Topological Properties Protect Quantum Memory, Computing

Theoretical physicists have formulated a new concept for engineering so-called topological states of matter in quantum mechanical many-body systems. They linked concepts of quantum optics and condensed matter physics that eventually could lead to a quantum computer that is immune against perturbations.

Three years ago, a research team led by Sebastian Diehl and Peter Zoller of the University of Innsbruck presented a completely new approach to engineering quantum states in many-body systems. They used a physical phenomenon that normally dramatically increases the degree of disorder in a system: dissipation. In classical physics, dissipation is the concept that explains the production of heat through friction. Surprisingly, in quantum physics, dissipation can also lead to order, and a completely pure many-body state can be realized.

This spring, an Innsbruck research team, led by physicist Rainer Blatt, demonstrated experimentally that by using dissipation, certain quantum effects can be generated and intensified. By linking concepts of quantum optics and condensed matter physics, physicists from the Institute of Theoretical Physics of the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences have pointed out a new direction for using dissipation in a beneficial way.

In condensed matter physics, a new concept describing order in many-body systems has gained in importance recently: topological order. Two examples of topological phenomena are the quantum Hall effect, which was demonstrated in the 1980s, and the topological insulator, which behaves as an electrical insulator in its interior while permitting the transport of charges on its surface. Diehl and Zoller's team now suggests realizing dissipation-induced Majorana fermions in a quantum system. This topological phenomenon was named after the Italian physicist Ettore Majorana and describes particles that are their own anti-particles.

“We show a new way of how Majorana fermions may be created in a controlled way in a quantum system,” said Diehl. “For this purpose, we use a dissipative dynamic that drives the system into this state in a targeted way and compels it back when affected by disturbances.”

Majorana fermions are generated at both ends of the atomic chain. (Image: H. Ritsch)

With this new approach, Diehl and his team combine the advantages of dissipation and topological order; both concepts are highly robust against perturbations such as disorder. Therefore, their suggestion to create Majorana fermions in an atomic quantum wire is of high interest for experimental implementation. It may be used for building a quantum computer whose basic building blocks consist of Majorana fermions. In quantum wires, atoms are confined to one-dimensional structures by optical lattices that are generated by laser beams. Majorana fermions are then generated at both ends of the atomic chain.

“We work at the interface between [condensed matter physics and quantum mechanics], which creates exciting new possibilities,” said Diehl. First, however, they had to prove beyond all doubt that the concept of topological order can be transferred to a dissipative context at all. “We were able to tick off all points on the topological checklist and show that its prerequisites are also valid in a system with dissipative dynamics.”

The scientists published their work in the journal Nature Physics.

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