Quantum hyperbole

In 2019, researchers from Google announced that their quantum computer Sycamore had performed a task that was beyond the abilities of the most powerful classical computers of the day. The much-anticipated milestone, termed 'quantum supremacy', is also the title of a book by theoretical physicist and prolific science communicator Michio Kaku.

Quantum computers work in a fundamentally different way compared to classical computers. Where n classical bits hold n bits of information, the quantum analogue, n qubits, exists in a superposition of 2ⁿ states, thanks to its quantum mechanical properties. This is the source of quantum computing's immense power. However, it is not simple to harness. The engineering effort required to build a working quantum computer is enormous and painstaking. Sycamore has 53 qubits, and the most powerful quantum machine existing today has a few hundred qubits. This needs to be scaled up by a factor of a thousand or more to realise much of the real-world potential of quantum computing.

Further, quantum algorithms need an entirely new approach to coax the required answers from a plethora of states. For most computing tasks, quantum computers are only somewhat more efficient. But for some kinds of problems – scheduling, computational chemistry, simulating quantum systems, factoring large numbers – they are dramatically faster than anything we have. And these happen to be problems with far-reaching consequences for us.

Kaku goes into all of this in the first third of Quantum Supremacy. While he does touch upon the challenges and limitations of quantum computing, they are easy to miss, given his almost rapturous excitement about the field. Kaku is at his best while providing a breezy and accessible introduction to how quantum computing evolved. He traces the development of ideas about computers and computing from Charles Babbage to Kurt Gödel to Alan Turing. And he explains in broad strokes how the work of scientists such as Albert Einstein, Max Planck, Erwin Schrödinger and Werner Heisenberg, and their interactions, shaped quantum mechanics and our understanding of matter and energy at the smallest scales. Richard Feynman brought those two strands together and proposed the idea of a quantum computer in 1981. A few years later, David Deutsch went on to conceptualise it.

Kaku's efforts to provide simple explanations sometimes feel simplistic or overly sweeping. The quantum world is famously unintuitive, but even by those standards it's puzzling to read that "In quantum theory, before you look at a tree it can exist in all possible states, such as firewood, lumber, ash, toothpicks, a house, or sawdust."

The bulk of Quantum Supremacy, two-thirds of it, is about the potential revolutionary applications of quantum computing. Here, Kaku writes as a futurist, a role he often plays in his books and in media appearances.

Quantum computers may one day be particularly effective at modelling chemical reactions. So, Kaku tells us, they may help with discovering the origins of life itself. Or they may lead to developing better mechanisms for fertilisers and spark a new green revolution that sustains even more people on Earth. Or lead to the manufacture of significantly improved batteries, which could help reduce carbon emissions and mitigate the climate crisis. Almost every current scientific challenge features in these pages with quantum computers coming in to address them: cancer, Alzheimer's, Parkinson's, fusion reactors. There's even a chapter titled Immortality.

All this sounds intriguing, except that Kaku spends most of these sections explaining the significance of the problem, only bringing up quantum computing towards the end as a kind of magic bullet. Inevitably for a book like this at the current time, there's a bit on pandemics. It ends, "So in the future, a network of sensors placed indoors might be able to detect aerosols in the air and then send the results to quantum computers, which can analyze this vast pool of information to find the early warning signs of the next pandemic." It's not clear at all that such a system is impossible without quantum computers. And many of these scenarios could be invoked for just about any other technology we're excited about. After all, those sensors could use nanotechnology. And the analysis might benefit from artificial intelligence or machine learning.

There's still some way to go before the promises of quantum computing are realised. But progress has been fast and the area has been attracting considerable investment. Quantum Supremacy may be an ideal read for someone interested in quantum computing from the business perspective. It evokes the science without getting into too much detail, and it paints an engaging picture of how quantum computing might change our world.

Srinath Perur writes on science, travel and culture.