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How to check if quantum computer is… quantum?

Posted September 3, 2013

This question may seem a little absurd at first: why someone would need to check if some certain computer is quantum or not? Isn’t it obvious from the start? In fact, it seems that similar scenarios are possible, at least in some not-far future, if not today. The main problem is, that the mainstream ‘classical’ computers which are used today are basically ‘incompatible’ with the quantum computing systems, the principles and first prototypes of which are currently being developed at major global centers of physics.

Certainly, someday in the bright future we will have the quantum computing technology developed to the sufficiently high degree of complexity so we could use it in our everyday lives and it would not be limited only to a relatively simple algorithms. And, even if we consider a hypothetical scenario where quantum and regular computers may communicate with each other (we certainly would like to have that), we could (and probably will) need tools to check if the computer is a quantum computer and if it performs its functions correctly.

This is the idea of work accomplished by team of scientists from University of Vienna (Austria), Singapore University of Technology and Design, National University of Singapore and University of Edinburgh. Research presented in a paper titled ‘Experimental verification of quantum computers’ presents a prototype protocol which could be used to verify the correctness of operation of such systems.

Authors describe the problem from practical point of view: it is possible to simulate small-scale quantum systems (including data processing-related functions or behavior of fundamental particles) using typical computers, although the processing power required to accomplish that is rather enormous, since quantum processors are inherently able to calculate all the possible combinations of the same experiment at once, thus increasing the computing power by several orders of magnitude in comparison to typical processors. So how do we check the performance of slightly more complex quantum system, especially when there is an obvious chance that there could be no tool suitable to do that?

Other scientists have discussed similar ideas previously, and their solutions included ideas to use a range of quantum resources, such as quantum memory or entangled quantum processors. In the current work, team presented a verification method suitable to verify larger quantum systems using smaller-scale quantum processors. Their approach consists of so-called interactive proof system based on ‘blind’ quantum computing. In essence, a verification device can be a ‘classical’ computer-based system, which should at least be able to present quantum computer with quantum bits – or qubits – with randomly-chosen initial physical states that are known only to the sender.

Qubits then are transmitted to the quantum computer under test, and quantum computations are made there without knowing anything about the physical states of qubits. Later the verification device obtains the information about the results of computation from the tested system. Thus, with the initial knowledge of qubits used in verification procedure the verificator can pack significantly less resources compared to the tested system.

Authors of the research also described their practical experiments involving verification of four-qubit quantum system. According to them, the developed protocol is suitable for any experimental quantum-computation platform and also can be scaled to more complex sets of qubits.

And, who knows: perhaps someday this principle will be used to test even our own personal quantum computers…

By Alius Noreika, source:

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