Just when it was looking like the underdog, classical computing is striking back. IBM has come up with a way to simulate quantum computers that have 56 quantum bits, or qubits, on a non-quantum supercomputer – a task previously thought to be impossible. The feat moves the goalposts in the fight for quantum supremacy, the effort to outstrip classical computers using quantum ones.

It used to be widely accepted that a classical computer cannot simulate more than 49 qubits because of memory limitations. The memory required for simulations increases exponentially with each additional qubit.

The closest anyone had come to putting the 49-qubit limit to a test was a 45-qubit simulation at the Swiss Federal Institute of Technology in Zurich, which needed 500 terabytes of memory. IBM’s new simulation upends the assumption by simulating 56 qubits with only 4.5 terabytes.

The simulation is based on a mathematical trick that allows a more compact numerical representation of different arrangements of qubits, known as quantum states.

A quantum computing operation is typically represented by a table of numbers indicating what should be done to each qubit to produce a new quantum state. Instead, researchers at IBM’s T. J. Watson Research Center in Yorktown Heights, New York, used tensors – effectively multidimensional tables augmented with axes beyond rows and columns.

Thanks to the additional axes, much more information can be squashed into a few tensors, so long as we know how to write it down in the language of tensors. The researchers found a way to do just that for quantum computing operations.

**Embarrassingly parallel**

While writing down the operations in tensor form, they also found out a way to divide the simulation task into what they call “embarrassingly parallel” chunks, which allowed them to use the many processors of a supercomputer simultaneously. This won them the final bit of efficiency needed to simulate a 56-qubit quantum computer.

“IBM pushed the envelope,” says Itay Hen at the University of Southern California. “It’s going to be much harder for quantum-device people to exhibit [quantum] supremacy.”

IBM now has a functional 56-qubit quantum computer living in their supercomputer. But while that’s an improvement on the previous record, Andrew Childs at the University of Maryland says it’s not a huge leap forward. “I don’t think they’re claiming that this is going to give them an efficient simulation of quantum systems on a classical computer,” he says.

Even so, they’ve upped the ante in the race to outperform classical computers with quantum systems. Google previously said they were on track to build a working 49-qubit processor by the end of 2017, but that will no longer win them the achievement of quantum supremacy.

Bob Wisnieff, the principal investigator of the IBM study, says their current simulation runs about “a billion times slower” than the theoretical estimates for an actual 56-qubit quantum computer.

Wisnieff’s team plans to experiment with supercomputers whose processors can communicate efficiently with one another. They expect to be able to squeeze out a few more qubits from these communication channels, which help speed up the parallel computation needed for the simulation.

IBM’s goal is to build a quantum computer that can “explore practical problems” such as quantum chemistry, says Wisnieff. He hopes to check the accuracy of quantum computers against his simulations before putting real quantum computers to the test.

“I want to be able to write algorithms that I know the answers for before I run them on a real quantum computer,” he says.