The team implemented the 'phase estimation algorithm' - a central quantum algorithm which achieves an exponential speedup over all classical algorithms. It lies at the heart of quantum computing and is a key sub-routine of many other important quantum algorithms, such as Shor's factoring algorithm and quantum simulations.

Dr Xiao-Qi Zhou, who led the project, said: "Before our experiment, there had been several demonstrations of quantum algorithms, however, none of them implemented the quantum algorithm without knowing the answer in advance.

This is because in the previous demonstrations the quantum circuits were simplified to make it more experimentally feasible. However, this simplification of circuits required knowledge of the answer in advance.

Unlike previous demonstrations, we built a full quantum circuit to implement the phase estimation algorithm without any simplification. We don't need to know the answer in advance and it is the first time the answer is truly calculated by a quantum circuit with a quantum algorithm."

Professor Jeremy O'Brien, director of the Centre for Quantum Photonics at the University of Bristol said: "Implementing a full quantum algorithm without knowing the answer in advance is an important step towards practical quantum computing.

It paves the way for important applications, including quantum simulations and quantum metrology in the near term, and factoring in the long term."

'Calculating unknown eigenvalues with a quantum algorithm' by Xiao-Qi Zhou, Pruet Kalasuwan, Timothy C. Ralph and Jeremy L. O'Brien in Nature Photonics.