Our goal is to build an optically-connected network of many (small) quantum computers. Such a network enables the exchange of quantum bits between any of the connected quantum processors in order to solve problems that are intractable classically.
A quantum network in which the processors are located at different geographical locations is called a quantum Internet. Our goal is to develop the technology to enable quantum communication between any two places on earth. One application of such a quantum internet is to provide a fundamentally secure way of communication in which privacy is guaranteed by the laws of physics.
Quantum processors can also be connected into a quantum network in order to assemble a large quantum computing cluster. This approach is called networked quantum computing and offers a natural path towards scalability. Combining a quantum internet and a networked quantum computer finally allows remote users/providers to perform secure quantum computing “in the cloud”.
Working towards a quantum Internet, the five-year objective of the roadmap is a rudimentary multi-node quantum Internet that connects four different cities in the Netherlands. Recently, we passed an important milestone towards this goal: the first “loophole-free Bell test”. In a loophole-free Bell test two remote systems – in our case 1.3 kilometers apart – show correlations that defy any explanation through classical physics. In addition to its fundamental importance for our worldview, this experiment directly distributes an intrinsically secure random key for communication. Importantly, because all loopholes are closed, it is impossible for an eavesdropper to intercept or fake the key without being noticed.
Along the road to a quantum Internet, there are several milestones. The most important is overcoming the inevitable imperfections in the quantum processors and connections through techniques such as quantum repeaters. A quantum repeater is the basic unit needed for extending quantum networks over longer distances. We aim to demonstrate the quantum repeater principle within the next years by connecting two nodes more than 1 km apart via several rounds of entanglement generation and purification.
Working towards a networked quantum computer, the first milestone is to develop quantum error correction. Quantum error correction is essential for large-scale quantum systems. By encoding a logical quantum bit in multiple physical qubits it becomes possible to detect and correct errors so that the quantum system becomes more stable with increasing size. We have recently demonstrated the first complete cycle of quantum error correction using a 4-qubit diamond processor. In the next years, we aim to realize a logical qubit that is protected against all types of quantum errors. A second milestone is to demonstrate the use of a networked quantum computer to execute a small-scale distributed quantum algorithm.