Universal quantum computer and fault-tolerant quantum computation——concepts, status and prospects

Quantum computing technology has developed rapidly in recent years and received wide attention. In this article, we review some basic concepts, current status, long-term and near-term challenges of quantum computing, so that readers can more accurately understand some recent progress and avoid misunderstanding. One of the main applications of universal quantum computers is to break RSA cryptographic systems. Without quantum error correction, it is difficult to achieve quantum computing in the scale of code breaking. Therefore, a primary challenge of quantum computing technology is to implement quantum computing protected by the quantum error correction, i.e. fault-tolerant quantum computation. By looking at the existing experimental technologies, we will find that quantum gates with error rates lower than the fault-tolerance threshold have been realised in experiments, but fault-tolerant quantum computation is still far from practical applications. The main difficulty is that quantum fault tolerance requires an enormous number of qubits with low error rates, beyond what can be achieved by state-of-the-art technologies; therefore, further development is needed. Noisy intermediate-scale quantum computation is likely to be realised in the near future, and there are still some theoretical and technical bottlenecks that need to be addressed. While we can see the huge potential value of quantum computing and recent significant progress, it is important to acknowledge the challenges, face the key problems, and overcome difficulties.