Eugene Wigner pointed out very interesting consequences of quantum physics in elegant gedanken experiments. As a result of technical progress, these gedanken experiments have become real experiments and contribute to the development of novel concepts in quantum information science, often called the second quantum revolution.
When Eugene Wigner conceived his Gedanken experiment in 1961 [1], he argued that in quantum theory two observers, Wigner and his friend, can experience two fundamentally different descriptions of reality. Yet, only six decades later, this question has been rigorously tackled independently by Brukner [2] and Frauchiger and Renner [3], leveraging on the Bell's theorem and on the Hardy's Paradox respectively. They exploit a new extended Wigner's friend scenario, whereby two superobservers (Alice and Bob) and two observers (Alice and Bob's friends) are now entangled, and record their own facts shown to be subjective to the observer who established them.
The second quantum revolution has been built on a foundation of fundamental research at the intersection of physics and information science, giving rise to the discipline we now call quantum information science (QIS).The quest for new knowledge and understanding drove the development of new experimental tools and rigorous theory, which defined the road map for second-wave quantum technologies, including quantum computers, quantum-enhanced sensors, and communication systems.As technology has matured, the race to develop and commercialize near-term applications has accelerated.In the current regime of Noisy Intermediate Scale Quantum (NISQ) devices [J.Preskill, Quantum 2, 79 ( 2018)], the continued necessity of basic research is manifest.Under what conditions can we truly harness quantum complexity and what are its implications for potential useful applications?These questions remain largely unanswered and as the QIS industry ramps up, a continuous feedback between basic science and technology is essential.In this Perspective, I review how curiosity-driven research has led to radical new technologies and why the quest for basic understanding is essential for further progress.
Quantum mechanics, which emerged in the 20th century, has achieved remarkable feats, giving rise to the first quantum revolution. Toward the end of the 20th century, the emergence of quantum information theory sparked the second quantum revolution. Unlike the first one, the second quantum revolution directly focuses on the development of quantum artifacts that are governed by the laws of quantum mechanics. This book delves deeply into the core concepts of the two quantum revolutions, including quantum reality, quantum information, quantum entanglement, quantum world, quantum computing, and quantum matter. By analyzing quantum mechanics from the perspectives of phenomenology, hermeneutics, and the trans-empirical concept, it expands the frontiers of understanding. This approach contributes to the formation of a trans-empirical reality and a trans-empirical cognition model.
In 1935, with co-authors Podolsky and Rosen, Einstein discovered a weird quantum situation, in which particles in a pair are so strongly correlated that Schrödinger called them “entangled”. By analyzing that situation, Einstein concluded that the quantum formalism is incomplete. Niels Bohr immediately opposed that conclusion, and the debate lasted until the death of these two giants of physics. In 1964, John Bell discovered that it is possible to settle the debate experimentally, by testing the now celebrated "Bell's inequalities", and to show directly that the revolutionary concept of entanglement is indeed a reality. A long series of experiments, started in 1972, have produced more and more precise results, in situations closer and closer to the ideal theoretical scheme.After explaining the debate, and describing some experiments, I will show how this conceptual discussion has prompted the emergence of the new field of quantum information, at the heart of the second quantum revolution.
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.
The quantum revolutions: from concepts to technology The development of quantum mechanics in the beginning of the twentieth century was a unique intellectual adventure, which obliged scientists and philosophers to change radically the concepts they used to describe the world. After these heroic efforts, it became possible to understand the stability of matter, the mechanical and thermal properties of materials, the interaction between radiation and matter, and many other properties of the microscopic world that had been impossible to understand with classical physics. A few decades later, that conceptual revolution enabled a technological revolution , at the root of our information-based society. It is indeed with the quantum mechanical understanding of the structure and properties of matter that physicists and engineers were able to invent and develop the transistor and the laser – two key technologies that now permit the high-bandwidth circulation of information, as well as many other scientific and commercial applications. After such an accumulation of conceptual – and eventually technological – successes, one might think that by 1960 all the interesting questions about quantum mechanics had been raised and answered. However, in his now-famous paper of 1964 – one of the most remarkable papers in the history of physics – John Bell drew the attention of physicists to the extraordinary features of entanglement: quantum mechanics describes a pair of entangled objects as a single global quantum system, impossible to be thought of as two individual objects, even if the two components are far apart.