In contrast to today's computers, quantum computers and information technologies may in future be able to store and transmit information not only in the state "0" or "1," but also in superpositions of the two; information will then be stored and transmitted in entangled quantum states. Zeilinger discusses recent advances toward using this principle for quantum cryptography and highlights studies into the entanglement (or controlled superposition) of several photons, atoms, or ions.
In contrast to today's computers, quantum computers and information technologies may in future be able to store and transmit information not only in the state "0" or "1," but also in superpositions of the two; information will then be stored and transmitted in entangled quantum states. Zeilinger discusses recent advances toward using this principle for quantum cryptography and highlights studies into the entanglement (or controlled superposition) of several photons, atoms, or ions.
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.
The aim of this chapter is not to focus one more time on quantum cryptography, which is extensively discussed in a collection of books and review papers, but rather to provide the reader with recent advances and challenges that concern experimental, entanglement-based, guided-wave quantum communication. In our everyday world, almost all the information exchanged, stored, and processed is encoded using elementary entities called bits, conventionally represented by the discrete values 0 or 1. In today's fiber-based telecommunications systems, these classical bits are carried by light pulses, corresponding to macroscopic packets of photons, allowing a classical description of their behavior and propagation. To draw a simple picture, each light pulse consists of at least hundreds, of photons to encode the bit value 1, or of no photons to encode the bit value 0.
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.
Quantum entanglement, one of the most intriguing phenomena in quantum mechanics, forms the cornerstone of emerging technologies in quantum information science. It describes the non-classical correlations between spatially separated particles, whereby the measurement outcome of one particle instantaneously determines the state of the other, regardless of the distance between them. This paper explores the fundamental principles of quantum entanglement and its critical role in advancing quantum cryptography and quantum computing. In cryptography, entanglement enables protocols such as Ekert’s E91 and BBM92 quantum key distribution schemes, which leverage nonlocal correlations to ensure unconditional security based on the violation of Bell’s inequalities. In quantum computing, entanglement serves as a powerful computational resource, facilitating speedups in algorithms, enabling quantum teleportation, and supporting robust quantum error correction techniques. The discussion integrates both theoretical underpinnings and experimental realizations, highlighting recent advances in photonic entanglement sources, superconducting qubits, and trapped-ion systems. Challenges related to decoherence, scalability, and entanglement distribution in quantum networks are examined, along with potential solutions. By bridging foundational theory with practical applications, this study underscores the pivotal role of entanglement in shaping the future of secure communications and computational paradigms beyond the capabilities of classical systems.
Today's computers work on bits that exist as either 0 or 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits, which can exist in superposition. Qubits represent atoms, ions, photons or electrons and their respective control devices that are working together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers. A processor that can use registers of qubits will be able to perform calculations using all the possible values of the input registers simultaneously. This superposition causes a phenomenon called quantum parallelism, and is the motivating force behind the research being carried out in quantum computing. Due to technical obstacles, till date, a quantum computer has not yet been realized. But the concepts and ideas of quantum computing has been demonstrated using various methods like NMR, Ion Trap, Quantum Dot, Optical Methods, etc. A quantum computer manipulates qubits by executing a series of quantum gates, each a unitary transformation acting on a single qubit or pair of qubits. In applying these gates in succession, a quantum computer can perform a complicated unitary transformation to a set of qubits in some initial state. The qubits can then be measured, with this measurement serving as the final computational result. Research must devise a way to maintain decoherence and other potential sources of error at an acceptable level. Probably the most important idea in this field is the application of error correction in phase coherence as a means to extract information and reduce error in a quantum system without actually measuring that system. Thereby, quantum computers will emerge as the superior computational devices and perhaps one day make today's modern computer obsolete.
Quantum computers are a revolutionary technology based on the principles of quantum mechanics. Unlike conventional computers that work with binary bits (0 or 1), quantum devices use qubits that can exist in multiple states (“superpositions”) and interact with each other through the phenomenon of “quantum entanglement.” Thanks to this, quanta can greatly accelerate the solution of a number of important problems that were previously considered insurmountable for classical machines. This review describes the principles of operation of quantum computers, considers their unique advantages, and demonstrates the potential for their impact on the future of computer science and the technology industry.