Photons, the fundamental quanta of the electromagnetic field, travel at the fastest possible speed, and interact relatively weakly with the environment. These features make them ideal for several practical applications, particularly in the transport of both classical and quantum information. One could also wonder whether nature itself realized the usefulness of these unique entities and through evolution made living beings rely on them for some physiological functions. In my thesis, I cover an example for each of these roles a photon can take. As the first example, I describe our theoretical work pertaining to an important practical application where photons can carry quantum information and mediate entanglement between distant quantum computing nodes. We designed a novel quantum repeater architecture using superconducting processors and optical links, which we believe is the first concrete proposal towards this goal. We compared our repeater’s performance with a few other promising approaches and showed that ours could yield higher entanglement distribution rates with good fidelities in appropriate regimes. Such an architecture could be pertinent to envision a quantum internet in the future, something that would be analogous to, but much more secure and powerful in certain aspects than today’s classical internet. As the second example, I describe our theoretical work where we speculate on the potential role of photons observed in mammalian brains. Could we be using these photons as information carriers in addition to the well-known electrochemical signals? We show, based on detailed theoretical modelling, that myelinated axons could serve as good optical waveguides in the brain, which can answer this question in the affirmative. Ours is the first proposal identifying myelinated axons as potential optical waveguides. There is indirect experimental evidence to support this hypothesis, and we propose precise experiments to test the waveguide capabilities directly. Since these photons can also in principle carry quantum information, we further speculate on the existence of quantum networks in the brain. Our work could help develop a better understanding of some of the biggest unsolved problems in neuroscience, possibly including the generation of our subjective conscious experience.
Photons, the fundamental quanta of the electromagnetic field, travel at the fastest possible speed, and interact relatively weakly with the environment. These features make them ideal for several practical applications, particularly in the transport of both classical and quantum information. One could also wonder whether nature itself realized the usefulness of these unique entities and through evolution made living beings rely on them for some physiological functions. In my thesis, I cover an example for each of these roles a photon can take. As the first example, I describe our theoretical work pertaining to an important practical application where photons can carry quantum information and mediate entanglement between distant quantum computing nodes. We designed a novel quantum repeater architecture using superconducting processors and optical links, which we believe is the first concrete proposal towards this goal. We compared our repeater’s performance with a few other promising approaches and showed that ours could yield higher entanglement distribution rates with good fidelities in appropriate regimes. Such an architecture could be pertinent to envision a quantum internet in the future, something that would be analogous to, but much more secure and powerful in certain aspects than today’s classical internet. As the second example, I describe our theoretical work where we speculate on the potential role of photons observed in mammalian brains. Could we be using these photons as information carriers in addition to the well-known electrochemical signals? We show, based on detailed theoretical modelling, that myelinated axons could serve as good optical waveguides in the brain, which can answer this question in the affirmative. Ours is the first proposal identifying myelinated axons as potential optical waveguides. There is indirect experimental evidence to support this hypothesis, and we propose precise experiments to test the waveguide capabilities directly. Since these photons can also in principle carry quantum information, we further speculate on the existence of quantum networks in the brain. Our work could help develop a better understanding of some of the biggest unsolved problems in neuroscience, possibly including the generation of our subjective conscious experience.
Understanding the fundamental quantum and classical properties of photons is crucial in a wide range of applications. Photons exhibit quantum entanglement, the strongest form of correlation that has been observed between subsystems. Entanglement is perhaps the most valuable resource for many quantum information and quantum computation tasks, empowering them to be superior to their classical counterparts. Undoubtedly, understanding how entanglement can be generated, manipulated, and measured are among the most pre-eminent research questions in quantum information processing. However, the realm of influence of photons is not just limited to information technologies. Photons are, surprisingly, being produced constantly in biological systems as well. Modeling and characterizing the involvement of photons in various biological processes are active areas of research. In the first part of my Ph.D. program, I focused on the problem of quantifying multi-partite entanglement in a solid, an atomic frequency comb (AFC) crystal, using a single photon. We proposed a novel entanglement witness and showed that our witness is well suited for demonstrating and quantifying multi-partite entanglement in AFC systems. Our results are the first demonstration of multi-partite entanglement in a solid between over two hundred ensembles each containing a billion atoms. The second project of my Ph.D. was motivated by the question of why brain cells are producing biophotons. Photons are known as perfect carriers for information. The brain is the computation centre where all the learning and decision makings are happening. Learning, in theory, is an informational task, and its biological realization in the brain is not completely understood. We asked if photons could play a role in the process of learning in the brain. A common form of learning involves multiple cycles of training, evaluating, and receiving feedback. Both forward and backward parts of these cycles require an information flow. Existing theories mainly focus on neural electrochemical signals. We proposed that biophotons can propagate the feedback teaching signals required for learning in the brain. By multiple numerical experiments simulated on an artificial neural network, we demonstrated that stochastic emissions of biophotons carrying partial backward information enable learning in the network.
Quantum information technology has been developing rapidly in last two decades. Very recently, quite a few optical quantum key distribution networks have been reported, enabling preliminary practical applications in secure information transfer. Meanwhile, a number of significant progresses based on quantum network have also been made in the research of linear optical quantum computing, simulation and metrology. However, on the way towards large-scale optical quantum network serious problems occur. On the one hand, the distance of fiber-based quantum communication is limited, due to intrinsic fiber loss and decreasing of entanglement quality caused by the noisy environment. On the other hand, the probabilistic feature of single and entangled photon sources would also cause an exponentially increasing overhead for large-scale quantum network. To solve the above problems, quantum repeaters and/or transmission of optical quantum bits over free space channel can be efficiently exploited for future wide-area realization of quantum communication. In addition, memory built-in quantum repeaters would also enable scalable linear optical quantum computing, simulation and high precision measurement. In this talk I will present some recent experiments from our group, including eight-photon entanglement, topological quantum error-correction, quantum repeater and efficient and long-lived quantum memory, and entanglement distribution and quantum teleportation over 100km-scale free-space quantum channels. These experiments show the promising future possibility towards scalable quantum network with photons and atoms.
Quantum mechanics shows that if one is able to generate and manipulate entanglement over a distance, one is able to perform certain tasks which are impossible using only classical communication. Classical communication refers to what is used in the Internet of today. A quantum internet would therefore bring new capabilities to our highly connected world. These capabilities both involve (1) the ability to perform tasks with are provably impossible in the current Internet, such as unconditionally secure communication, and (2) the ability to perform certain tasks much more efficient, such as distributed (quantum) computing or extending the baseline of telescopes. To be able to build a quantum internet, two main components are needed: (i) hardware that can store, manipulate and entangle qubits and (ii) a software stack to control the hardware. The core task of both of these is to generate entanglement to be used by applications. In this thesis we focus on the latter, i.e. the development of software and protocols that enable entanglement generation using capable hardware. To enable a certain application, one can certainly, in theory, manually specify each operation the hardware should perform, involving micro-wave pulses, lasers etc. However, in practice this is not feasible, if not to say impossible, due to the complexity of the operations needed, especially in a distributed system such as a quantum network. What is needed is a software stack, which can help with abstracting complexity away in multiple layers. This allows for someone to program a protocol in one layer without knowing all the details of the lower layers. In particular, one can abstract away the hardware details, in order to make higher-layer protocols and applications hardware-agnostic. Therefore, to be able to build a universal, efficient and scalable quantum internet, a software stack is crucial. In chapter 2 we start discussing the networking part of a software stack. Namely, we introduce a network stack for a quantum internet, drawing parallels to the IP/TCP-suite of the classical Internet. We continue with proposing a service and interface of the lowest layer of the network stack: the link layer. The link layer is here responsible for generating entanglement between nodes in a quantum network which are directly connected by a quantum link, i.e. a fiber cable. When developing a protocol or application it is very useful to be able to run it. Both to see if the intended ideas make sense and also to check that the implementation is actually correct. Currently we do not have quantum hardware that exposes a full-fledge API that can be used to execute applications. For this reason, it is very useful to be able to instead simulate the hardware in a way that exposes the same API as the hardware being developed. In chapter 3 we introduce SimulaQron for this exact purpose. Any application of a quantum internet will need entanglement in one way or another. However, entanglement is generally hard to generate and is usually the bottleneck when executing an application. We would therefore like to make use of the generated entanglement in the most optimal way. To be able to do this we need to understand how entanglement can be transformed and distributed in a quantum network. We study the entanglement of a particular class of states called graph states in chapters 4 to 9 and how these states can be transformed in a quantum network.
Quantum networking enables the distribution of quantum information across distant nodes, unlocking capabilities such as secure communication, distributed quantum computing, enhanced metrology, and tests of fundamental physics. Photons serve as the natural carriers of quantum information over long distances due to their low loss in optical fibers and robustness to environmental decoherence. However, the practical realization of quantum networks remains limited by the lack of photon sources that are simultaneously compatible with diverse quantum systems, stable over long timescales, and suitable for deployment in real-world fiber infrastructure. This dissertation addresses these challenges through the co-development of photon-pair sources and a field-deployed quantum networking testbed. We present two complementary chip-scale sources of correlated photon pairs tailored for distinct bottlenecks in quantum networking. The first is a hybrid visible–telecom photon-pair source designed to address a fundamental spectral mismatch between quantum nodes and communication channels. While most quantum memories and processors operate at visible to near- nfrared wavelengths, optical fibers exhibit minimal loss in the telecommunications band. By integrating periodically poled nonlinear waveguides with photonic integrated circuits, we demonstrate a platform capable of directly generating visible–telecom photon pairs, enabling efficient interfacing between heterogeneous quantum systems and existing fiber networks. The second source is a fully packaged, broadband telecom photon-pair source based on silicon waveguide spirals, designed for stable, long-duration operation. This stability enables extended measurements necessary to overcome channel losses and environmental drifts in deployed fiber networks. These sources are evaluated within RoQNET (Rochester Quantum Network), a ∼20 km fiber link between the Rochester Institute of Technology and the University of Rochester. Using this platform, we demonstrate the transmission and on-chip modulation of heralded telecom single photons, verify their nonclassical statistics, and characterize long-term channel behavior. Additionally, we explore programmable time-bin entanglement distribution using spectral waveshapers, providing a flexible approach to quantum communication. Together, this work establishes a co-designed source–network framework for scalable, heterogeneous quantum networking.
Quantum networking, heralded as the next frontier in communication networks, envisions a realm where quantum computers and devices collaborate to unlock capabilities beyond what is possible with the Internet. A critical component for realizing a long-distance quantum network, and ultimately, the Quantum Internet, is the quantum repeater. As with the race to build a scalable quantum computer with different technologies, various schemes exist for building quantum repeaters. This article offers a gentle introduction to the two-way “all-photonic quantum repeaters,” a recent addition to quantum repeater technologies. In contrast to conventional approaches, these repeaters eliminate the need for quantum memories, offering the dual benefits of higher repetition rates and intrinsic tolerance to both quantum operational errors and photon losses. Using visualization and simple rules for manipulating graph states, we describe how all-photonic quantum repeaters work. We discuss the problem of the increased volume of classical communication required by this scheme, which places a huge processing requirement on the end nodes. We address this problem by presenting a solution that decreases the amount of classical communication by three orders of magnitude. We conclude by highlighting other key open challenges in translating the theoretical all-photonic framework into real-world implementation, providing insights into the practical considerations and future research directions of all-photonic quantum repeater technology.
In today’s digital era, the need for communication systems that are both faster and more secure continues to grow, pushing innovation toward the development of the Quantum Internet. This paper investigates how advancements in optical switching and networking technologies are shaping this evolution by combining classical optical communication with the principles of quantum mechanics. It begins by reviewing how optical fiber systems have advanced to support high speed, low loss data transmission, forming the backbone for quantum communication infrastructure. The study explains the working principles of qubits, entanglement, and Quantum Key Distribution (QKD), which allow the transfer of data with unparalleled security through photon based signaling. Furthermore, it explores modern optical switching methods such as all optical, electro optic, and MEMS based switches, emphasizing their role in managing quantum signals efficiently while reducing energy consumption and transmission latency. The paper also highlights key network components like quantum routers, repeaters, and photonic integrated circuits that enable seamless interaction between classical and quantum networks. In addition, it reviews emerging technologies such as AI-assisted routing, low noise photon detectors, and nanophotonic materials that enhance the scalability and performance of quantum systems. The research identifies persistent challenges, including photon loss, decoherence, fabrication cost, and standardization issues, while outlining strategies to overcome them through advanced design and material innovation. Overall, this work provides a comprehensive understanding of how optical switching advancements are paving the way for a reliable, large scale, and globally connected Quantum Internet capable of transforming future communication and information security.