In the race to build a working quantum computer, IBM has just overtaken Google, with a step researchers previously thought was impossible. The company has simulated a quantum computer with 56 qubits on a classical computer.

Simulating anything more than 49 qubits on a classical computer was thought impossible because of the limited memory classical computers have compared to their quantum counterparts. Because of the difference between classical and quantum computing, each time you add a qubit to a simulation it increases the necessary memory exponentially in the classical simulation.

But now, IBM has a simulation of a 56–quantum computer. “IBM pushed the envelope,” Itay Hen at the University of Southern California, told New Scientist. “It’s going to be much harder for quantum-device people to exhibit [quantum] supremacy.”

Not only this, but the new simulation has managed to dramatically reduce the amount of memory used in the previous efforts. A 45-qubit simulation at the Institute of Technology in Zurich revealed in July this year took 500 terabytes of memory, whereas IBM’s latest attempt took only 4.5. This is thanks to a breakthrough in thinking, related to the way quantum computers work.

In computing, information is stored in ‘bits’ in either the state 1 or 0, like a light switch either turned on or off. By contrast, in quantum computing, the unit of information can be 1 or 0, or a superposition of the two states.

Think of it like a sphere, with a 1 written at the north pole and a 0 at the south. A classical bit can be found at either pole, but a quantum bit, or qubit, could be found on any point on the surface of the sphere. Before a qubit is measured, it can be 0 and 1 at the same time, until it is measured.

“My seemingly inconsequential moment came one night while washing dishes and using a bristle brush to clean a tall glass,” said IBM’s Edwin Pednault. “It suddenly occurred to me that if one looks at the gates applied to a given qubit in a grid circuit, the gates form a bristle-brush pattern where the bristles are the entangling gates that are being applied to that qubit.”

He then took this idea and applied it to simulating qubits.

“Mathematically, that ‘bristle brush’ of gates corresponds to a tensor and the bristles to tensor indices. A tensor in mathematics essentially corresponds to an n-dimensional array in computer science.”

Basically, instead of using a number to represent a qubit, the team at IBM started to use tensors – multidimensional tables with more axes than just rows and columns. This gives them much more room to squeeze information in.

Pednault hopes this will help pave the way for functioning quantum computers.

“As device technology advances, we will move into a period of quantum advantage where a broad range of enterprises, scientists and engineers will make full use of the hardware and the power of quantum computing to continue to solve increasingly difficult and complex problems,” he says. But there will still be a need for simulations.

“During this quantum-advantage phase, advanced simulation capabilities will be needed to support both the research and development of new quantum algorithms as well as the advancement of the device technology itself.”