WASHINGTON -A new academic study has raised doubts about the performance of a commercial quantum computer in certain circumstances.  In some tests devised by a team of researchers, the commercial quantum computer has performed no faster than a standard desktop machine.

The team set random maths problems for the D-Wave Two machine and a regular computer with an optimised algorithm. Google and Nasa share a D-Wave unit at a space agency facility in California. The comparison found no evidence D-Wave’s $15m (£9.1m) computer was exploiting quantum mechanics to calculate faster than a regular machine.

But the team only looked at one type of computing problem and the D-Wave Two may perform better in other tasks. The study has been submitted to a journal, but has not yet completed the peer review process to verify the findings. And D-Wave told BBC News the tests set by the scientists were not the kinds of problems where quantum computers offered any advantage over classical types.

Quantum computers promise to carry out fast, complex calculations by tapping into the principles of quantum mechanics. In conventional computers, “bits” of data are stored as a string of 1s and 0s. But in a quantum system, “qubits” can be both 1s and 0s at the same time - enabling multiple calculations to be performed simultaneously. Small-scale, laboratory-bound quantum computers supporting a limited number of qubits can perform simple calculations.

But building large-scale versions poses a daunting engineering challenge. Thus, Canada-based D-Wave Systems drew scepticism when, in 2011, they started selling their machines, which appeared to use a non-mainstream method known as adiabatic quantum computing.

But last year, two separate studies showed indirect evidence for a quantum effect known as entanglement in the computers. And in a separate study released in 2013, Catherine McGeoch of Amherst College in Massachusetts, a consultant for D-Wave, found the machine was 3,600 times faster on some tests than a desktop computer. Last year, it was announced that Google, Nasa and other scientists would share time on a D-Wave Two - which has a liquid helium-cooled processor operating close to the temperature known as absolute zero - at the US space agency’s Ames facility in California. In the latest research, Prof Matthias Troyer of ETH Zurich and colleagues set random maths problems for a D-Wave machine owned by defence giant Lockheed Martin, pitting it against a desktop machine.

Their results revealed that there were some instances in which D-Wave Two was faster than the “classical” computer, but likewise there were others where it performed more slowly.