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Sunday, June 28, 2026

The Topological Gap Protocol: A Test Built to Not Fool Itself

How Microsoft tries to test whether a chip hosts an exotic, error-resistant particle, without fooling itself the way this field has before.

What the protocol is

The topological gap protocolMicrosoft's pre-registered pass/fail test for deciding whether a device has truly entered a topological state hosting Majorana modes, built to reject look-alike false signals. Its reliability is still debated. is a pass/fail test. Microsoft wrote it to settle one question about a chip: has this device entered a topological state, the strange phase of matter supposed to hold Majorana zero modesAn exotic, hard-to-create quantum state that Microsoft's topological qubit depends on. Its existence in these devices is still scientifically disputed., the building blocks of its qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once.? The protocol fixes the criteria in advance, runs them on the data, and returns a verdict. Pass, or fail.1

For years, seeing the signal was the claim. The protocol exists because the signal was never enough.

That sounds bureaucratic for a physics experiment, and it is, on purpose. For years the field looked for a signal: a bump in the reading at zero voltage that a Majorana should produce. The trouble is that dull, ordinary flaws in the wire produce the same bump, so the signal alone was never proof.1 Earlier claims rested on exactly that thin evidence and did not survive scrutiny.2

How the test works

The device is a hybrid: a thin semiconductor nanowireAn extremely thin wire, billionths of a meter wide, used as the body of Microsoft's qubit device. laid against an ordinary superconductor, which lends the wire superconductivity it would not have alone, the proximity effectWhen a superconductor placed against another material lends it superconducting behaviour it would not have on its own..1 Older devices had two electrical contacts; this one has three, and that extra terminal is the trick. With three contacts you can measure not just what happens at each end of the wire, the local conductance, but what crosses its middle, the non-local conductance.

A real topological phase announces itself in a particular way. As you sweep the magnetic field and the gate voltages, the energy gap in the bulk of the wire closes and then reopens, and that reopening must line up, at the same settings, with steady zero-voltage signals at both ends.2 Demanding both at once is meant to reduce false positives from trivial Andreev bound statesA harmless zero-energy state caused by ordinary disorder in the wire that can mimic a Majorana signal. It is exactly the false positive the protocol is built to reject., the look-alikes that disorder can throw off.

A three-terminal nanowire device on a superconductor. Below it three small graphs: a zero-voltage peak at the left end, the bulk energy gap closing and reopening in the middle, and a zero-voltage peak at the right end. A red PASS stamp marks that the device passes only when all three signatures agree.
Figure 1: The protocol’s logic. A device passes only when the end signals and the bulk gap closing then reopening line up together, not from a single signature alone.

The numbers, and why it is hard

When Microsoft ran it, the devices that passed showed a topological gap of 20 to 60 microelectronvolts, a minuscule energy, over a finite range of magnetic fields and gate settings.2 Getting there is a materials problem more than a physics one: a single charged impurity at the wrong interface can erase the phase, so the wires demand exceptionally pure material, with electron mobilities above 60,000 in the usual units.2

Passing the protocol is a gate, not a finish line.

On top of devices screened this way, Microsoft later reported reading a device’s parityWhether the number of particles in a system is even or odd. It is the quantity Microsoft's qubit reads out. in a single shot, with about a 1 percent error.3 I will be honest about where this sits: the result is contested. A published Matters Arising article challenged the interpretation, and Microsoft published a reply defending it. The existence and robustness of these Majorana modes in the devices is still not settled science.3

What the verdict actually buys

Here is why a protocol most investors will never read still matters. Every other kind of qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once. fights noise by catching and fixing errors after they happen. A topological qubitA theorized qubit that stores information in a special, spread-out quantum state, making it naturally resistant to errors. Microsoft's bet. would instead store its information non-locally, smeared across non-Abelian anyonsExotic particle-like objects whose record of being moved around one another changes the system's state, which is how a topological qubit would store and process information., so local noise has nothing to grab.4 If it works, the prize is a much cheaper path to fault toleranceThe milestone where a quantum computer can run long calculations correctly despite ongoing errors. It is the field's holy grail.. That is Microsoft’s whole bet, and this protocol is the gate it has to pass first.

So I read a passing grade for what it is. It says the device passed Microsoft’s stated test for the right phase. It does not hand you a working qubit, let alone the braidingMoving defects in a 2D code around one another so the path itself performs a protected logical operation. of those modes that real computation needs. Microsoft has built a disciplined way to avoid fooling itself; the results that would settle the question are still ahead of it, not behind.

Sources

  1. Pikulin, D. I., van Heck, B., Karzig, T. et al. “Protocol to Identify a Topological Superconducting Phase in a Three-Terminal Device.” Preprint: arXiv:2103.12217 (2021).
  2. Microsoft Quantum (Aghaee, M. et al.). “InAs-Al Hybrid Devices Passing the Topological Gap Protocol.” Physical Review B 107, 245423 (2023). DOI: 10.1103/PhysRevB.107.245423. Preprint: arXiv:2207.02472.
  3. Microsoft Quantum. “Interferometric Single-Shot Parity Measurement in InAs-Al Hybrid Devices.” Nature 638, 651–655 (2025). DOI: 10.1038/s41586-024-08445-2. The interpretation was challenged in a published Matters Arising (Legg, H. F., “On the Robustness of Topological Gap Detection via Transport,” Nature 654, E22–E26, 2026; preprint arXiv:2503.08944), with a Reply from Microsoft Quantum in the same issue.
  4. Nayak, C., Simon, S. H., Stern, A., Freedman, M. & Das Sarma, S. “Non-Abelian Anyons and Topological Quantum Computation.” Reviews of Modern Physics 80, 1083–1159 (2008). DOI: 10.1103/RevModPhys.80.1083. Preprint: arXiv:0707.1889.