New Protocol Allows a Quantum Computer to Check its Own Answer to Difficult Problems

A novel test was recently performed to assess whether a quantum computer can provide accurate answers to queries that are beyond the capabilities of conventional computing. Such a test can result in the development of the first quantum computer that would supersede a traditional computer.

A research team from the University of Warwick has developed a new protocol that enables a quantum computer to verity its own answers to challenging issues. With this protocol, the researchers have developed a new way to validate whether a quantum computer is operating properly without using too many resources.

This issue was recently addressed by Samuele Ferracin, Theodoros Kapourniotis, and Dr Animesh Datta from the Department of Physics of the University of Warwick, in a study published in The New Journal of Physics on November 18th, 2019.

The scientists have created a protocol to determine how noise influences the outputs of quantum computers. Noise can be described as anything that impacts the hardware of a quantum machine but is beyond the scope of a user’s control, for example, flaws in the fabrication or variations in temperatures. This can impact the precision of the result produced by a quantum computer.

When the investigators’ test was applied, it created two percentages—how close it predicts the quantum computer is to the right result and how assured a user can be of that closeness.

Such a test will assist the developers of quantum computers to find out whether their quantum machine is working accurately to further improve their performance—an important step in establishing the utility of quantum computing in the days to come.

A quantum computer is only useful if it does two things: first, that it solves a difficult problem; the second, which I think is less appreciated, is that it solves the hard problem correctly. If it solves it incorrectly, we had no way of finding out. So what our paper provides is a way of deciding how close the outcome of a computation is to being correct.

Dr Animesh Datta, Department of Physics, University of Warwick

It is very difficult to determine whether a quantum computer has generated the right answer to a challenging issue because, by definition, such issues are beyond the capability of existing traditional computers.

Checking whether the answer produced by the quantum computer is correct would normally require a large number of conventional computers to deal with the problem. However, it is not possible to achieve this because these computers deal with ever more complicated problems.

Therefore, the scientists have suggested another technique in which the quantum computer is used to run several easy calculations with already known answers and precision of those results can be subsequently established.

Using this technique, the scientists can then create a statistical boundary on how far the quantum computer is from the right answer in the challenging problem that the researchers require this machine to answer, called the target computation.

It is analogous to the process used by computer programmers to check massive computer programs, by adding in minor functions with known answers. If the program answers a sufficient number of these functions correctly, the researchers can be confident that the entire program is accurate.

The whole point of having a quantum computer is to not spend an exponential amount of time-solving problems, so taking an exponential amount of time to check whether it’s correct or not defeats the point of it. So our method is efficient in that it doesn’t require an exponential amount of resources.

Dr Animesh Datta, Department of Physics, University of Warwick

Datta continued, “We do not need a classical computer to check our quantum computer. Our method is self-contained within a quantum system that can be used independently of large servers.”

Samuele Ferracin, the study lead author, has been creating ways for researchers working on quantum computers to integrate the test into their work.

We have spent the last few years thinking about new methods to check the answers of quantum computers and proposing them to experimentalists. The first methods turned out to be too demanding for the existing quantum computers, which can only implement ‘small’ computations and perform restricted tasks.

Samuele Ferracin, Study Lead Author, Department of Physics, University of Warwick

Ferracin continued, “With our latest work we have successfully developed a method that suits existing quantum computers and encompasses all their main limitations. We are now collaborating with experimentalists to understand how it performs on a real machine.”

By harnessing the extraordinary characteristics of quantum physics, quantum computing processes data in an entirely different manner to traditional computers. Leveraging the behavior of quantum systems, like those existing in various different states simultaneously, this innovative form of computing is developed to simultaneously process information in all of those states, thus providing it with a major advantage over traditional computing.

Some kinds of challenges, like those seen in chemistry and in codebreaking, are specifically appropriate for manipulating this trait.

Unparalleled experimental advancements have been made in the last few years. The size of the biggest quantum computers is doubling every six months, and these computers currently appear to be extremely close to achieving quantum supremacy.

Quantum supremacy can be described as a milestone in the advancement of quantum computers, where a quantum computer initially executes a function that would need an irrationally large amount of time using a traditional computer.

Dr Datta added, “What we are interested in is designing or identifying ways of using these quantum machines to solve hard problems in physics and chemistry, to design new chemicals and materials, or identify materials with interesting or exotic properties. And that is why we are particularly interested in the correctness of the computation.”

The study was supported by the Engineering and Physical Sciences Research Council, part of UK Research and Innovation, in the United Kingdom.

The study titled “Accrediting outputs of noisy intermediate-scale quantum computing devices” has been published in the New Journal of Physics.


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