Since 2000, several landmark results in quantum information have catapulted the field to one of the most exciting in science. From the demonstration of Shor’s factorisation algorithm that threatens the long-term security of public-key cryptography, to the teleportation of super-position states, the demonstration of error-correcting codes and the development of topologically protected qubits, our ability to control the quantum world is ever increasing. Matched by elegant experiments to test the fundamentals of quantum physics, such as the recent demonstration of the loop-hole free violation of Bell's inequality, 1 , 2 these exciting achievements are all emerging just at the time when information technology is beginning to enter the quantum regime.
Since 2000, several landmark results in quantum information have catapulted the field to one of the most exciting in science. From the demonstration of Shor’s factorisation algorithm that threatens the long-term security of public-key cryptography, to the teleportation of super-position states, the demonstration of error-correcting codes and the development of topologically protected qubits, our ability to control the quantum world is ever increasing. Matched by elegant experiments to test the fundamentals of quantum physics, such as the recent demonstration of the loop-hole free violation of Bell's inequality, 1 , 2 these exciting achievements are all emerging just at the time when information technology is beginning to enter the quantum regime.
Quantum physics is well known for being counter-intuitive, or even bizarre. Quantum correlations have no equivalence in classical physics. This was all well known for years. But several discoveries in the 1990's changed the world. First, in 1991 Artur Ekert, from Oxford University, discovered that quantum correlations could be used to distribute cryptographic keys. Suddenly physicists realized that quantum correlations and its associated bizarre non-locality, could be exploited to achieve a useful task that would be impossible without quantum physics. What a revolution! And this was not the end. Three years later, Peter Shor, from the AT&T Laboratories, discovered an algorithm that breaks the most used public key crypto-systems. Shor's algorithm requires a quantum computer, yet another bizarre quantum device, a kind of computer that heavily exploits the quantum superposition principle. The following year, in 1995, a collaboration between six physicists and computer scientists from three continents led to the discovery of quantum teleportation, a process with a science-fiction flavour. These discoveries and others led to the emergence of a new science, marrying quantum physics and theoretical computer science, called quantum information science. Today, a steadily growing community of physicists, mathematicians and computer scientists develop the tools of this new science. This leads to new experiments and new insights into physics and information theory. It is a fascinating time. Quantum information is still a very young science. In particular, the technology required to build a quantum computer is still unknown. Only quantum key distribution has reached a certain level of maturity, with a few start-ups already offering complete systems. Nevertheless, quantum information has been widely recognised as a source for truly innovative ideas and disruptive technologies.
Abstract Quantum computing represents a fundamentally new computational paradigm based on quantum-mechanical phenomena such as superposition, entanglement, and quantum tunneling. These effects enable certain computational tasks to be performed more efficiently than on classical digital computers. In theory, quantum computing promises significant reductions in execution time and potentially improved energy efficiency for specific classes of problems. The slowing of traditional hardware scaling trends — often associated with Moore’s Law — combined with advances in nanoscale control technologies, has led to substantial commercial and national-security investments in quantum technologies throughout the 2010s. A landmark moment occurred when Google announced the demonstration of so-called quantum supremacy using its superconducting quantum processor Sycamore. The experiment showed that a quantum device could perform a specific computational task beyond the practical reach of classical supercomputers. While the task itself had no immediate practical application, the demonstration marked a critical milestone in the evolution of quantum computing and provided a concrete benchmark for future developments. Quantum Computing and Information Security The rapid progress of quantum technologies introduces significant implications for information security. Many modern cryptographic systems rely on the computational difficulty of mathematical problems such as: Integer factorization Discrete logarithms Elliptic curve problems Quantum algorithms — most notably Shor’s algorithm — threaten to efficiently solve these problems once sufficiently large, fault-tolerant quantum computers become available. This creates a long-term risk to widely deployed public-key cryptosystems, including RSA and elliptic curve cryptography. Through practical experimentation on the IBM Q platform, it becomes possible to: Implement and test fundamental quantum algorithms. Simulate cryptographic primitives under quantum attack models. Evaluate the scalability and limitations of current hardware. Explore post-quantum mitigation strategies. Although current quantum devices remain in the NISQ (Noisy Intermediate-Scale Quantum) era and are not yet capable of breaking real-world cryptographic systems, the trajectory of development suggests that cryptographic transition planning must begin well before large-scale quantum computers emerge. Strategic Implications The quantum threat to information security is not immediate but is strategically significant: Encrypted data intercepted today may be decrypted in the future ("harvest now, decrypt later"). Critical infrastructure and national security systems require long-term protection horizons. Migration to quantum-safe cryptographic standards must occur proactively. Practical experience using IBM Q demonstrates both the power and current limitations of quantum systems, providing valuable insight into how theoretical quantum threats translate into real-world technological development.
The quantum physics of light is a most fascinating field. Here I present a very personal viewpoint, focusing on my own path to quantum entanglement and then on to applications. I have been fascinated by quantum physics ever since I heard about it for the first time in school. The theory struck me immediately for two reasons: (1) its immense mathematical beauty, and (2) the unparalleled precision to which its predictions have been verified again and again. Particularly fascinating for me were the predictions of quantum mechanics for individual particles, individual quantum systems. Surprisingly, the experimental realization of many of these fundamental phenomena has led to novel ideas for applications. Starting from my early experiments with neutrons, I later became interested in quantum entanglement, initially focusing on multi-particle entanglement like GHZ states. This work opened the experimental possibility to do quantum teleportation and quantum hyper-dense coding. The latter became the first entanglement-based quantum experiment breaking a classical limitation. One of the most fascinating phenomena is entanglement swapping, the teleportation of an entangled state. This phenomenon is fundamentally interesting because it can entangle two pairs of particles which do not share any common past. Surprisingly, it also became an important ingredient in a number of applications, including quantum repeaters which will connect future quantum computers with each other. Another application is entanglement-based quantum cryptography where I present some recent long-distance experiments. Entanglement swapping has also been applied in very recent so-called loophole-free tests of Bell's theorem. Within the physics community such loophole-free experiments are perceived as providing nearly definitive proof that local realism is untenable. While, out of principle, local realism can never be excluded entirely, the 2015 achievements narrow down the remaining possibilities for local realistic explanations of the quantum phenomenon of entanglement in a significant way. These experiments may go down in the history books of science. Future experiments will address particularly the freedom-of-choice loophole using cosmic sources of randomness. Such experiments confirm that unconditionally secure quantum cryptography is possible, since quantum cryptography based on Bell's theorem can provide unconditional security. The fact that the experiments were loophole-free proves that an eavesdropper cannot avoid detection in an experiment that correctly follows the protocol. I finally discuss some recent experiments with single- and entangled-photon states in higher dimensions. Such experiments realized quantum entanglement between two photons, each with quantum numbers beyond 10 000 and also simultaneous entanglement of two photons where each carries more than 100 dimensions. Thus they offer the possibility of quantum communication with more than one bit or qubit per photon. The paper concludes discussing Einstein's contributions and viewpoints of quantum mechanics. Even if some of his positions are not supported by recent experiments, he has to be given credit for the fact that his analysis of fundamental issues gave rise to developments which led to a new information technology. Finally, I reflect on some of the lessons learned by the fact that nature cannot be local, that objective randomness exists and about the emergence of a classical world. It is suggestive that information plays a fundamental role also in the foundations of quantum physics.
The subject of quantum computing brings together ideas from classical information theory, computer science, and quantum physics. This review aims to summarize not just quantum computing, but the whole subject of quantum information theory. Information can be identified as the most general thing which must propagate from a cause to an effect. It therefore has a fundamentally important role in the science of physics. However, the mathematical treatment of information, especially information processing, is quite recent, dating from the mid-20th century. This has meant that the full significance of information as a basic concept in physics is only now being discovered. This is especially true in quantum mechanics. The theory of quantum information and computing puts this significance on a firm footing, and has led to some profound and exciting new insights into the natural world. Among these are the use of quantum states to permit the secure transmission of classical information (quantum cryptography), the use of quantum entanglement to permit reliable transmission of quantum states (teleportation), the possibility of preserving quantum coherence in the presence of irreversible noise processes (quantum error correction), and the use of controlled quantum evolution for efficient computation (quantum computation). The common theme of all these insights is the use of quantum entanglement as a computational resource. It turns out that information theory and quantum mechanics fit together very well. In order to explain their relationship, this review begins with an introduction to classical information theory and computer science, including Shannon's theorem, error correcting codes, Turing machines and computational complexity. The principles of quantum mechanics are then outlined, and the Einstein, Podolsky and Rosen (EPR) experiment described. The EPR-Bell correlations, and quantum entanglement in general, form the essential new ingredient which distinguishes quantum from classical information theory and, arguably, quantum from classical physics. Basic quantum information ideas are next outlined, including qubits and data compression, quantum gates, the `no cloning' property and teleportation. Quantum cryptography is briefly sketched. The universal quantum computer (QC) is described, based on the Church-Turing principle and a network model of computation. Algorithms for such a computer are discussed, especially those for finding the period of a function, and searching a random list. Such algorithms prove that a QC of sufficiently precise construction is not only fundamentally different from any computer which can only manipulate classical information, but can compute a small class of functions with greater efficiency. This implies that some important computational tasks are impossible for any device apart from a QC. To build a universal QC is well beyond the abilities of current technology. However, the principles of quantum information physics can be tested on smaller devices. The current experimental situation is reviewed, with emphasis on the linear ion trap, high- Q optical cavities, and nuclear magnetic resonance methods. These allow coherent control in a Hilbert space of eight dimensions (three qubits) and should be extendable up to a thousand or more dimensions (10 qubits). Among other things, these systems will allow the feasibility of quantum computing to be assessed. In fact such experiments are so difficult that it seemed likely until recently that a practically useful QC (requiring, say, 1000 qubits) was actually ruled out by considerations of experimental imprecision and the unavoidable coupling between any system and its environment. However, a further fundamental part of quantum information physics provides a solution to this impasse. This is quantum error correction (QEC). An introduction to QEC is provided. The evolution of the QC is restricted to a carefully chosen subspace of its Hilbert space. Errors are almost certain to cause a departure from this subspace. QEC provides a means to detect and undo such departures without upsetting the quantum computation. This achieves the apparently impossible, since the computation preserves quantum coherence even though during its course all the qubits in the computer will have relaxed spontaneously many times. The review concludes with an outline of the main features of quantum information physics and avenues for future research.