EXECUTIVE SUMMARY

Quantum Information Processing and Communication (QIPC) has the potential to revolutionize many areas of science and technology. It exploits fundamentally new modes of computation and communication, because it is based on the physical laws of quantum mechanics instead of classical physics. It holds the promise of immense computing power beyond the capabilities of any classical computer, it guarantees absolutely secure communication, and it is directly linked to emerging quantum technologies, such as, for example, quantum based sensors. The worldwide interest in the subject may be gauged by the recent significant increase of funding in quantum information technology; in particular in the United States, Canada, Australia and in some countries in Asia (see section 2.2). Europe has played a leading role in the early development of QIPC, and, given appropriate research infrastructure and suitable funding, European researchers are well positioned to maintain Europe at the forefront of the field. However, this requires a significant effort at national level and a consolidation, coordination and unification of many national projects and initiatives under one common European umbrella with the lead of the research program of the European Commission. For Europe to remain competitive in this field in the future there is an urgent need for a substantial EU-programme in QIPC.

OVERVIEW OF QIPC RESEARCH AND ITS GOALS FOR THE COMING FIVE TO TEN YEARS

QUANTUM COMPUTING

Classical physics is at the root of present-day information processing: strings of bits (discrete digital states) are represented and processed in electronic devices (registers, logic gates etc.) through quantities such as charges, voltages, or currents. In Quantum Computing and more generally in Quantum Information Processing (QIP), one makes instead use of the laws of quantum mechanics replacing bits with qubits, two-state quantum systems that do not possess in general the definite values of 0 or 1 of classical bits, but rather are in a so-called ‘coherent superposition’ of the two. Full exploitation of this additional freedom implies that new processing devices (quantum registers, quantum logic gates etc.) need to be designed and implemented. As several sets of universal quantum gates acting on one and two qubits are known, a large scale quantum computer can in principle be built, provided the quantum physical system used meets some basic requirements (the so-called DiVincenzo criteria) on scalability, faithful initialization, manipulation, transmission and readout of qubits, and long coherence times with respect to the gate operation time. At present, a number of physical systems are under investigations for their suitability to implement a quantum computer. These include trapped ions and neutral atoms, cavity quantum electrodynamics (CQED), solid state devices (such as superconducting qubits, possibly in combination with circuit CQED, and spin qubits), all-optical devices, as well as impurity spins in solids, single molecular magnets etc.. During the last few years remarkable progress, measured in terms of the aforementioned DiVincenzo criteria, towards demonstrating the basic building blocks of a quantum computer have been reported in these systems. At present no fundamental physical roadblocks seem in sight for building a scalable quantum computer including error correction. However, a mixture of significant technological challenges and some open physical questions remain to be answered. At the same time it is premature to select a winner, rather research should progress on a broad front across all physical disciplines which studies these systems in view of scalability, coherence and speed of QIP, in particular also concerning their reliability, fault tolerance and use of error correction. Finally, development of a computer architecture must be complemented by interfacing with quantum communication to allow building of quantum networks. Ultimately, the goal must be to transfer this academic knowledge about the control and measurement of quantum systems to industry. Major international companies have shown interest and support for developing and providing systems suitable for quantum manipulation.

Few-qubit applications. A first short range goal is the realization of a few-qubit general purpose quantum computer including error correction, as a test bed for demonstrating operation of a quantum computer. In parallel, however, special effort must be made to further develop few qubit applications which range from quantum information processing and quantum communication all the way to quantum assisted precision measurements.

Many-qubit specialized applications. As a second short range goal, special purpose quantum computers with a large number of qubits should be developed. A highly relevant example is provided by quantum simulators, programmable quantum systems whose dynamics can be engineered such that it reproduces the dynamics of other many body quantum systems of interest, e.g., atoms in optical lattices simulating high temperature superconducting systems and/or quantum phase transitions. Full simulation of a quantum mechanical system consisting only of a few hundred particles (spins) requires in fact classical computing resources in terms of memory of the order of the number of atoms in the visible universe – clearly demonstrating the inadequacy of any classical computer for this task. Quantum simulators could be the first nontrivial applications of quantum information, providing answers to problems which are fundamentally beyond classical computing capacities, such as the study of microscopic properties of materials permitting free variation of system parameters, an accurate description of chemical compounds and reactions, or find out the reason why free quarks are not found in Nature.

Quantum interfaces. In the long term a first goal is the development of hybrid technologies and architectures for quantum computation, including interfaces between them. This will stretch the theoretical and experimental resources of many branches of physics, from quantum optics and atomic physics to solid state devices. It is likely that there will not be a single winner in this search, but rather a number of different technologies complementing each other: some will be more suitable for quantum memories, some for quantum processing, and some for quantum communication and so on. Therefore, in addition to developing individual technologies, interfaces between the latter are also needed, so that different qubit ‘memories’ (atoms/ions, quantum-dots, squids) and carriers of quantum information (atoms/ions, photons, phonons, electrons) can be interconnected.

Fault-tolerant gates and architectures. A second long range goal is the demonstration of fault-tolerant quantum logic gates, by the engineering of sub-microscopic systems in which qubits affect each other in a controllable way, while avoiding at the same time undesired couplings with the environment leading to decoherence. Applying to quantum computers the traditional network model, simple quantum logic gates would be connected up into quantum networks. However, the more interacting qubits are involved, the harder it tends to be to engineer the interaction that would display the quantum behaviour, and the more components there are, the more likely it is that quantum information will spread outside the quantum computer and be lost into the environment, thus spoiling the computation. It has been proven that if decoherence-induced errors are small (and satisfy certain other achievable conditions), they can be corrected faster than they occur, even if the error correction machinery itself is error-prone. The requirements for the physical implementation of quantum fault tolerance are, however, very stringent, and can be met either by improving technology or by going beyond the network model of computation and designing new, inherently fault-tolerant, architectures for quantum computation. One candidate for such an alternative architecture, e.g., might be the one-way quantum computer model, in which errors can classically be fed-forward and corrected. At the end however, a fault-tolerant quantum computer will most likely be achieved by an optimized combination of both strategies.

Implementation theory. Theory must continue to play a leading role in guiding and supporting experimental developments. Aside from finding and investigating fundamentally new algorithms especially suited for quantum computing, the various implementations require continuous theoretical work especially finding physical solutions where mere technology is yet too cumbersome. For example, operations in specially designed “decoherence free subspaces”, i.e., physically tailored systems less susceptible to technical errors, will be an important feature in finding an optimum system and optimized algorithms. Therefore, the theoretical work will have to cover a wide range of physical systems and technologies.

QUANTUM COMMUNICATION

Quantum Communication is the art of transferring quantum states from one place to another. The general idea is that quantum states encode quantum information: hence quantum communication also implies transmission of quantum information. Quantum Communication covers aspects of basic physics as well as of practical relevance. Additionally, it will take care of the whole “wiring” inside a quantum computer, i.e., contribute to the quantum interface. Already now, one of its outstanding results is the emerging technology of quantum cryptography, which promises absolute secure transmission of the key codes that are essential to encrypt messages with tamper proof security. More specifically, any encryption scheme entails the distribution of a secret key among legitimate users; as the key must be transmitted between sender and recipient, it is susceptible to interception by an eavesdropper. For a secret key made of classical bits, none of the two parties will ever know that their communication has been intercepted. Not so if the key is carried out by a quantum communication channel. As said before, qubits do not possess definite values such as the 0 or 1 of classical bits; rather, they represent a so-called coherent superposition of physical states (e.g., the polarizations of a photon). The laws of quantum mechanics imply that the mere act of observing a quantum bit modifies it, causing it to change its quantum state. The eavesdropper’s attempt to intercept the secret key made of qubits will therefore be manifest to both parties. Quantum cryptography is now developing from the initial approach known as point-to-point Quantum Key Distribution (QKD), towards the management of quantum-based security over many-nodes networks, that are being built in various places worldwide (USA, Europe, Japan). Presently, technical problems are controlled well enough so that secure transmissions over a few tens of kilometers can be implemented. However, non-trivial problems emerge for really long-distance communication (hundreds to thousand of kilometers), and in the quest for higher bit rates. High-flux single photon sources as well as entangled photon sources should be developed in order to enhance the secure medium range quantum communication. At present photons are the only suitable system for medium-distance quantum communication, as they maintain a robust quantum state throughout transmission, can be detected efficiently and with low levels of noise (other systems, such as atoms or ions, can be used for building quantum memories but not to propagate qubits over long distances). But even light signals, whether viewed classically or quantum-mechanically, are dampened exponentially with distance in both optical fibres and free space. More fundamental basic research is needed in particular in production, detection and distribution of qubits. In classical optical telecommunications, this problem is solved by using simple devices known as repeaters, which amplify and reshape the transmitted signal. But the latter are of no use for quantum communication: they are intrinsically noisy and create so many errors that any quantum key being transmitted would not survive. This is related to the fact that a classical repeater breaks down quantum entanglement, a purely quantum phenomenon associated with very strong, non-classical correlations between the states of two widely separated qubits. However, novel protocols (for instance based on entangled qudits) could enhance the fault-tolerance of quantum communication schemes and need further investigation. Entanglement is a crucial element in quantum communication schemes, which allows one to ‘teleport’ qubits directly to their destination, avoiding transmission losses. So quantum communication must reinvent the repeater concept, using quantum hardware that preserves entanglement.

Real world medium-distance quantum communication. If Quantum Communication is to become, on the 5 to 10 year time-scale, an established technology, backing up the quantum cryptography “boxes” which are already commercialized, several scientific as well as technological gaps have to be filled. In particular one needs to demonstrate the feasibility of ‘real world’ medium-distance quantum communication both in optical fibres and in free space, purely terrestrial and to and from flying quantum communication nodes, e.g., high altitude platforms (HAP) or satellite, and to increase the qubit transfer rate by several orders of magnitude. These two goals, together with the one of realizing long-distance secure quantum networks will be significantly advanced by developing quantum repeaters Achieving these goals will require facing a number of non-trivial challenges, needing very strong interaction between fundamental and applied research.

Quantum repeaters. In the long term a quantum repeater would actually be a small dedicated quantum processor, incorporating quantum memories, which, whilst feasible, requires a significant effort and is perhaps the most important technological hurdle facing QIPC. So far we have seen some first experimental steps towards elements needed for a quantum repeater, but there is much work to be done. The basic elements to be developed are: medium range entanglement between memories, teleportation between different memories. The exact number of qubits that would have to be stored and processed in such a repeater to ensure high-fidelity quantum communication over thousands of kilometres is an open issue. But it is likely to be in the range of tens or hundreds – much lower than the number required for a fully fledged quantum computer. Therefore it is more likely that we will have secure global quantum communication before quantum code breaking.

QUANTUM INFORMATION SCIENCES – THEORY

Our conception of what a computation is has been altered drastically during history, since the times of Leibniz, Babbage and Turing. The result of this remarkable history of ideas – computers as we know them today – has changed our modern society significantly. Yet, the development of computing and communication devices has not come to a stop. Recent developments have shown, in fact, that we are at the beginning of a new era of harnessing the laws of nature, using quantum physics for unprecedented and very powerful ways of information processing. The development of Quantum Information Science (QIS) has been driven by theoretical work of scientists working on the boundary between Physics, Computer Science, Mathematics, and Information Theory. In the early stages of this development, theoretical work has often been far ahead of experimental realization of these ideas. At the same time, theory has provided a number of proposals of how to implement basic ideas and concepts from quantum information in specific physical systems. These ideas are now forming the basis for successful experimental work in the laboratory, driving forward the development of tools that will in turn form the basis for all future technologies which employ, control and manipulate matter and radiation at the quantum level. While the development of QIS has started as early as in the 80’s, the field has gained significant momentum in the last decade. Major triggers were the discovery of fast quantum algorithms and the identification of concrete physical systems in which a quantum computer could be realized. In the meantime, a broad spectrum of research activities can be observed, ranging from the study of fundamental concepts such as quantum entanglement, to novel applications such as quantum simulators, and with significant spin-off also to other fields of research. In many of these activities, European research has played a leading role and has established a strong set of world leading centres. It is important to realize that theoretical activities are often interdisciplinary in nature and span a broad spectrum of research in which the different activities are benefiting from each other to a large degree. Thus it does not seem to be advisable to concentrate research on too narrowly defined topics only. The following list nevertheless tries to highlight the main current areas of quantum information theory as it has been described in more detail in the strategic report.

Quantum algorithms & complexity. Quantum algorithms will be one of the most powerful applications of quantum computers. We know only a few examples up to date, such as Shor’s factoring algorithm, but new techniques and protocols are currently being developed. This area remains one of the cornerstones of research in QIC.

Computational models & architectures. There are many different ideas of how to make quantum systems compute. New computer models, which have only recently been developed, are providing new agendas to formulate quantum algorithms. At the same time, they have opened new ideas for physical implementations of a quantum computer, and we expect new methods for fault-tolerant computation that will make it technologically less challenging to realize scalable devices in the laboratory.

Geometric and topological methods. These methods represent an alternative approach to the realization of quantum computing. They have intrinsic fault-tolerant properties that do not need an active error detection and recovery; however, the overhead that one has to pay are longer operation times, so that much work must still be done to identify which of the available schemes suit better to quantum computation.

Quantum simulations. Quantum simulators may become the first short-term application of quantum computers, since with modest requirements one may be able to perform simulations which are impossible with classical computers. They could be used for a variety of purposes, e.g., to obtain an accurate description of chemical compounds and reactions, to gain deeper understanding of high temperature superconductivity, or to find out the reason why quarks are always confined.

Quantum error correction & purification. Despite its amazing power, a quantum computer will be a rather fragile device, susceptible to disturbances and errors. Fortunately, methods have been developed to protect such a device against disturbances and imperfections, as long as these are small enough. These methods are constantly being improved and refined, but there is still a lot of work to be done until we can run a quantum computer reliably.

Theory of entanglement. Entanglement represents a novel and particularly strong form of correlations which is not present in classical systems. It is a key resource in quantum information science and, at the same time, one of the most prominent features of quantum physics. Insights in the theory of entanglement will continue to have broad implications, and applications will lie not only within the field of QIS itself, but also in other areas of physics, such as field theory and condensed matter physics.

Multi-partite entanglement & applications. Research on multi-particle entanglement has emerged recently, and it is expected to have an impact on novel protocols for quantum information processing. Multi-partite entangled states represent keys resources, both for quantum computers and for novel communication schemes with several users such as quantum-secret sharing, quantum voting etc. Alternatively one can consider multi-partite fingerprinting schemes that would allow for the determination of whether or not a number of databases are identical with very little resources.

Noisy communication channels. In practice, all communication channels such as optical fibres are subject to some level of noise. Such noise can destroy the crucial entanglement or other quantum properties that are needed, e.g., for security or to reduce communication complexity. A proper understanding of how one can communicate via noisy quantum channels and of the capacities of such channels is at the heart of the study of quantum communication tasks. Fundamental quantum mechanics and decoherence. Quantum information was born, in part, via research on the famous Einstein-Podolski-Rosen paradox and the issue of quantum non-locality. It is now understood that non-locality is one of the central aspects of quantum mechanics. More generally, quantum information profits substantially from studying the fundamental aspects of quantum mechanics and, at the same time, it yields new perspectives, raising hopes of gaining a deeper understanding of the very basis of quantum mechanics. In particular, quantum information theory can provide deeper understanding of dynamics of open quantum systems.

Spin-off to other fields. A very exciting aspect of theoretical work in QIS is the impact that it is beginning to gain on other fields of science. Examples are given by the theory of classical computing, by field theory, and by condensed matter physics. Many of the questions that are now being asked in this area can only be answered or even formulated correctly because of the many insights and techniques gained in the research in entanglement theory in recent years. Theoretical research in QIS in Europe has prospered through the efficient support for collaboration by the European Union, the European Science Foundation and the national funding bodies. In the face of significantly growing international competition from North America, Japan and Australia it will be essential that flexible support compatible with innovative work will continue to be provided

SUMMARY OF SHORT- AND LONG-TERM GOALS

For convenience of synthesis, we summarize in a table a short list of objectives for the next and more distant future of quantum computing and quantum communication (the internal ordering of such lists does not necessarily reflect chronology). The great diversity and openness of the field of quantum information theory prevents from drawing a similar list for that particular subfield.

SYNERGIES AND INTEGRATION

QIPC is a new conceptual framework, a new way of looking at things with deep reaching consequences from network security to understanding the structure of the physical reality. It covers a broad spectrum of activities, from researching the foundations of quantum mechanics between the microscopic and the macroscopic level, to the development of patented industrial applications like quantum key distribution devices. The three domains of QIPC, quantum computation, communication and theory, are all closely connected, and within these domains there are a variety of different approaches that are all striving towards the same goal - integrated quantum systems. This integration will provide the next great challenge and inspiration for QIPC. In recent years tremendous progress has been made in all three fields, improved distances and fidelities in quantum cryptography and teleportation, coherent control of atomic systems for processing and theory is making daily advances in developing a basis for the theory of quantum computer science. Characteristic of the work within QIPC is that proof-of-principle advances in each of the sub-domains are used when pursuing the work in the other sub-domains and this is a key issue for developing QIPC as an integrated science and the basis of future and emerging technology. The experimental demands on the next phase of QIPC research will have a larger focus on integration of components and their reliability as the field moves from research oriented problems to applied and even commercial quantum technologies. Still an even closer interplay between theory and experiment will be needed in order to achieve complete realistic schemes for coherent manipulation and high-precision performance. These efforts will eventually lead to a pool of reliable technologies for the different components of a quantum architecture, much like it happens now for classical computers where magnetic, optical and electric bits are used for storage, transmission and processing of information, respectively. Clearly, it is too early to pick the winner implementation for the practical realization of a working quantum device: it is even possible that the best technology is still to be developed. The already ongoing integration among different research communities (for instance those working on solid-state and on atom/quantum optical systems) is a solid basis for further pushing these effort to integrating actual devices. An avenue that theory needs to embrace in order for efficient implementations to be developed is a deeper understanding of entanglement, which is a quantum feature that permeates the whole QIPC; its complexity just started being appreciated and much is left to investigate both in terms of formal theoretical description and of its applications. One also needs to fully explore the potentials of the available physical systems in order to invent new communication protocols, to investigate algorithmic consequences of physical assumptions, and to develop new computational algorithms, both implementable with a small-scale quantum computer and exploiting the immense power entailed in quantum parallelism.

THE LEADING ROLE OF EUROPEAN RESEARCHERS IN QIPC

European researchers have been from the outset prominent in setting the agenda of, and leading, the worldwide research efforts in quantum information science, in friendly competition with similar efforts and programs in the US, Australia, Canada, Japan and China. This includes pioneering work on the foundations of the quantum theory of computation, quantum algorithms, and the discovery of entangled state quantum cryptography, which generated a spate of new research that established a vigorously active new area of physics, computer science and cryptology. Many subsequent seminal contributions, inspired by the 1994 Shor’s quantum factoring algorithm, such as ways of implementing quantum computation using ion traps, quantum dots, cavity QED, optical lattices and a number of other technologies, novel computational architectures, methods of error correction and fault tolerant quantum computation originated in Europe. A unique feature and strength of European research is the broad range of activities and expertise, linking coherently efforts from experimental realization all the way to basic theoretical questions in quantum information science and quantum physics.

RECOMMENDATIONS FOR FUNDING ON THE EU AND NATIONAL LEVEL

QIPC has established itself as one of the key new multidisciplinary fields between theoretical and experimental physics, computer science and mathematics. Continued competitiveness of the EU and its member nations requires a significant effort both on the European and national level

• QIPC must take a prominent and established position in EU research efforts, e.g., in the Seventh Framework Programme for Research of the European Commission (FP 7), and find its counterpart in national programs.
• The structure of the funding must account for the interdisciplinary character of the field, and must support a spectrum of activities across different disciplines from experimental to theoretical physics, computer sciences and mathematics.
• Links with industry must be developed, both on the level of possible commercial exploitation, and in research programs making new technologies available, outside the capabilities and know-how of traditional QIPC basic-research oriented laboratories. In particular, links with micro- and nano-fabrication facilities and related technology centers must be strengthened, and QIPC spin-off new quantum technologies like quantum sensors and high precision measurement devices ought to be encouraged.

References: The US Roadmap for Quantum Computing and Quantum Cryptography is available at http://qist.lanl.gov.