Current information technology relies on representing data as zeros or ones in memory elements called bits that obey the laws of classical physics familiar from every day life. Conceptually, it does not matter if we use electronic circuits or pebbles in boxes to represent bits. Using instead objects that obey the laws of quantum mechanics, which usually only manifest themselves in the microscopic world of atoms and molecules, promises qualtitatively new capabilities. For example, cryptographic procedures to transmit secret messages can be protected against undetected eavesdropping by the laws of physics. In computing, practically important tasks such as optimization problems or the simulation of catalysts for chemical processes could be made drastically faster, in some cases allowing us to tackle currently intractable problems.
While first quantum communication and quantum computing systems are already available, their usefulness is still limited by their moderate size and accuracy. Substantial improvements are needed to unleash their full potential. This project tackled two important aspects for semiconductor-based quantum bits (short qubits), which are an attractive approach because of their similarity with the semiconductor technology used for current computers. First, highly accurate methods to execute elementary operations with pairs of qubits, which are the fundamental steps for composing larger algorithms, were developed. The inaccuracy of such operations is a key limiting factor for the performance of current quantum processors. Our simulation results show that with a careful optimization, semiconductor qubits can reach figures of merit that are sufficient for large scale quantum computing. Furthermore, we have realized and demonstrated a device to couple qubits over larger distances than commonly achieved. The key idea here is to move individual electrons across a chip using electric signals. Based on this concept, we have developed a complete architecture that promises to be much better scalable than other approaches.
A second outcome of the project is an approach to optically interlink qubits that are suitable for realizing quantum circuits and processors. Such quantum links can be seen as the first step towards a quantum internet. Small quantum computers could be connected to tackle problems that are too large for each individual ones, and the range of quantum-secure communication could be extended beyond the limit of about 100 km encountered by current commercial devices, which do not yet dispose of fully operational qubits at each communicating node. A long term vision is that every home will connected to a quantum network to secure communication for online banking, purchases, messaging and other sensitive applications. The concrete step completed in the project is the realization of a challenging device design that can equip a promising type of qubit with an optical interface, i.e. a receiver or emitter of quantum states.
