Quantum computers: Trust is good, proof is better

A quantum computer can solve tasks where a classical computer fails. The question 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 using quantum technology. 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 a quantum 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 case 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", explain Elham Kashefi (Edinburgh) and Joseph Fitzsimons (Singapore), theoreticians and co-authors of the paper. The more traps the user builds into the tasks the better the user can be sure 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" says Stefanie Barz (Vienna), first author of the study. 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

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

International Research Collaboration:
The project is an international cooperation of researchers from the University of Vienna, the Centre for Quantum Technologies at the National University of Singapore and the Singapore University of Technology and Design as well as the University of
Edinburgh. The research has been partly funded by the European Commission (Q-ESSENCE and QUILMI), the Austrian Science Fund (SFB-FoQuS, START Y585-N20), the ERA-Net CHISTERA project (QUASAR), the Vienna Science and Technology Fund (ICT12-041), the
Air Force Office of Scientific Research (FA8655-11-1), and from the Singapore National Research Foundation and the Ministry of Education (NRF-NRFF2013-01) as well as the UK Engineering and Physical Sciences Research Council (EP/E059600/1).

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