
An international research group has for the first time demonstrated a quantum algorithm that performs a true calculation, without needing to know the answer before the start.
The team's 'phase estimation algorithm', they say, is a central quantum algorithm which achieves an exponential speedup over all classical algorithms. It's 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 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," says the University of Bristol's Dr XiaoQi Zhou, who led the project.
"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."
The team says its approach can be scaled up to multiqubit systems, since both the algorithm and the number of qubits can be scaled.
"Implementing a full quantum algorithm without knowing the answer in advance is an important step towards practical quantum computing," says Professor Jeremy O'Brien, director of the Centre for Quantum Photonics at the University of Bristol.
"It paves the way for important applications, including quantum simulations and quantum metrology in the near term, and factoring in the long term."