The post Developing a topological qubit appeared first on Microsoft Azure Quantum Blog.

]]>The fragile nature of qubits is well-known as one of the most significant hurdles in quantum computing. Even the slightest interference can cause qubits to collapse, making the solutions we're pursuing impossible to identify because the computations cannot be completed.

Microsoft is addressing this challenge by developing a topological qubit. Topological qubits are protected from noise due to their values existing at two distinct points, making our quantum computer more robust against outside interference.This increased stability will help the quantum computer scale to complete longer, more complex computations, bringing the solutions we need within reach.

Topology is a branch of mathematics describing structures that experience physical changes such as being bent, twisted, compacted, or stretched, yet still maintain the properties of the original form. When applied to quantum computing, topological properties create a level of protection that helps a qubit retain information despite what's happening in the environment. The topological qubit achieves this extra protection in two different ways:

**Electron fractionalization.**By splitting the electron, quantum information is stored in both halves, behaving similarly to data redundancy. If one halfof the electron runs into interference, there is still enough information stored in the other half to allow the computation to continue.**Ground state degeneracy.**Topological qubits are engineered to have two ground statesknown as ground state degeneracymaking them much more resistant to environmental noise. Normally, achieving this protection isn’t feasible because there’s no way to discriminate between the two ground states. However, topological systems can use braiding or measurement to distinguish the difference, allowing them to achieve this additional protection.

Currently years into the development of the topological qubit, the journey began with a single question, “Could a topological qubit be achieved”? Working with theory as a starting point, Microsoft brought together mathematicians, computer scientists, physicists, and engineers to explore possible approaches. These experts collaborated, discussed methods, and completed countless equations to take the first steps on the path toward realizing a topological qubit.

Modeling and experimentation work hand-in-hand as an ongoing, iterative cycle, guiding the design of the topological qubit. Throughout this process, the Microsoft team explored possible materials, ways to apply control structure, and methods to stabilize the topological qubit.

A team member proposed the use of a superconductor in conjunction with a strong magnetic field to create a topological phase of matteran approach that has been adopted toward realizing the topological qubit. While bridging these properties has been long-taught,it had never been done in such a controlled way prior to this work.

To create the exact surface layer needed for the qubit, chemical compounds are currently being grown in Microsoft labs using a technique called "selective area growth." Chosen for its atomic-level precision, this unique method can be described as spraying atoms in the exact arrangement needed to achieve the properties required.

The team continues testing functional accuracy through device simulation, to ensure that every qubit will be properly tuned, characterized, and validated.

Many fields of knowledge have come together to realize the topological qubit, including mathematics, theoretical physics, solid state physics, materials science, instrumentation and measurement technology, computer science, quantum algorithms, quantum error correction, and software applications development.

Bridging these fields has led to breakthrough techniques across all aspects of realizing a topological qubit, including:

**Theory and simulation****–**Turning a vision into reality by creating a rapid design, simulation, and prototyping process**Fabrication****–**Pioneering unique fabrication approaches and finding new ways to bridge properties**Materials growth**– Developing inventive methods to create materials using special growth techniques to create the exact properties required at nanoscale**Measurement and quantum control****–**Tuning devices for accuracy in function and measurement

At Microsoft, the development of the topological qubit continues, bringing us closer to scalable quantum computing and finding solutions to some of the world's most challenging problems.

Follow along on our journey to scalable quantum computing by signing up for the Microsoft Quantum newsletter.

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We are pleased to announce that the American Physical Society (APS) journal, *Physical Review Letters*, has selected the Station Q paper, Transport Signatures of Quasiparticle Poisoning in a Majorana Island, as an Editors' Suggestion. The paper details how, working with theorists in Copenhagen, we found a way to measure the quasiparticle poisoning rate of a Majorana wire by using the strength of a shadow of the main Coulomb blockade peaks. By operating with a nearly open device, the Coulomb energy was reduced, and we could see Coulomb peak motion much more clearly than in previous studies. Microsoft is supporting this fundamental science both for its intrinsic interest and as a stepping stone toward building a topological quantum computer.

*Physical Review Letters *editors highlight only one in six submitted papers as an Editors' Suggestion, based on the paper's particular importance, innovation, and/or broad appeal. Ranked first among physics and mathematics journals by the Google Scholar five-year h-index, *Physical Review Letters* accepts fewer than one quarter of the submissions it receives. We are honored and thank *Physical Review Letters* for highlighting our work.

Read the abstract of the paper.

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]]>As its name implies, the poisoning of Majorana devices by normal electrons is fatal to topological computation, so much effort is now focused on characterizing the degree of poisoning either by the creation of quasiparticle pairs within the device, or by electrons entering the device through the leads.

A recent experiment (see https://arxiv.org/abs/1612.05748), led by Sven Albrecht and carried out at Station Q Copenhagen, demonstrates that when a Majorana device is strongly coupled to normal-metal leads, poisoning has a distinct experimental signaturea set of "shadow" Coulomb blockade diamonds, offset from the main diamonds by one electron charge from the main Coulomb diamonds associated with Cooper-pair tunneling.

Detailed theoretical modeling by Esben Hansen, Jeroen Danon, and Karsten Flensberg, also presented in the paper, was in good quantitative agreement with the experiment, and allowed the strength of the shadow peaks to be converted into a poisoning rate. The rate was measured at, and calculated at, strong tunneling, which is not where one would operate a Majorana device. Extrapolating the theory to the point where the shadows would be invisible allows a bound to be placed on the poisoning rate from the leads, even when they are more closed than in the experiment. The fact that we don't see shadows in the more closed devices means that poisoning must occur with a characteristic time of around 10 microseconds, given the parameters of these nanowire devices. We expect that the situation is much better than this conservative bound, an assumption we will test in future experiments.

Read the full paper.

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]]>The post A clear view of emerging and hybridizing Majorana zero modes using epitaxial InAs-Al nanowires appeared first on Microsoft Azure Quantum Blog.

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The first signature of Majorana physics, identified experimentally at TU Delft in 2012, focused on a characteristic conductance peak at zero voltage. It bore many signatures of Majorana zero modes, but had a sizable background signal that obscured how the peak arose out of coalescing Andreev bound states. Recently, Mingtang Deng and a Station Q Copenhagen (QDev) team considered a similar geometrynow fabricated with epitaxial semiconductor-superconductor nanowires grown by Peter Krogstrup, also of QDev.

Published in *Science* (December 2016), this new material system provides a clearer view of how Majorana zero modes emerge, adding support for the Delft interpretation and exploring new, previously unconsidered regimes, including how the Majorana zero mode hybridizes with a quantum dot at the end of the wire. The measurements, led by postdoctoral fellow Mingtang Deng, along with a team of experimentalists and theorists, are complemented by extensive numerical simulations, showing very good agreement between theory and experiment. The case for Majoranas is stronger now, and furthers our understanding of the relationship between Andreev states and Majorana zero modes in proximitized nanowires.

Read the *Science *article.

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In the paper, "Double semions in arbitrary dimension," published in *Communications in Mathematical Physics*, Michael Freedman and Matthew Hastings present a new construction of topological phases of matter in higher dimensions, generalizing the double semion theory in two dimensions. This theory is distinct from the Dijkgraaf-Witten model and generalized toric code models.

Read the published version.

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