Microsoft Achieves Milestone in Topological Quantum Computing Quest
Microsoft has announced significant progress in its two-decade pursuit of building topological quantum bits, also known as qubits. This innovative approach aims to create quantum computers that are more stable and easier to scale up, potentially revolutionizing the field.
For years, researchers and companies have been racing to build quantum computers. These powerful machines promise to unlock unprecedented capabilities in areas such as materials simulation and discovery. A key challenge, however, is constructing systems that are both large enough and stable enough to perform complex computations reliably.
Many current quantum computing technologies, such as the superconducting qubits employed by Google and IBM, are highly susceptible to errors. This necessitates the use of extensive error correction schemes, which require a large number of extra qubits, increasing system overhead. Microsoft has been exploring an alternative, using components that are inherently more stable to reduce the need for extensive error correction.
These components, called Majorana quasiparticles, are not actual particles but rather specific behavioral patterns that emerge under particular physical conditions within certain systems. Although the research has faced setbacks, including a notable paper retraction in 2018, the Microsoft team, which has since brought this research in-house, now aims to build a fault-tolerant quantum computer containing a few thousand qubits. Furthermore, they have a blueprint for chips containing about a million qubits, a target that could unlock the full potential of these machines.
This week, the company unveiled some early accomplishments. Building upon a recent Nature paper that validates the system, Microsoft has been testing a topological qubit and has wired up a chip with eight of them. Dr. Chetan Nayak, a Microsoft technical fellow and leader of the program, stated, “You don’t get to a million qubits without a lot of blood, sweat, and tears and solving a lot of really difficult technical challenges along the way. And I do not want to understate any of that.” Dr. Nayak added, “I think that we have a path that we very much believe in, and we see a line of sight.”
Researchers not directly involved with Microsoft are cautiously optimistic. Scott Aaronson, a computer scientist and head of the Quantum Information Center at the University of Texas at Austin, observed, “I’m very glad that [this research] seems to have hit a very important milestone.” Aaronson added, “I hope that this stands, and I hope that it’s built up.”
The Foundation of Quantum Computing: Qubits
The initial step in building a quantum computer involves constructing qubits that can exist in fragile quantum states, representing a mixture of both 0s and 1s, unlike the bits in classical computers. Maintaining qubits in these complex states while also connecting them is challenging. A significant amount of research has been dedicated to developing precise error correction schemes due to noisy hardware.
For years, both theorists and experimentalists have been intrigued by the concept of topological qubits. These qubits, which use mathematical twists and turns, are designed with error protection built directly into their underlying physics. Aaronson noted, “It’s been such an appealing idea to people since the early 2000s. The only problem with it is that it requires, in a sense, creating a new state of matter that’s never been seen in nature.”
Microsoft has been working to synthesize this state, called a Majorana fermion, in the form of quasiparticles. The Majorana fermion was first proposed nearly 90 years ago as a particle that is its own antiparticle. This means that if two Majorana fermions come into contact, they annihilate each other.
Microsoft has been working to achieve the behavior of the Majorana fermion within materials by setting up the right conditions and physical setup. Over the past few years, Microsoft’s method has focused on creating a very thin wire (nanowire) from the semiconductor indium arsenide. This material is then placed close to aluminum, which becomes a superconductor near absolute zero and can be used to create superconductivity in the nanowire.
Under ordinary circumstances, there are typically no unpaired electrons in a superconductor because electrons prefer to exist in pairs. However, it is theoretically possible for an electron to hide itself under the right conditions in the nanowire, one half at each end of the wire. These complex entities, called Majorana zero modes, are difficult to destroy, making them intrinsically stable.
Dr. Sankar Das Sarma, a theoretical physicist at the University of Maryland, commented on an earlier concept, saying, “Now you can see the advantage. You cannot destroy a half electron, right? If you try to destroy a half electron, that means only a half electron is left. That’s not allowed.”
In 2023, the Microsoft team published a paper in the journal Physical Review B, stating that the system had passed a specific protocol designed to assess the presence of Majorana zero modes. This week in Nature, the researchers reported that they can “read out” the information in these nanowires – specifically, whether Majorana zero modes exist at the ends of the wires. If present, this indicates that the wire possesses an extra, unpaired electron. “What we did in the Nature paper is we showed how to measure the even or oddness,” says Nayak. “To be able to tell whether there’s 10 million or 10 million and one electrons in one of these wires.”
This represents a significant step, as the company aims to use these two states—an even or odd number of electrons in the nanowire—as the 0s and 1s in their qubits. If these quasiparticles exist, the team hopes to “braid” the Majorana zero modes in pairs of nanowires by making certain measurements in a particular way. The result would be a qubit with a combination of these two states.
Nayak indicated the team has, in fact, created a two-level quantum system. He stated a paper on the results is in preparation. Researchers outside Microsoft cannot comment on the qubit results until the paper is available. However, some have shared hopeful sentiments about the presented findings. Travis Humble, the director of the Quantum Science Center at Oak Ridge National Laboratory in Tennessee, stated, “I find it very encouraging. It is not yet enough to claim that they have created topological qubits. There’s still more work to be done there… But this is a good first step toward validating the type of protection that they hope to create.”
Others have been more skeptical. Physicist Henry Legg of the University of St Andrews in Scotland, who previously criticized a paper from 2023 for not including enough data, is not convinced that the company is seeing evidence of Majorana zero modes in its Nature paper. “The optimism is definitely there, but the science isn’t there,” he says.
A potential complication is the presence of impurities in the device, which can create conditions that mimic Majorana particles. However, Nayak says the evidence has grown as the research has progressed, stating, “This gives us confidence: We are manipulating sophisticated devices and seeing results consistent with a Majorana interpretation.”
Das Sarma, after reviewing initial results of the qubit, stated, “They have satisfied many of the necessary conditions for a Majorana qubit, but there are still a few more boxes to check. The progress has been impressive and concrete.”
Scaling Up and the Future
At first glance, Microsoft’s topological efforts may appear to lag in the world of quantum computing, since the company is only now working with qubits in the single digits. Other companies have already connected more than 1,000. Nayak and Das Sarma say the other efforts had a head start due to systems already having a solid grounding in physics. Work on the topological qubit, on the other hand, has meant starting from the ground up.
“We really were reinventing the wheel,” Nayak says, comparing the team’s efforts to the early days of semiconductors. He noted, “It’s the longest-running R&D program in Microsoft history.”
Support from the US Defense Advanced Research Projects Agency may help the company catch up. Early this month, Microsoft was one of two companies selected to continue working on the design of a scaled-up system. The other company selected is PsiQuantum, a startup aiming to build a quantum computer containing up to a million qubits using photons.
Many researchers the MIT Technology Review spoke with are eager to see further scientific publications. Das Sarma commented, “The biggest disadvantage of the topological qubit is that it’s still kind of a physics problem. If everything Microsoft is claiming today is correct … then maybe right now the physics is coming to an end, and engineering could begin.”
