QUROPE Quantum Information Processing and Communication in Europe

Objectives: A quantum computer is a device that harnesses some of the basic laws of quantum mechanics in order to solve problems in more efficient ways than classical (standard) computers. The main objective in the field of quantum computation is to build such a device. Other objectives include the development of quantum algorithms to solve specific problems, and the creation of interfaces between quantum computers and communication systems. The construction of a quantum computer with thousands of quantum bits would have tremendous consequences on the security in communications (like the internet), by breaking most of everyday used cryptography. It would also allow us to solve certain problems that the most powerful super computers are not able to solve now or in the near future, and possibly never; in particular, those dealing with quantum many-body systems, as they appear in different fields of physics, chemistry, and material science.

State of the art: We already know that the basic principles of quantum computation are correct and there is no fundamental obstacle in constructing such a powerful machine. The basic building blocks of a quantum computer have been demonstrated with many different technologies, including trapped ions, neutral atoms, photons, NV-centers in diamonds, quantum dots, and superconducting devices. Small prototypes have been built using some of those technologies, and some of the quantum algorithms have been demonstrated. The most advanced technologies at the moment are trapped ions and superconducting qubits. With the first one, coherent control has been achieved with up to 15 qubits. Although the control of the latter is still not at the level of the first, it has the potentiality of being scaled up much more easily. With both technologies, proof-of-principle experiments on quantum error correction have been carried out.

Future directions: Despite the strong efforts devoted by many scientists during the last years, the objective of building a quantum computer remains as a central challenge in science. The main obstacle to build a quantum computer is the presence of decoherence, i.e., undesired interactions between the computer’s constituents and the environment. Standard isolation is not a valid solution, since it seems impossible to reach the levels of isolation that are required in large computations. Therefore, the construction of such a device will require the use of quantum error correction techniques. It is not clear, however, which (already or not yet existing) technology will be optimally suited for the implementation of such techniques in a scalable way and/or in distributed settings. On a different note, we only know a limited class of problems where a quantum computer could overcome the limitations of classical ones, and thus theoretical studies for applications of such devices need to be further pursued.

Some specific future directions of research include:

• Further development of all current technologies to understand their limitations and find ways around them.
• Assessment of the capabilities of different technologies for being scaled up.
• Optimization of the performance of error correcting codes, by both increasing the error threshold and decreasing the overhead of required qubits.
• Investigation of new ways of performing quantum computation, in particular based on self-correcting codes (as they appear in topological systems).
• Development of new quantum algorithms and search for problems where quantum computers will be required.
• Development of quantum complexity theory and its application to many body physics.
• Building interfaces between quantum computers and communication systems.
• Development of quantum-proof cryptography to achieve forward-in-time security against possible future decryption (by quantum computers) of encrypted stored data.