The team implemented the “phase estimation algorithm” — a central quantum algorithm that  achieves an exponential speedup over all classical algorithms.

It lies at the heart of quantum computing and is a key subroutine of many other important quantum algorithms, such as Shor’s factoring algorithm and quantum simulations.

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

According to Professor Jeremy O’Brien, director of the Centre for Quantum Photonics at the University of Bristol,”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.”