Original URL: http://www.theregister.co.uk/2013/02/25/quantum_calculation_phase_estimation/

## Quantum computer one step closer after ‘true’ quantum calculation

Baby steps

Posted in Science, 25th February 2013 01:08 GMT

An international group of physicists has had an important “first” acknowledged by the journal *Nature Photonics*: they built the first complete single-qubit system to implement a key algorithm in quantum computing.

The research, abstract here, is an end-to-end implementation of a quantum phase estimation algorithm. Quantum phase estimation is one of the important sub-classes of the discipline, since it’s a problem that only a quantum computer can, in theory, solve in polynomial time.

While prior experiments have demonstrated that the quantum states produced in a phase estimation could be read out, what the *Nature: Photonics* publication recognises is that this experiment runs the whole process: input, processing, and output.

As the researchers write in their paper: “all demonstrations to date have required already knowing the answer to construct the algorithm.”

This has been done, according to project leader Dr Xiao-Qi Zhou, so that experimenters could create quantum circuits simple enough to actually test. To get around the need to “look in the back of the book”, so to speak, his team – comprising researchers from the University of Bristol and the University of Queensland – embarked on the more ambitious project of building the entire quantum circuit “to implement the phase estimation algorithm without simplification”

As stated in the paper: “We have implemented the complete quantum phase estimation algorithm for a single qubit unitary in which the answer is calculated by the algorithm.”

Just as interesting is the claim made by the researchers that their approach is also scalable to multi-qubit systems, since both the algorithm and the number of qubits can be scaled.

The original work was carried out prior to 2011, and the pre-publication paper released at Arxiv, here. ®