一種常見的量子儲存方法是基於冷原子的平台下所建立的電磁波誘發透明機制，電磁波誘發透明機制藉由一道強的控制光場將待儲存的光場量子態轉換到原子的基態相干性中並儲存在原子介質中，在經過一段儲存時間後，重新開啟控制光後，光場的量子態可以重新從原子系統被重建回來。不過由於此量子記憶體是基於一個原子系統所建立的，這使得待儲存的光源之線寬以及頻率被強烈限制。
另一方面，非線性晶體中自發性參量下轉換是一種常見且實用的方法用於製備非古典光源如光子對的非線性過程。在這個過程中，一個高頻率的光子被轉換為一對時間－頻率糾纏的光子對。另外，其中製造的閒置光子也同時能夠用於通知光子對中另一個信號光子的產生，因此這個光子源也被稱為預示單光子源。然而，自發性參量下轉換所產生的光子對之頻寬通常遠大於原子線寬的等級，也因此大大的降低的光與原子的交互作用進而增加對單光子儲存的困難度。
在這個工作中，我們克服了上述所提到的困難並利用原子量子記憶體實現了對固態晶體的光子源之量子儲存。這篇博士論文中主要被分為兩個主題，首先，為了克服自發性參數下轉換的高線寬問題，我們採用共振腔增強形式自發性參數下轉換以建構一個窄線寬﹑單模且非簡併的光子對光源。為了維持系統穩定，我們發展了一套分時多工的鎖定機制將系統維持在最佳狀態，並同時將產生的光子對之頻率鎖定在原子躍遷中。由於鎖定機制的穩定性，這使得光子源的產生率被大大的提升，同時也讓此光源成為一個非常適合運用在原子系統的量子光源。在這光子源中，我們分別得到了7.24x10^5 和 6142 s^-1 mW^-1 的光子對產生率及計數率，所製備的光子對之關聯時間為21.6(2.2)奈秒，對應的頻寬為2πx6.6(6) MHz。根據以上的數據，我們估計光子對光源的光譜亮度為1.06x10^5 s^-1 mW^-1 MHz^-1，這個數值對於一個單模運作的光子對光源而言是一個相對高的數值。
完成了光子對的製備後，在實驗的下一個階段中，我們進一步將產生的非古典光源送往原子系統量子記憶體。在這個實驗中，我們利用基於電磁波誘發透明機制的量子記憶體實現了對共振腔增強型式光子源所產生的光子對的量子儲存以及操控。首先，為了確保光子源與原子系統之間的相容性，我們測試了一系列的慢光實驗並估計其量子保真度，結果顯示理論與實驗的良好吻合。在驗證了光子源與量子記憶體的相容性後，我們進一步執行了對預示單光子的量子儲存與操控，根據不同的儲存與操控條件，光子對的時間關聯性或是波形能夠被量子記憶體控制。這個操控過程不只是能夠操控光子對的古典性質，如頻寬與群速度。另一方面，操控過程也同時能夠提升讀取光子的非古典關聯和量子保真度。在量子記憶體系統中，我們得到了大約40%的儲存效率以及g^(2)s,i=5.87的非古典關聯。另外，藉由操控過程，我們可以將非古典關聯進一步被提升至g^(2)s,i=7.5，同時量子保真度也能被有效的提升到該條件的最大上限。
由於我們所製備的光子源是基於共振腔型式的固態晶體以及其單模運作的特性，這些性質使得系統的複雜性被大大的簡化。此外，我們也演示了利用原子量子記憶體儲存與操控光子對特性，這些結果一方面顯示了光子源的良好性能，另一方面也實現了原子系統與固態光源系統的連結。這個工作為原子記憶體系統提供了一個緊緻的非古典光源解決方案，此光源的架設也能簡易的被擴展到長距離以及大尺度的量子通訊系統，我們相信這些工作將對量子通訊領域有所幫助，尤其是量子中繼站的實現。

關鍵字

量子通訊 ； 量子記憶體 ； 單光子光源 ； 光子對光源 ； 量子儲存 ； 電磁波誘發透明 ； 自發性參量下轉換

並列摘要

Photon-pair source plays an important role in long-distance quantum communication. On the one hand, photons propagate at the speed of light to transfer the information. On the other hand, the strong quantum correlation between the two photons in a photon pair also allows one to distribute quantum information to different nodes while maintaining absolute security. However, in long-distance quantum communication, the communication distance is limited by the loss in the optical fiber for guiding photons. To solve this problem, the concept of the quantum repeater was proposed. In a quantum repeater, the quantum entanglement transfers to another group of photon pairs through entanglement swapping in each repeater station to extend the entanglement distribution distance, before it becomes weak and noisy. By repeating such a process in each two nearby nodes, the entanglement can be extended to the full distance and thus realized the long-distance quantum communication. In the quantum repeater protocol, an important process is to temporarily store the flying photons into the quantum memories at the repeater station, in order to synchronize another set of photons that are ready to perform the entanglement swapping. For this reason, the realization of quantum storage of single photons is an important milestone in achieving long-distance quantum communication.
One quantum storage method is to utilize the electromagnetically-induced-transparency(EIT) mechanism established in the platform of cold atoms. In the EIT mechanism, the quantum state of light is converted into the ground-state coherence of atoms and stored in the atomic medium through adiabatic ramping of the control field. After a period of storage, the control light is switched on, the quantum state of the light can be reconstructed back from the atomic system. However, since quantum memories are built on the basis of an atomic system, the linewidth and frequency of the single photons prepare be stored are strongly limited.
The spontaneous parametric down-conversion (SPDC) in nonlinear crystals provide a common and practical method for preparing nonclassical light such as photon pairs. In this process, a high-frequency photon is converted into a pair of time-frequency entangled photon pairs, usually called idler and signal. The idler photons are used to inform the generation of another signal photon in the photon pair. Therefore, this type of photon source is also called the heralded single-photon source. However, the bandwidth of photon pairs produced by SPDC is typically much larger than the level of atomic linewidth, and thus greatly reduces the interaction strength between light and atoms, thereby increasing the difficulty of storing the single photons.
In this work, we overcome the difficulties mentioned above and demonstrate the quantum storage of single photons from SPDC in the atomic memories. This thesis is mainly divided into two parts. First, in order to conquer the high bandwidth of SPDC, we use cavity-enhanced SPDC to construct a narrow-band, single-mode, and nondegenerate photon source. To maintain system stability, we have developed a time-division multiplexing locking scheme to maintain the double resonant condition for cavity and simultaneously lock the frequency of the generated photon pairs at atomic transition. Thanks for the stability of the locking system, the photon source generation rate is greatly improved, and that also makes the quantum light source become a very suitable source for use in an atomic system. In this photon source, we get the photon pair generation rate and the count rate of 7.24x10^5 and 6142 s^-1 mW^-1, respectively. The correlation time of the photon pairs are 21.6 (2.2) ns, and the corresponding bandwidth is 2πx6.6(6) MHz. Based on the above data, we estimate the spectral brightness of the photon source to be 1.06x10^5 s^-1 mW^-1 MHz^-1, which is a relatively high value for the photon-pair source in single-mode operation.
After completing the preparation of the photon pair, we further send the generated non-classical light to the atomic quantum memories in the next phase of the experiment. In this experiment, we use quantum memories based on EIT to achieve quantum storage and manipulation of photon pairs generated by the cavity-enhanced SPDC. First, in order to ensure the compatibility between the photon source and the atomic system, we tested a series of slow light experiments and estimated their quantum fidelity. The results show a good agreement between the theory and the experiment. After that, we further perform the quantum storage and manipulation of the heralded single photon. According to different storage and manipulation conditions, the temporal correlation or waveform of the photon pairs can be controlled by the quantum memories. This manipulation process not only allows us to manipulate the classical properties of photon pairs, such as bandwidth and group velocity but also enhances the non-classical correlation and quantum fidelity of reading photons. We achieve quantum storage with a storage efficiency of about 40% and the non-classical correlation of g^(2)s,i=5.87. In addition, though the manipulation process, we can further increase the non-classical correlation to g^(2)s,i=7.5, and the quantum fidelity can be effectively raised to the maximum limit of the condition.
Our photon-pair source is very compact and its output is the single-mode operation which allows direct application without the complication of adding external filters. In addition, we demonstrate the source can be deployed for the atomic quantum memory to store a manipulate photon pair properties. Our scheme provides a compact non-classical light source solution for atomic memories systems. The setup of the photon source can also be easily extended to long-distance and large-scale quantum-communication systems. We believe that these efforts will be helpful in the field of quantum communication, especially the implementation of the quantum repeater.

並列關鍵字

quantum communication ； quantum memories ； heralded single-photon source ； photon-pair source ； quantum storage ； electromagnetically-induced-transparency ； spontaneous parametric down-conversion

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