March 15, 2022 — Microsoft announced it has demonstrated the underlying physics required to create a new kind of qubit. The following is a blog post from Microsoft Distinguished Engineer Dr. Chetan Nayak explaining the details.

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Quantum computing promises to help us solve some of humanity’s greatest challenges. Yet as an industry, we are still in the early days of discovering what’s possible. Today’s quantum computers are enabling researchers to do interesting work. However, these researchers often find themselves limited by the inadequate scale of these systems and are eager to do more. Today’s quantum computers are based on a variety of qubit types, but none so far have been able to scale to enough qubits to fully realize the promise of quantum.

Microsoft is taking a more challenging, but ultimately more promising approach to scaled quantum computing with topological qubits that are theorized to be inherently more stable than qubits produced with existing methods without sacrificing size or speed. We have discovered that we can produce the topological superconducting phase and its concomitant Majorana zero modes, clearing a significant hurdle toward building a scaled quantum machine. The explanation of our work and methods below shows that the underlying physics behind a topological qubit are sound—the observation of a 30 μeV topological gap is a first in this work, and one that lays groundwork for the potential future of topological quantum computing. While engineering challenges remain, this discovery proves out a fundamental building block for our approach to a scaled quantum computer and puts Microsoft on the path to deliver a quantum machine in Azure that will help solve some of the world’s toughest problems.

Dr. Chetan Nayak and Dr. Sankar Das Sarma recently sat down to discuss these results and why they matter in the video below. Learn more about our journey and visit Azure Quantum to get started with quantum computing today.

Microsoft Quantum Team Reports Observation of a 30 μeV Topological Gap in Indium Arsenide-Aluminum Heterostructures

Topological quantum computation is a route to hardware-level fault tolerance, potentially enabling a quantum computing system with high fidelity qubits, fast gate operations, and a single module architecture. The fidelity, speed, and size of a topological qubit is controlled by a characteristic energy called the topological gap. This path is only open if one can reliably produce a topological phase of matter and experimentally verify that the sub-components of a qubit are in a topological phase (and ready for quantum information processing). Doing so is not trivial because topological phases are characterized by the long-ranged entanglement of their ground states, which is not readily accessible to conventional experimental probes.

This difficulty was addressed by the “topological gap protocol” (TGP), which our team set forth a year ago as a criterion for identifying the topological phase with quantum transport measurements. Topological superconducting wires have Majorana zero modes at their ends. There is a real fermionic operator localized at each end of the wire, analogous to the real fermionic wave equation constructed by Ettore Majorana in 1937.

Consequently, there are two quantum states of opposite fermion parity that can only be measured through a phase-coherent probe coupled to both ends. In electrical measurements, the Majorana zero modes (see Figure 1) cause zero-bias peaks (ZBPs) in the local conductance. However, local Andreev bound states and disorder can also cause zero-bias peaks. Thus, the TGP focuses on ZBPs that are highly stable and, crucially, uses the non-local conductance to detect a bulk phase transition. Such a transition must be present at the boundary between the trivial superconducting phase and the topological phase because these are two distinct phases of matter, as different as water and ice.