Experimental fiber-optic quantum communication using hyper-entanglement

Abstract

Quantum mechanics, which was developed in the quarter 20th century, has revolutionized the way of recognizing our real worlds on a fundamental level. Significantly, the onset of the new physical notions made our understanding of the scientific phenomena of “light” more complete in terms of photon quanta, thus resulting in the invention of light technologies such as laser systems, MRI images, solar cells, and more. Recently, the innovative attempts to utilize quantum properties, e.g., an entanglement or uncertainty principle, have led to an emerging field of quantum information science and could have combined the photonic quantum effects with the study of Information science in communication. Thus, the quantum-enhanced field of quantum communication (QC) has appeared, which could further enhance performance features such as capacity, sensitivity, speed, and, especially, “security” for numerous end uses.

Given the guaranteed advantage, significant progress has been achieved in QKD development, reaching the level of maturity required for deployment in real-world scenarios especially on fiber-optic network. Significantly, It has already been demonstrated to operate with conventional fiber-optic technologies on the same telecommunications fiber and over large distances through fiber channel. The transmission distance has exceeded hundreds of kilometers of optical fiber in recent years, and the secure bit rate achievable has reached megabits per second, making QKD applicable for metro-city networks. In this thesis, related aspects of fiber-based QKD technologies are in-depth investigated from the practical perspective of the view. There still remains a critical unsolved gap toward constructing the global fiber quantum network, and many unsolved problems to address. Figuring them out, innovative strategies to address the unsolved issues in fiber-optic channels are proposed and conducted experimentally. The quantum resources required for the fiber-based application in QC systems are studied in-depth. We restrict ourselves to its most promising application, QKD, both point to point and in futuristic networks, and the majority of discussions are intended for its deployment in the most widely-used fiber-optic network. In the following, we represent how the main contents are organized through the thesis.

In Chapter 1, the brief history and key concepts of quantum communications are reviewed for background information, including its most-famous QKD. In addition, the quantum resources for the fiber-based application are also represented for the readers to easily understand throughout the thesis. It is noteworthy that all the experiments and discussions in the thesis entirely depend on entanglement-based quantum communication. Therefore, photon resources like spontaneous four-wave mixing (SFWM), and spontaneous parametric down-conversion (SPDC) sources are critically important and are separately reviewed in Chapter 2.

In Chapter 3, we experimentally demonstrate a well-designed photon source which is built up on a micro/nanofiber. One of the biggest hurdles to adopting the QKD protocols into a well-established classical network lies in insufficiently developed photon resources; quantum communications have required robust, versatile, and bright photon sources. Over the past few decades, the spontaneous parametric down-conversion (SPDC)-based (2) non-linear crystals have been used for quantum optics experiments due to their advantages. However, the SPDC scheme operates primarily in the free space environment, therefore exhibiting a non-compatible nature in fiber-based systems, e.g., high coupling loss. Here, we address these issues by effectively designing a fiber-based entangled photon source generated by the spontaneous four-wave mixing process in a micro/nanofiber (MNF). The performances as a photon source are theoretically and experimentally investigated.

In a quantum network involving multiple communicating parties, an important goal is to establish high-quality pairwise entanglement among the users without introducing multiple entangled-photon sources which would necessarily complicate the overall network setup. Moreover, it is preferable that the pairwise entanglement of photons is in the time-bin degree of freedom as the photonic time-bin qubit is ideally suited for fiber-optic distribution. Ahead of proving a field-deployable quantum communication network involving multiple users, an experimental distribution of high-quality entangled qubits over long-distance fiber channels, especially using the time- bin modes due to its outstanding robustness, is proposed and conducted in Chapter 4, which proves the significance of our study in long-range quantum communication. Then, we report an experimental demonstration of a field-deployable quantum communication network involving multiple users in Chapter 5, all of whom share pairwise entanglement in the time-bin degree of freedom of photons. In particular, by utilizing a single spontaneous-parametric down-conversion (SPDC) source which produces a broadband pair of photons and the wavelength-division demultiplexing/multiplexing technology, all the communicating parties within the network are always simultaneously ready for quantum communication.

In the last chapter 6, we present a way of conducting a noise-resistant quantum communication through noisy quantum channels by effectively employing hyper-entanglement of photons. Quantum information protocols are being deployed in increasingly practical scenarios, via optical fibers or free space, alongside classical communication channels. However, entanglement, the most critical resource to deploy to the communicating parties, is also the most fragile to the noise-induced degradations. In particular, we demonstrate that our hyperentanglement-based scheme results in orders of magnitude increase in the signal-to-noise ratio for distribution of polarization-entangled qubit pairs, enabling quantum communication even in the presence of strong noise that would otherwise preclude quantum operations due to noise-induced entanglement sudden death.