A quantum computer can solve tasks not tractable with conventional supercomputers. The question of how one can, nevertheless, verify the reliability of a quantum computer was recently answered in an experiment at the University of Vienna. The conclusions are published in the reputed scientific journal Nature Physics.

The harnessing of quantum phenomena, such as superposition and entanglement, holds great promise for constructing future supercomputers. One huge advantage of such quantum computers is that they are capable of performing a variety of tasks much quicker than their conventional counterparts.

The use of quantum computers for these purposes raises a significant challenge: how can one verify the results provided by such a computer?

It is only recently that theoretical developments have provided methods to test a quantum computer without having an additional quantum computer at hand. The international research team around Philip Walther at the University of Vienna have now demonstrated a new protocol, where the quantum computational results can be verified without using additional quantum computer resources.

Laying traps for a quantum computer
In order to test the quantum computer the scientists inserted "traps" into the tasks. The traps are short intermediate calculations to which the user knows the result in advance. In the case that the quantum computer does not do its job properly the trap delivers a result that differs from the expected one.

"In this way, the user can verify how reliable the quantum computer really is", explains Stefanie Barz (Vienna), first author of the study. The more traps the user builds into the tasks the more sure the user can be that the quantum computer indeed computes accurately.

"We designed the test in such a way that the quantum computer cannot distinguish the trap from its normal tasks" say Elham Kashefi (Edinburgh) and Joseph Fitzsimons (Singapore), theorists and co-authors of the paper.

This is an important requirement to guarantee that the quantum computer is not able to tweak the test result. The researchers have also tested whether the quantum computer really resorts to quantum resources. Thereby, they can sure that even a maliciously constructed quantum computer cannot fool them into accepting incorrect results.

Implementing the idea with photons
For this first demonstration the researchers used an optical quantum computer, where single light particles, so-called photons, carried the information. The demonstrated protocol is generic, but optical quantum computers seem to be ideally suited for this task.

The mobility of photons allows for easy interactions with the quantum computer. Philip Walther is optimistic about the prospects raised by this experiment which shows promising control mechanisms for future quantum computers. And, moreover, that it might lead to new tools for probing even complex quantum resources.

Publication in "Nature Physics": Experimental verification of quantum computations: Stefanie Barz, Joseph F. Fitzsimons, Elham Kashefi, Philip Walther. Nature Physics, September 2013 Doi: 10.1038/nphys2763