Thread by @theeczoo

Chetan Nayak overviews unpublished results by @MSFTResearch on Majorana qubits.

The goal is to build Majorana wire qubits and add a layer of qec on top. All computation would be measurement based.

One can induce topological superconductivity in a semiconductor wire by opening a gap and cranking a magnetic field.

But disorder changes the phase diagram substantially, see right picture vs previous photo.

As a result, the device has to be as clean as possible. Scattering length has to be longer than the length of the Cooper pairs. "Considerable amount of development over the years."

Aluminum quality has improved, as evident by photos on right over the years.

These improvements culminate in localization lengths over a micron, which they believe is suitable for finding a topological phase.

To find the phase, there is a topological gap protocol with criteria on the left of photo. "Can't be 100% certain from transport data" so stat analysis (4th point) has to be performed to yield confidence estimates of whether this is disorder or a true topological phase.

As devices get better, more and more pass the protocol (right table). The exptal phase diagram is also shown.

Once in the topological region, there is a way to readout fermion parity via tunneling that is sensitive to it.

Here is a simulation of capacitance vs flux of interferometer setup, with two curves representing the two parities.

The capacitance is measured via a simple RLC circuit. It's value is taken from a distribution because of noise. But it does seem to depend on parity.

And periodicity is observed as well. Here is comparison to simulation.

While in trivial phase there is no periodicity observed.

Recent designs include two such wires for a true qubit of fixed parity.

Here is the labeled STM photo of the tetron device. errorcorrectionzoo.org/c/mbq

Key result: red region is the topological phase of each wire, and both wires are observed to be in said phase.

To define a qubit, X and Z measurements are performed natively.

Z measurement shows jumps and seems clear.

X measurement is messier and requires auto-correlation analysis. Telegraph signal is claimed.

Processing the data and classifying in a binary way yields jumps, just like with the Z measurement.

Measurements and their associated noise processes differ substantially, justifying the differences between their data.

Eight qubit device is being tested right now.

@Dr_Chetan_Nayak @MSFTQuantum