量子糾纏對於探索基礎量子物理和實現量子技術至為重要，我們的研究目的是：區別“以量子糾纏為媒介的真正量子資訊處理過程”與“基於古典物理學的任何模仿”，藉以發揮量子技術中真正量子優勢與其價值，確保量子資訊之安全。基於此目標，我們同時透過實驗和理論方法研究以下三個主題：“實驗糾纏光子與多光子糾纏的產生與識別”、“實驗光子量子糾纏於量子計算與通訊中的應用”，以及“量子資訊處理過程之識別理論”。主要研究成果概述如下：
Quantum entanglement is crucial to exploring essential quantum physics and realizing quantum technology. The purpose of our research is to distinguish between “real quantum information processing using quantum entanglement as a medium” and “any imitation based on classical physics.” This distinction will help us unleash the actual quantum advantages and value of quantum technology and ensure the security of quantum information. For this purpose, we study the following three projects experimentally and theoretically: “Generation and identification of experimental entangled photons and multi-photon entanglement,” “Application of experimental quantum entanglement in quantum computation and quantum communication,” and “Theory of identifying quantum information processing processes.” The selected research results are summarized below:
實驗糾纏光子與多光子糾纏的產生與識別 Generation and identification of experimental entangled photons and multi-photon entanglement
“光子融合”是一種將聯合光子探測事件的不同、但無法區分的雙光子振幅疊加的非古典過程。這種雙光子干涉已被廣泛用於產生多光子糾纏，從長距離到晶片尺度、從測試量子物理基礎到處理光子量子資訊等，都可以見到它不可或缺之角色。雖然重要，但沒有實驗證據顯示整個光子融合的能力可以像量子實體一樣完全被量化。在這裡，我們展示了實驗光子融合的首次能力量化（圖中i與ii為兩個光子融合單元）；我們的定量描述忠實地測量了實驗中光子融合產生和保存糾纏光子對的能力。此外，透過實驗上產生四光子和六光子糾纏態，我們表明量子過程能力量化，提供了對用以產生真正的多光子糾纏態之多光子干涉的精確評估。這些結果證明了一種實用的診斷方法，可以對通用量子光子網路和設備中基本操作的光子融合進行基準測試。
Photon fusion is a nonclassical process of superposing two-photon amplitudes of different yet indistinguishable alternatives of a joint photodetection event. This two-photon interference has been extensively utilized in creating multiphoton entanglement, from long-distance to chip-scale implementations, from testing the foundation of quantum physics to processing photonic quantum information. While significant, no experimental evidence exists that the capability of entire photon fusion can be utterly quantified like a quantum entity. Herein, we demonstrate the first capability quantification of experimental photon fusion. (As depicted, i and ii are two units of photon fusion.) Our quantitative characterization faithfully measures the abilities of photon fusion in the experiment to create and preserve entangled photon pairs. Moreover, with the created four- and six-photon entangled states, we show that capability quantification provides a precise assessment of interferometry for generating genuine multiphoton entanglement. These results reveal a practical diagnostic method to benchmark photon fusion underlying the primitive operations in general quantum photonics networks and devices.
利用量子糾纏節點所建構之量子網路（a圖）比古典網路之相關性更強，可用於嶄新之量子聯網之相關應用。然而，實際的量子網路存在噪聲，最糟糕的情況是，導致量子節點可以被預先所存在的古典數據描述（b-f圖）。在這種不受信任的網路中，確定量子網路的保真度和真正的多節點糾纏成為至關重要的問題。在這裡，我們證明，在不可信星形網路中確定量子網路保真度和真正的 N 節點糾纏僅需要 N+1 測量。我們的網路確定方法是透過偵測真正的 N 節點 Einstein-Podolsky-Rosen 量子可操縱性來實現。在實驗上，我們使用自發參數下轉換糾纏光子源（下圖），展示了真正的 3 光子和 4 光子量子網絡的確定，以及目前被廣泛使用的糾纏見證所出現的糾纏誤判，即 1/2 的保真度標準。我們的結果提供了一種可擴展的方法來確定現實量子網路中的多節點糾纏。
Quantum networks of entangled end nodes (Fig. a) serve stronger than the classical correlation for unparalleled quantum internet applications. However, practical quantum networking exists noise, at worst, causing end nodes to be described in pre-existing classical data (Figs. b-f). In such untrusted networks, determining quantum network fidelity and genuine multi-node entanglement becomes crucial problems. Here, we show that deciding quantum network fidelity and genuine N-node entanglement in an untrusted star network requires only N+1 measurement settings. Our network determination method is enabled by detecting genuine N-node Einstein-Podolsky-Rosen steerability. Experimentally, using spontaneous parametric down-conversion entanglement sources (the bottom figure), we demonstrate the determinations of genuine 3-photon and 4-photon quantum networks and the false positives of the widely used entanglement witness, the fidelity criterion of 1/2. Our results provide a scalable method for determining multipartite entanglement in realistic quantum networks.
實驗光子量子糾纏於量子計算與通訊中的應用 Application of experimental quantum entanglement in quantum computation and quantum communication
量子同調特性是量子力學預測的非古典特徵之一。作為量子技術的資源，從量子計算到量子通訊，從晶片級到遠距離實現，量子同調的傳輸過程在各種情況下都是必要的。然而，如何使用最小的實驗成本明確驗證從同調準備，到後續同調傳輸的成功傳輸過程仍有待探究。在這裡，我們從理論上和實驗證明，只需一種同調產生操作，和一種測量設定，即可見證相干性傳輸過程（圖a）。我們使用多光子糾纏和干涉測量技術證明了四光子和六光子糾纏網絡中相干傳輸過程的有效見證（圖b，c）。上述空間場景中的見證，可以應用於見證量子動力學在時間上的相干性創建和保存。所引入的概念和方法，可用於明確有效驗證相關量子工程中所需之同調量子操縱過程。
Quantum coherence is one of the nonclassical signatures predicted by quantum mechanics. As a resource for quantum technologies, coherence transmission process is necessary in various circumstances, from quantum computation to quantum communication, from chip-scale to large-distance implementations. However, how to unambiguously verify successful transmission processes from coherence preparation to subsequent coherence transfer using minimum experimental costs remains to be discovered. Here, we theoretically and experimentally show that a witness to coherence transmission processes can be performed with only one coherence creation operation and one measurement setting (Fig. a). We demonstrate such efficient witnesses to coherence transmission processes in four-photon and six-photon entangled networks using multiphoton entanglement and interferometry (Figs. b,c). The above witness in spatial scenarios can be applied to witness the coherence creation and preservation of quantum dynamics temporally. The introduced concept and method can be used to unambiguously verify the primitives of coherence manipulation processes in related quantum engineering efficiently.
量子資訊發送方可以使用預先共享的糾纏對、僅發送方的單量子位元測量以及發送方通知的接收方的簡單校正，來為遠端接收方準備量子態。與量子隱形傳態相比，它為量子資訊提供了資源高效的優勢。在這裡，我們提出了一種最有效的方法來檢測遠端狀態準備（RSP），該方法基於量子相干共享對的靜態資源和 RSP 參與者輸入的動態資源所提供的量子效益。它只需要接收器最少的一次額外同調產生操作來驗證RSP。在實驗上，我們使用高品質偏振薩尼亞克干涉儀產生的不同光子對狀態來實現引入的RSP評估，確認了靜態和動態量子相干資源所發揮的必要和充分的作用（如圖所示），並展示了有效的RSP驗證。我們的結果提供了一種在量子網路中的量子資訊等實際場景中有效評RSP的途徑。
A sender can prepare a quantum state for a remote receiver using preshared entangled pairs, only the sender’s single-qubit measurement, and the receiver’s simple correction informed by the sender. It provides resource-efficient advantages over quantum teleportation for quantum information. Here, we propose the most efficient approach to detect the remote state preparation (RSP) based on the quantum benefits powered by quantum coherence’s static resources of the shared pairs and the dynamic resources both the RSP participants input. It requires only the receiver’s minimum of one additional coherence creation operation to verify RSP. Experimentally, we implement the introduced RSP assessment using different photon pair states generated from a high-quality polarization Sagnac interferometer, confirming the necessary and sufficient role played by the static and dynamic quantum coherence resources (as shown in the figure) and demonstrating efficient RSP verification. Our results provide a route to efficiently assess RSP in practical scenarios such as quantum information in quantum networks.
以光子作為媒介的量子網路通常由量子通道、中繼器和終端節點組成。遠程狀態準備 (RSP) 使一個端節點能夠遠程準備其他端節點的狀態。 RSP 還充當網路通訊的確定性單光子源。在此研究中，我們從理論上和實驗上研究了這種網路 RSP 如何超越任何沒有糾纏和量子位元操作的古典模擬過程。我們提出了一種新型的量子資源，稱為 RSP 能力，以驗證真正的量子網路中非古典狀態準備和傳輸所需的所有靜態和動態元素，例如量子通道和中繼器，因此該量子資源超越量子關聯之靜態資源。我們透過實驗證明了偏振薩尼亞克干涉儀產生的光子對的 RSP 能力測量，包括根據光子對品質在古典和非古典 RSP 之間的轉變。我們的結果有助於透過確認網路 RSP 以發揮以其為基本運作單元的量子聯網之量子優勢。
Photon-mediated quantum networks generally consist of quantum channels, repeaters, and end nodes. Remote state preparation (RSP) enables one of the end nodes to prepare the states of the other end nodes remotely. RSP also serves as a deterministic single-photon source for networking communications. Herein, we theoretically and experimentally investigate how such networking RSP surpasses any classical emulation without entanglement and qubit unitaries. We introduce a new type of quantum resource, which we refer to as RSP capability, to validate all the static and dynamic elements required for nonclassical state preparation and transmission in a genuine quantum network, such as quantum channels and repeaters. This goes beyond the static resources of quantum correlations. We experimentally demonstrate the RSP capability measurement of the photon pairs created by a polarization Sagnac interferometer, including the transition between classical and nonclassical RSP depending on the photon-pair qualities. Our results help reveal the quantum advantages arising when networking RSP plays a role.
量子資訊處理過程之識別理論 Theory of identifying quantum information processing processes
與設備無關的量子密鑰分發（DIQKD）是一種密鑰分發方案，其安全性基於量子物理定律，但不需要對協定中使用的設備進行任何假設。現有的基於糾纏的DIQKD協議的安全性依賴於貝爾測試。在這裡，我們提出了一種高效的設備無關量子密鑰分發（EDIQKD）協議，其中一個參與者準備狀態並通過量子通道將其傳輸給另一個參與者進行測量。在這個準備和測量協議中，參與者之間的傳輸過程根據安全過程斷層掃描進行確認，排除使用古典初始狀態、傳輸狀態和最終狀態的任何模仿。比較確保安全免受集體攻擊的所需密鑰產生最小回合數，對於允許量子誤碼率高達6.5%的可靠密鑰，EDIQKD協定的效率比DIQKD協定高兩個數量級（如圖所示）。這項優勢將使參與者能夠大幅節省糾纏對所需的資源和測量。根據最近最先進的光子實驗中的最高檢測效率，我們的協議可以以非零密鑰率實現，並保持比通常的 DIQKD 更有效率。我們的協議及其安全分析可以為識別與設備無關的場景中典型的準備和測量量子資訊任務提供有用的見解。
Device-independent quantum key distribution (DIQKD) is a key distribution scheme whose security is based on the laws of quantum physics but does not require any assumptions about the devices used in the protocol. The security of the existing entanglement-based DIQKD protocol relies on the Bell test. Here, we propose an efficient device-independent quantum key distribution (EDIQKD) protocol in which one participant prepares states and transmits them through a quantum channel to another participant to measure. In this prepare-and-measure protocol, the transmission process between participants is characterized according to the process tomography for security, ruling out any mimicry using the classical initial, transmission, and final state. Comparing the minimum number of rounds to guarantee security against collective attacks, the efficiency of the EDIQKD protocol is two orders of magnitude more than that of the DIQKD protocol for the reliable key of which quantum bit error rate is allowed up to 6.5%, as shown in the figure. This advantage will enable participants to substantially conserve the entangled pair’s demanded resources and the measurement. According to the highest detection efficiency in the recent most advanced photonic experiment, our protocol can be realized with a non-zero key rate and remains more efficient than usual DIQKD. Our protocol and its security analysis may offer helpful insight into identifying the typical prepare-and-measure quantum information tasks with the device-independent scenario.
我們提出了一種實驗上可實施之方法，藉由簡單地準備分離狀態作為測試輸入，然後對相應輸出的單個量子位進行簡單的局域測量，就可識別實驗過程中量子關聯性（包括EPR操控性和貝爾非定域性）的產生能力。這一發現可用於通用量子計算中，雙量子位元控制邏輯閘客觀之基準測試。
A method is introduced for identifying the creation of quantum correlations, including EPR steering and Bell nonlocality, for experimental processes, by simply preparing separable states as test inputs and then performing local measurements on single qubits of the corresponding outputs. This finding enables the construction of objective benchmarks for the two-qubit controlled operations used to perform universal quantum computation.