A century after Einstein's revolutionary suggestion that light is composed of particles, the quantum information revolution seeks to use the almost magical properties of non-classical physics to enable new feats in information processing. The critical quantum resource is entanglement, which can now be produced at high rates with exquisite precision, enabling such feats as quantum cryptography and teleportation. I will describe some of these micracles, and our investigations into how the usual benefits can be further extended, by using more complex quantum states (e.g., hyper-entanglement), and by incorporating other elements of modern physics (e.g., special relativity). Time and appetites permitting, a brief lesson in quantum cooking may be forthcoming.
A century after Einstein's revolutionary suggestion that light is composed of particles, the quantum information revolution seeks to use the almost magical properties of non-classical physics to enable new feats in information processing. The critical quantum resource is entanglement, which can now be produced at high rates with exquisite precision, enabling such feats as quantum cryptography and teleportation. I will describe some of these micracles, and our investigations into how the usual benefits can be further extended, by using more complex quantum states (e.g., hyper-entanglement), and by incorporating other elements of modern physics (e.g., special relativity). Time and appetites permitting, a brief lesson in quantum cooking may be forthcoming.
For a long time the conceptually challenging predictions of quantum physics for individual experiments have been the issue of such bizarre discussions as Schrodinger's cat gedanken experiment. There, quantum superposition would imply the co-existence of the states of a live cat and a dead cat, quantum randomness would imply the fundamental unpredictability of the results of an observation of the cat's real and factual properties, and entanglement means that the state of the cat, dead or alive, is entangled with the state of an radioactive atom. Interestingly these same concepts, randomness, superposition, and entanglement, have become cornerstone concepts in a new quantum information technology. The most important applications are quantum cryptography, quantum computation, and quantum teleportation. Quantum cryptography permits the encoding of confidential information in a way that its security against eavesdropping is guaranteed by the laws of physics. A quantum computer would be able to operate in a quantum superposition of many different states thus enabling exponential speedup for some problems. Quantum teleportation allows transferring directly the quantum state of an individual system onto a distant one thus providing an ideal way for quantum computers to communicate with each other.
Information and computation theory have undergone a spurt of new growth, and a renewal of their historic connection to basic physics, as they have expanded to treat the intact transmission and processing of quantum states, and the interaction of such 'quantum information' with traditional forms of information. We may wonder why this did not happen earlier, as quantum principles have long been accepted as fundamental to all of physics. Perhaps the founders of information and computation theory, such as Shannon, Turing and von Neumann, were too accustomed to thinking of information processing in macroscopic terms, not yet having before them the powerful examples of the genetic code and ever-shrinking microelectronics. Be that as it may, information until recently has largely been thought of in classical terms, with quantum mechanics playing a supporting role in the design of the equipment to process it, and setting limits on the rate at which it could be sent through certain channels. Now we know that a fully quantum theory of information and information processing offers, among other benefits, a brand of cryptography whose security rests on fundamental physics, and a reasonable hope of constructing quan- tum computers that could dramatically speed up the solution of certain mathematical problems. These benefits depend on distinc- tively quantum properties such as uncertainty, interference and entanglement. At a more fundamental level, it has become clear that an information theory based on quantum principles extends and completes classical information theory, just as complex numbers extend and complete the reals. Besides quantum generalizations of classical notions such as sources, channels and codes, the new theory includes two complementary, quantifiable kinds of information— classical information and quantum entanglement. Classical infor- mation can be copied at will, but can only be transmitted forward in time, to a receiver in the sender's forward light cone. Entanglement in contrast, cannot be copied, but can connect any two points in space-time. Conventional data processing operations destroy entanglement, but quantum operations can create it and use it for various purposes, such as speeding up certain classical computa- tions and assisting in the transmission of classical information or intact quantum states. Part of the new quantum information theory is the qualitative and quantitative study of entanglement, and its interactions with classical information. Any means, such as an optical fibre, of delivering quantum systems more or less intact from one place to another, may be viewed as a quantum channel. Unlike classical channels, which are well characterized by a single capacity, quantum channels have several distinct capacities, depending on what one is trying to use them for, and what auxiliary resources are brought into play. New effects involving quantum information continue to be discovered, not only in the traditional areas of computation, channel capacity, and cryptography, but in areas such as commu- nication complexity and game theory.
In recent years, due to advances in technology that allow for precise manipulation of quantum states, we can control superposition and entanglement of quantum states for an increasing number of states. These advances sparked an explosive growth of new technologies that led to an effective implementation of quantum encryption protocols and the development of quantum computers. This rapid development, sometimes referred to as “the second quantum revolution,” was made possible by the development of precise laser manipulation, increased precision in solid-state experiments, and the promise of novel computational tools that could solve problems currently beyond the reach of supercomputers. We start this chapter by introducing the fundamental unit of quantum information, the quantum bit or qubit. We then discuss entanglement and the conceptual conundrum it raised, the EPR paradox, and Bell's theorem. Afterward, we delve into modern applications of these fundamental ideas, such as quantum teleportation, quantum cryptography, and elements of quantum computation.
Over the last sixty years, classical information theory has revolutionized the understanding of the nature of information, and how it can be quantified and manipulated. Quantum information processing extends these lessons to quantum systems, where the properties of intrinsic uncertainty and entanglement fundamentally defy classical explanation. This growing field has many potential applications, including computing, cryptography, communication, and metrology. As inherently mobile quantum particles, photons are likely to play an important role in any mature large-scale quantum information processing system. However, the available methods for producing and detecting complex multi-photon states place practical limits on the feasibility of sophisticated optical quantum information processing experiments. In a typical quantum information protocol, a source first produces an interesting or useful quantum state (or set of states), perhaps involving superposition or entanglement. Then, some manipulations are performed on this state, perhaps involving quantum logic gates which further manipulate or entangle the intial state. Finally, the state must be detected, obtaining some desired measurement result, e.g., for secure communication or computationally efficient factoring. The work presented here concerns the first and last stages of this process as they relate to photons: sources and detectors. Our work on sources is based on the need for optimized non-classical states of light delivered at high rates, particularly of single photons in a pure quantum state. We seek to better understand the properties of spontaneous parameteric downconversion (SPDC) sources of photon pairs, and in doing so, produce such an optimized source. We report an SPDC source which produces pure heralded single photons with little or no spectral filtering, allowing a significant rate enhancement. Our work on detectors is based on the need to reliably measure single-photon states. We have focused on optimizing the detection efficiency of visible light photon counters (VLPCs), a single-photon detection technology that is also capable of resolving photon number states. We report a record-breaking quantum efficiency of 91 +/- 3% observed with our detection system. Both sources and detectors are independently interesting physical systems worthy of study, but together they promise to enable entire new classes and applications of information based on quantum mechanics.
Quantum information has dramatically changed information science and technology, looking at the quantum nature of the information carrier as a resource for building new information protocols, designing radically new communication and computation algorithms, and ultra-sensitive measurements in metrology, with a wealth of applications. From a fundamental perspective, this new discipline has led us to regard quantum theory itself as a special theory of information, and has opened routes for exploring solutions to the tension with general relativity, based, for example, on the holographic principle, on non-causal variations of the theory, or else on the powerful algorithm of the quantum cellular automaton, which has revealed new routes for exploring quantum fields theory, both as a new microscopic mechanism on the fundamental side, and as a tool for efficient physical quantum simulations for practical purposes. In this golden age of foundations, an astonishing number of new ideas, frameworks, and results, spawned by the quantum information theory experience, have revolutionized the way we think about the subject, with a new research community emerging worldwide, including scientists from computer science and mathematics.
A lot of people have been interested in quantum entanglement recently, especially because of the possible uses it could have in contemporary encryption. It's the phenomena wherein quantum particles stay connected no matter how far apart they are. the principles and applications of quantum entanglement in creating safe protocols for data transmission. The most visible use of quantum entanglement is quantum key distribution (QKD), a quantum mechanically based approach to secure information exchange. Concepts of QKD, with an emphasis on important protocols like BB84 and E91 that use entanglement to generate cryptographically sound keys. Our investigation also delves into quantum teleportation and how it could be incorporated into forthcoming encryption protocols. We also go into the pros and cons of using quantum entanglement in practical cryptographic contexts, including topics like quantum channel security, noise mitigation, and the necessity of quantum repeaters. Through a thorough examination, this work seeks to highlight the revolutionary power of quantum entanglement in protecting digital communication from the impending danger of classical computing power and future quantum assaults. A new age of private and secure communication may be on the horizon as a result of cryptographic systems that incorporate quantum entanglement.