In this dialogue, the physicist Ignacio Cirac, director of the Theoretical Division of the Max Planck Institute for Quantum Optics, outlines why quantum physics has brought about a much greater change than that caused by Einstein’s theory of relativity, how quantum physics takes free will into account and how it combines with philosophy. He describes why quantum theory defines “everything else,” yet is unable to define itself. Explaining how, together with Peter Zoller, he developed and presented the first theoretical description of a quantum computing architecture based on trapped ions, and, how this quantum architecture will be viable and capable of performing calculations we cannot perform at present. Their quantum computer calculates in qubits, which would require at least 100,000 qubits to function, rising to 1,000,000 if error correction is implemented. It will be able to perform calculations previously unachievable and create encrypted messages impossible to decipher. Building a functional quantum computer still requires a huge technological change, which has yet to come about. Lastly, Cirac explains the differences between European and American visions of science and why mathematicians are even more conservative than physicists.
In this dialogue, the physicist Ignacio Cirac, director of the Theoretical Division of the Max Planck Institute for Quantum Optics, outlines why quantum physics has brought about a much greater change than that caused by Einstein’s theory of relativity, how quantum physics takes free will into account and how it combines with philosophy. He describes why quantum theory defines “everything else,” yet is unable to define itself. Explaining how, together with Peter Zoller, he developed and presented the first theoretical description of a quantum computing architecture based on trapped ions, and, how this quantum architecture will be viable and capable of performing calculations we cannot perform at present. Their quantum computer calculates in qubits, which would require at least 100,000 qubits to function, rising to 1,000,000 if error correction is implemented. It will be able to perform calculations previously unachievable and create encrypted messages impossible to decipher. Building a functional quantum computer still requires a huge technological change, which has yet to come about. Lastly, Cirac explains the differences between European and American visions of science and why mathematicians are even more conservative than physicists.
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.
The development of quantum mechanics and quantum physics in general is one of the greatest intellectual achievements of the 20th century. Initially, quantum physicists focused on mathematically describing and understanding the laws of a micro-world, comprised of atoms, molecules and photons. Nuclei and sub-atomic particles were discovered, particle physics evolved, and complex quantum systems in chemistry and solid-state physics are now described in terms of quantum physics. Within a century, quantum physics became the framework, which allows us to describe and predict, albeit with probabilities, the behavior of the microscopic world as we know it. The powerful quantum framework led to revolutionary technical advances for a century, such as semiconductor technology and the entire electronics industry based on it, enabling computers and the information age. The development of lasers found applications in many areas of our current technologies; we use them for printers, storing and retrieving information using CDs, industrial welding, medical surgery, and for range finding and communication purposes. Medical imaging using MRI is now a routine instrument, as are precision measurements using tunneling and atomic force microscopes. Superconducting technologies are used for high magnetic fields and many more current technologies are based on and enabled by the laws of quantum physics. While the framework of quantum physics has thus proven to be extremely successful and powerful, throughout the last century there were many conceptual, and to some extent philosophical, discussions on our understanding of quantum physics. As early as the 1930s, Einstein and Schrödinger realized that quantum physics allows for some peculiar states, which Schrödinger called “entangled”, which have a non-local character and thus do find an easy interpretation in our classical world. It took a few more decades until John Bell conceived a way to quantify such states, which thus became amenable to measurements. Only in the eighties and nineties of the last century did the generation of entangled states, their characterization and their subsequent application become feasible and accessible in laboratories. Early on, Bell states were proposed for applications in quantum communication, employing protocols based on teleportation. Soon, entanglement was recognized to be useful also for sensing, for precision measurements and eventually for quantum information processing. Starting with the early concept of a trapped-ion quantum computer introduced by Ignacio Cirac and Peter Zoller in 1995, many other platforms were proposed for quantum information processing, based on superconducting quantum bits, photonic processes, engineered quantum dots and many more. The first decade of the 21st century saw an explosive development of quantum information physics, which is by now a mature field and increasingly focuses on applications. Worldwide efforts are ongoing to realize quantum networks for secure communication and to implement quantum computers. Enhanced sensing has been demonstrated using advanced quantum states, and quantum simulators have been developed, already tackling problems that are hard to compute with classical computers. While architectures for full-fledged quantum computers are already available, the realization of such computers still requires the routine implementation of error correction, which is not yet available, for full scalability. Currently, noisy intermediate-scale quantum (NISQ) processors are already used for computations and are even accessible via cloud services. Since about 2000, quantum technologies have been developed for many platforms with the aim to surpass the capablities of their classical counterparts. Many such devices now make use of entangled states, which are seen as the new resource for advanced quantum technologies. For this, describing, understanding and generally mastering the generation and manipulation of entanglement is a key element. Moreover, both hardware and software technologies have to be improved in order to better control quantum devices. Mastering quantum phenomena for communication, metrology, simulation and computation purposes is currently one of the hottest topics in science since the wide applicability of quantum technologies will revolutionize these fields. The development of advanced quantum technologies is thus seen worldwide as a strategic area for generating technical knowledge and industrial evolution. This special issue intends to showcase the breadth and impact of the ongoing research in advancing quantum technologies. We have invited a selection of renowned scientists to present their work, and we are confident that this issue will find a broad readership and attract much interest within the scientific community.
Quantum many-body systems, as they appear in different branches of physics and chemistry, are very hard to simulate. The number of parameters defining their quantum states grows exponentially with the number of constituents. This prevents us from exploring and investigating relevant problems involving relatively small numbers of subsystems even with the help of the most powerful supercomputers. Richard Feynman already noticed this difficulty back in 1981 when he wrote his visionary paper “Simulating Physics with Computers” [Int. J. Theor. Phys. 21:6/7, 467 (1982)]. There, he proposed to use quantum systems (as opposed to classical computers) to perform the simulation and in this way to circumvent the need to store and compute a colossal number of parameters, necessary to describe the pertaining superpositions. What was a visionary idea at that time has become a very active field of research in the last few years. The extraordinary progress of both theoretical and experimental research in quantum physics allows us now to tame, control, and manipulate various quantum systems with unprecedented precision. Laser, magnetic, electronic and other technologies enable us to arrange quantum subsystems in different geometries, modify their interactions, and detect them. This allows us to engineer Hamiltonians and emulate systems displaying some of the most intriguing quantum phenomena. This tremendous progress is, to a large extent, due to the pioneering work of many researchers during the last twenty to thirty years. Here, the investigations of basic phenomena with one or few atoms or photons were of particular impact. The degree of control achieved in the experiments has enabled the observation of mind-boggling quantum effects, related to the superposition principle or to entangled states, and their disappearance due to decoherence. In fact, the 2012 Nobel Prize has been awarded to two scientists, S. Haroche and D. Wineland, for such ground-breaking experiments. Such experiments, often in combination with laser cooling of atoms or ions and Bose-Einstein condensation, have formed one of the pillars on which the field of quantum simulations is being established. Quantum simulations with cold atoms, either in magnetic traps or in optical lattices, with trapped ions, with photons, with superconducting devices, with quantum dots, etc., have been theoretically proposed and experimentally considered by many researchers. Most of the proposals so far deal with problems in condensed matter physics that are either difficult to tackle with modern supercomputers or are difficult to observe in solid state systems. This has established a very close link between different disciplines, like atomic, molecular, and optical (AMO) physics, and condensed matter physics. Although some of the experiments are still in their infancy, and a lot of research is still needed, the extraordinary progress made in all these research areas during the last few years makes us feel confident that in the not too distant future quantum simulators will provide a key tool to investigate many-body quantum systems. Furthermore, we also expect that quantum simulators will help quantum physicists to establish new and fruitful links to other scientific communities, like high-energy physics or quantum chemistry. The present special issue contains various review papers as well as research papers on quantum simulations. It starts out with two papers written by last year's Nobel Prize winners describing their foundational work (Wineland, p. 739 and Haroche, p. 763). The issue continues with three thorough, invited review papers covering different topics on quantum simulations. The first one reviews recent theoretical proposals to use cold atoms in optical lattices to simulate lattice gauge theories of the sort that appear in high-energy physics (U.-J. Wiese, p. 783). The second covers both theoretical proposals and experimental demonstrations of cold-atom systems to simulate the physics of matter in the presence of gauge fields, displaying so-called topological phenomena, which so far have only been observed in solid-state systems (I. Spielman, p. 794). The third one contains an extensive overview and new proposals to perform a broad range of quantum simulations using quantum dot systems (P. Barthelmy et al., p. 813). The issue also includes six original papers with a variety of theoretical proposals as well as the introduction of novel theoretical techniques related to the field of quantum simulation. The field of quantum simulations is just starting, but we are confident that it will soon generate an even larger plethora of new ideas and discoveries, trigger interdisciplinary research, and lead us to the adventure of describing and understanding the quantum world. We hope that this special issue will help future researchers in such an exciting adventure. Rainer Blatt received his PhD degree in physics from the University of Mainz in 1981. As a postdoctoral fellow he worked on atomic beam cooling in Boulder, CO and later as a research assistant in Hamburg with single trapped ions. In 1995 he accepted a chair in experimental physics at the University of Innsbruck working on quantum information processing with trapped ions. Since 2003 he is also scientific director at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences. Immanuel Bloch received his PhD degree in physics from the Ludwig Maximilians University of Munich in 2000. As a junior group leader, he continued in Munich starting his work on ultracold quantum gases in optical lattices. In 2008 he was appointed scientific director of the newly founded division on Quantum Many-Body Systems at the Max Planck Institute of Quantum Optics. Since 2009 he also holds a chair in quantum optics at the Ludwig-Maximilians University of Munich. Ignacio Cirac received his PhD degree in physics from the Universidad Complutense de Madrid in 1991. He was professor at the Universities of Castilla-La Mancha and Innsbruck from 1991–1996 and 1996–2001, respectively. In 2001 he joined the Max-Planck Institute of Quantum Optics, where he leads the Theory Division. His current research interests include quantum information theory and quantum optics. Peter Zoller obtained his doctorate from the University of Innsbruck in 1977. He was Max Kade Fellow at the University of Southern California from 1978/79, and was JILA visiting Fellow 1981/82 and 1988. In 1990 he became JILA Fellow and professor of physics at the University of Colorado. At the end of 1994, he accepted a chair at the University of Innsbruck, where he has worked ever since. Since 2003, he has also held the position of Scientific Director at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences.
Quantum theory has created a fundamental change in our ideas as relativity theory had done before. But while relativity theory is essentially classical in its conceptions—it is in fact the culmination of classical physics—quantum mechanics is founded on some new principles. The quantum laws are very different in character from the laws we find in classical theory. In particular, quantum mechanics is a strictly statistical type of theory : from this arises the different conception of causality that marks the new outlook of modern physics. Classical theory describes the so-called macroscopic phenomena, that is, things and events which are more or less of the same size and kind as we find them in ordinary life; and so a description can be given in terms of concepts close to the ones we use in ordinary language. The language of quantum mechanics is based upon a duality principle ; and most of our ordinary language is equally dualistic.
Quantum mechanics is one of the cornerstones of modern physics. It governs the behavior and the properties of matter in a fundamental way, in particular on the microscopic scale of atoms and molecules. Hence, what we may call a classical computer, i.e., those machines on or under the desktops in our offices together with all their potential descendants, are themselves following the rules of quantum mechanics. However, they are no quantum computers in the sense that all the inside information processing can perfectly be described within classical information theory. In fact, we do not need quantum mechanics in order to explain how the zeros and ones – the bits – inside a classical computer evolve. The reason for this is that the architecture of classical computers does not make use of one of the most fundamental features of quantum mechanics, namely the possibility of superpositions. Throughout the entire processing of any program on a classical computer, each of the involved bits takes on either the value zero or one. Quantum mechanics, however, would in addition allow superpositions of zeros one ones, that is bits – now called qubits (quantum-bits) – which are somehow in the state zero and one at the