An international research group led by scientists from the University of Bristol, UK, and the University of Queensland, Australia, has demonstrated a quantum algorithm that performs a true calculation for the first time. Quantum algorithms could one day enable the design of new materials, pharmaceuticals or clean energy devices.

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."

The research is published in *Nature Photonics*.

**Explore further:**
Dramatic simplification paves the way for building a quantum computer

**More information:**
'Calculating unknown eigenvalues with a quantum algorithm' by Xiao-Qi Zhou, Pruet Kalasuwan, Timothy C. Ralph and Jeremy L. O'Brien in *Nature Photonics*. DOI: 10.1038/NPHOTON.2012.360

## javjav

Politician answers. They never answer what they are asked for

## LizR

Are they suggesting that experimenters who already knew the prime factors of 15 cheated, and just published a paper _saying_ they'd done it?

How does knowing the answer in advance stop the quantum computer performing a "true calculation" ?

Not trying to detract from their achievement, but whenever someone starts dissing previous attempts at what they've achieved, I get a hackle-raising reaction. (Possibly this dates back to so-called improved versions of "Willy Wonka and the Chocolate Factory", "Psycho" and "Doctor Who"...)

## tkjtkj

As i misunderstand the article, knowing the answer allowed previous workers to 'tune' their process , nudging it more-efficiently towards the answer they wanted. These workers, however, in their own work, then ignored their own a priori knowledge of correctness of the result of their own assignment task they presented to the 'device'.

So, in their own minds, they did not 'tune' the algorithm.

Unfortunately, nothing about the algorithm,the problem, or the answer is presented (which, actually, might be parts of their own algorithm: the last step of which might be: 'deny all fudgings' ...

Now, the price they pay would be not knowing if their own algorithm's result is correct .. but then, nor do we.

## Meyer

Well there's this. http://arxiv.org/...76v1.pdf

## Roach

## antialias_physorg

If I'm reading the paper correctly (and I'm not at all sure that I am.) then the pre-knowledge of the answer in previous works has more to do with how the reading mechanism at the end was constructed (the one that transforms the qbits of the answer into regular bits).

It's not so much that this was 'hard coded' to give a certain result but that it would more easily read the expected result than any other (i.e. if the algorithm had 'failed' it wouldn't read anything. It would only read a result if the algorihm succeeded.)

They're not dissing previous works. They are just showing that they have made an extension that will read any result equally well - not just the one that is expected.

Previous works were all about getting quantum calculations to work at all. This work is a step towards more general usefulness.

## jibbles