Synopsis: Toward Topological Protection with Qubits

A chain of superconducting qubits reproduces two key features of topological insulators.
Synopsis figure
W. Cai et al., Phys. Rev. Lett. (2019)

Reliable quantum computation will require devices that can protect fragile quantum states from environmental disturbances. To that end, some researchers have pinned their hopes on topological insulators, materials that intrinsically protect their novel electrical behavior in the face of external perturbations. However, no one has yet engineered a topological insulator in an ensemble of qubits, the quantum equivalent of digital bits. Now, Luyan Sun of Tsinghua University in Beijing and collaborators have done just that by crafting a string of qubits in which topological protection can be switched on and off at will.

The team fabricated a chain of qubits out of superconducting circuits. In the device’s default state, a spin-excitation (magnon) generated in one qubit would propagate back and forth along the chain. By tuning local magnetic fluxes to adjust the relative coupling strength between the qubits, however, the team engineered a topological magnon insulator that sustained the excitation and constrained it to a single qubit.

By observing the magnon dynamics over time, the team was able to characterize two hallmarks of any topological insulator: the winding number, which is a parameter in momentum space that does not change if the system is deformed, and edge states, in which qubit excitations crowd together at the boundaries of the system. Each has been seen before in other qubit systems but never simultaneously. The team says that these results show that superconducting qubit chains can not only be easily engineered to have topological protection but can also provide a flexible platform for investigating a variety of topological behaviors.

This research is published in Physical Review Letters.

–Christopher Crockett

Christopher Crockett is a freelance writer based in Arlington, Virginia.


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