IBM quantum system outpaces classic computer in real-world demo

1 Jul 2021

A 3D illustration of a quantum computer. Image: © Guajillo studio/

IBM researchers have demonstrated a new kind of quantum advantage.

As IBM’s quantum computers are being set up across the world, the tech company has now set its sights on proving what these machines can do.

In a paper published in Nature Physics, IBM quantum scientists set out to demonstrate that there are functions a classical computer cannot compute, but that a quantum computer can.

They reported a simultaneous proof and experimental verification of a new kind of quantum advantage. “Specifically, we show that qubits, even today’s noisy qubits, offer more value than bits as a medium of storage during computations,” they wrote in an accompanying blog.

A central feature of quantum computing is that unlike a classical computer that uses binary bits, which can be either one or zero, a quantum computer uses quantum bits, or qubits, which can be one, zero or both at the same time.

The IBM researchers explored the power and limitations of both classical and quantum technologies by engaging in what they called “limited space computations”.

“These are circuits restricted to using two-input gates, and limited to using one bit of computational or scrap space,” they wrote.

“This restriction allows us to establish a fair comparison between the power of classical and quantum computational space, and in particular demonstrate a fundamental advantage of computing with qubits over classical bits.”

The systems were tasked with finding the majority of three bits, returning zero if more than half of the bits were zero and returning one if more than half the bits were one.

In a theoretical scenario, the researchers calculated an accuracy of 100pc for their noiseless quantum system, whereas a classical computer would only succeed 87.5pc of the time.

However, today’s real-life quantum computers are noisy, so the task was repeated after introducing noise into the quantum system. In this scenario, it returned a 93pc accuracy.

“This result demonstrates that a noisy quantum computer with as few as four qubits – three input qubits to encode the input; one qubit to use as computational or scrap space – can perform calculations inaccessible to a perfect classical computer,” the researchers wrote.

They added that the result seems to break ‘Holevo’s boundary’, a theorem that states one qubit can only store one bit of information. “But our paper demonstrates that you can store much more than one bit as the intermediate results of a computation.”

While it’s still early days for the technology, this computational proof and experimental verification give hope for the possibility of solving computational problems far beyond the reach of traditional computers.

Sam Cox was a journalist at Silicon Republic covering sci-tech news