Quantum computing; a whole new field of bewilderment

At primary school in London I was taught about different number bases, concentrating on binary arithmetic. This was part of an attempt in the 1960s to bring mathematics up to date. The introduction of binary to the curriculum was prompted by the realisation that computers would become hugely more significant. It was fascinating to realise that there was nothing inevitable or sacrosanct about using base 10 and I enjoyed it all.

When I started working in IT I quickly picked up the technical side. I understood binary and with a bit of work, keeping a clear head, everything made sense. It was mostly based on Boolean logic. Everything was true or false, on or off, pass or fail, 1 or 0.

Now this simple state of affairs applied only to the technology. When you started to deal with humans, with organisations, with messy social reality it was important to step back from a simple binary worldview. You can’t develop worthwhile applications, or test them, if you assume that the job simply requires stepping through a process that is akin to a computer program, with every decision being a straightforward binary, yes/no, pass/fail. I’ve talked about that at length elsewhere, eg here “binary opinions – yes or no?“.

Recently I’ve been reading about quantum computing. I’d vaguely known about the subject before. I knew it was a radically different approach to computing, but I hadn’t thought through just how radical the differences are, or the implications for testing. It’s not just testing of course. When a completely new form of computing comes on the scene, one that leaves all our expectations, assumptions and beliefs in tatters, there will be huge ramifications throughout IT. Traditional computers won’t vanish; there will still be plenty of jobs working with them. People will continue to specialise in specific areas of computing. The problem for testers will be that quantum computers are going to be used for new applications, to do things that were previously impossible, and traditional testing techniques won’t necessarily be readily transferable.

What’s so different about quantum computing?

I’m not going to try and provide an introduction to quantum computers. If you think you understand them and you haven’t done some serious study of quantum physics then you’re kidding yourself. And as Richard Feynman said;


If a primary school introduction to binary arithmetic allowed me to get to grips with traditional, digital computers, then the equivalent for quantum computers would be at least an undergraduate degree in physics. Here is a good overview.

I’m just going to run through three features of quantum computers that make my head spin, that have persuaded me that everything I knew about computers will be useless and I will be as well prepared as a peasant trying to get to grips with Excel after stumbling through a time portal from the Middle Ages.

First, instead of bits we have qubits. Bits can be 0 or 1. Once set to a value they don’t change until we perform some operation. The possible states of a qubit are excited or relaxed, which could be considered equivalent to 0 or 1. The fundamental difference is that they can be both at the same time, or rather a mixture of the two. This property is superposition. However, when the qubit is measured it will be frozen as one or the other.

The second weird feature of qubits is that different qubits influence each other. In traditional computing if we operate on a bit then no other bits will change. If they do it’s because the logic of the program controls these changes. In quantum computing that doesn’t apply. Changing a qubit will cause other qubits to change. Qubits are not independent of each other; they are all linked to each other. This is called entanglement.

The third weird thing about quantum computing is that algorithms are probabilistic, not deterministic. A classical digital computer will run through an algorithm and produce the right answer, or rather it will produce a predictable answer that is consistent with the algorithm’s logic. A quantum computer won’t always give the right answer straight away. It might have to send the same input through the same process many times and the results should converge on the right answer, probably.

I’m telling you this not because I think it will give you any sort of useful understanding of quantum computing. A spot of humility is in order. I pass on this information in the hope you realise you have as little idea as I do.

In summary, quantum computing is moving way beyond dear old familiar Boolean logic. Boolean algebra is still valid, but it is only relevant to one part of a wider reality and quantum computers will allow us to explore beyond the Boolean boundaries.

I have done my share of white box testing, delving into the guts of applications to understand what is going on. I know I will never be in a position to do that with quantum computers, even assuming that they are in widespread use before I retire. I doubt if there are many, if any testers, who will be better placed than I am.

What will quantum computers be used for?

If white box testing poses a tough challenge what about black box testing? As I said earlier, quantum computers will be used for problems that traditional computers can’t help with. One application will be the transformation of cryptography. Quantum computing will probably render existing cryptographing techniques obsolete. Other likely uses will be new ways to search and interrogate massive databases, and financial modelling that requires fiendishly intricate calculations on huge volumes of data. These are the applications that caught my eye, because of my background in financial services. A big problem for testers will be, how do you evaluate the output if the computer is doing something that can’t be replicated by other means? This isn’t a new problem, but quantum computers will massively increase the difficulty.

Blind computing guiding the blind?

I wrote about the problems I’d encountered working with big data a few years ago. Our approach could be summed up simply as;

  • build a deep understanding of the domain and the data, and the essential relationships or rules,
  • get the business or domain experts heavily involved and pick their brains,
  • ensure you influence the design so that it is testable, ideally so that the solution enables testing of separate segments which can be isolated so you can bring your acquired knowledge and understanding to bear.

The first two points will always be relevant regardless of the technology. The third one has been picked up by quantum computing scientists who have developed a technique called blind quantum computing.

The blind technique involves running a separate, simpler(!), quantum computer called a verifier alongside the server running the test application. The verifier feeds the server with small, discreet calculations to perform. It does this in a way so that only the verifier, and the testers, can know what the answer should be, and the server knows only what operations it should perform. This is the simplest explanation of blind quantum computing I’ve been able to find.

An obvious observation is that this is hardly black box testing as we have known it. Who programs and controls the verifier? It is a quantum computer, and I doubt if ISTQB accreditation will ever get you very far here.

Fields of bewilderment

I don’t have any answers here, and I strongly suspect easy answers will never be available to testers who work with quantum computers. That, perhaps, is the point to take away from my article. Writing this has felt less like trying to spread some understanding of an immensely difficult subject, and more like an account of a bewildered ramble through my freshly expanded fields of ignorance.

I can’t state with confidence that software testers will ever move into quantum computing. Perhaps the serious testing will be done by quantum computing scientists. That raises its own dangers. Will they have the right instincts, an appropriate sceptical and questioning approach? There is already talk of quantum computers permitting the development of perfect software. Apparently they will be able to work their way through every conceivable permutation of data to establish that the application was built to its specification. Of course there is a huge, familiar old question being begged there; how does anyone know the spec was right?
Opening of the poem 'Youth and Hope' from which the quote 'perplexity is the beginning of knowledge' comes

I worry that good testers may be frozen out of working with quantum computers, and that the skills and questions they do have, and that will remain relevant, will be lost. Whatever happens I am confident that testers imbued with the principles of Context-Driven Testing will be better prepared than those who know only what they learned to pass ISTQB exams, or who are accustomed to following prescriptive standards. Whatever the future holds for us it will be interesting! After all, perplexity is the beginning of knowledge.


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