From theoretical to practical

Part of Freedman’s challenge is that Microsoft isn’t just interested in building a quantum computer it can show off in a lab somewhere. Instead, the company is embarking on a plan to deliver a full-fledged topological quantum computing system. That includes everything from hardware capable of consistently running calculations that require tens of thousands of logical qubits to a complete software stack that can program and control the quantum computer.

“We’re doing everything,” Holmdahl said, “from the physics to the control plane to the software that runs the computer to the algorithms that you need to do interesting things like quantum chemistry, to the applications for personalized medicine or helping with climate change.”

Microsoft has even had a companion research project focused on cryptography and security in a post-quantum world, and has participated in industrywide efforts to prepare quantum-resistant cryptographic algorithms.

The centerpiece of Microsoft’s efforts is the topological qubit.

A dozen years ago, when Freedman came to Mundie looking for backing for his quantum computing idea, Mundie said quantum computing was in a bit of a doldrums. Although physicists had been talking about the possibility of building quantum computers for years, they were struggling to create a working qubit with high enough fidelity to be useful in building a working computer.

For researchers using physical qubits with only a minimum degree of fidelity, it requires roughly 10,000 of them to make one “logical” qubit – which is a qubit reliable enough for any truly useful computation.

The problem is that qubits are extremely finicky. If you disrupt them in even the smallest way, they “decohere,” which in layman’s terms basically means they stop being in a physical state where they can be used for computation.

Freedman had been exploring the idea that topological qubits would be more robust because their topological properties would make them more stable and provide more innate error protection. That’s because, by definition, a topological state of matter is one in which an electron can be fractionalized and appear in different places within a system.

Once the electron is divided, it’s harder for it to be disturbed because you must alter all the different places the information is being stored.

As a longtime supercomputer designer and software engineer, Mundie said he was instantly sold on the idea of a quantum bit that was both more robust and had built-in fault-tolerance. It would make the task of designing a scalable, useful machine dramatically more manageable.

“Computing itself was already transforming every sector of society and the economy,” he said. “I realized that if you could create a new class of computing that was capable of altering these fundamental building blocks, you might be able to do over again what computing has done over the past 50 or 60 years.”