Optical Control of Chiral Superconductors

The use of strong electro­magnetic radiation to push materials out of their normal equilibrium is a new frontier in this field. Such mani­pulation leads to the emergence of new phases with novel, useful and control­lable properties. Now, a team of scientists in the USA and Germany has shown that tailored laser pulses can be used to control the properties of chiral topo­logical super­conductors.

Switching of a two-component chiral order parameter represented on the Bloch sphere. (Source: M. Claassen, MPSD / NPG)

Chiral topological super­conductors are a particular class which hosts a Majorana fermion. This can be used to encode quantum bits and perform error-resilient computation. However, controlling and manipulating these emergent properties poses a significant challenge. Funda­mentally, the chiral topo­logical nature of these materials relies on the rotation and reflection symmetries of the crystal lattice to maintain a subtle balance between competing super­conducting states. The researchers from the Flatiron Institute’s Center for Compu­tational Quantum Physics (CCQ) in New York City, Freie Universität Berlin and the Max Planck Institute for the Structure and Dynamics of Matter found that a weak pulse can disrupt this balance and induce a dramatic change in the underlying electronic order. This occurs because the pulse selec­tively breaks these symmetries via choice of polari­zation.

In particular, the team showed numeri­cally that an appro­priately-tuned pulse sequence can selectively reverse the chirally-super­conducting region on a very fast time scale on the order of picoseconds. This handedness is an intrinsic topo­logical property of such materials and sets the propa­gation direction – clockwise or counter-clockwise – of Majorana fermions that are induced along its boundary.

An intriguing consequence of their work is the possi­bility to optically program topo­logically-protected quantum circuits, to perform compu­tation on the charging states of single electrons injected into the Majorana boundary modes. Further­more, the underlying mechanism is robust and relies solely on symmetry, not on the materials’ details. It could be applied to any material with multi-component order parameters.

The scientists predict that topo­logical super­conductivity can be detected in time-resolved pump-probe experiments where an initial laser pulse alters the super­conducting state in the material and a second reads these changes after a short delay. This establishes pump-probe experiments as a new experi­mental tool to reveal the putative chiral topo­logical nature of super­conductivity in a wide array of candidate materials such as twisted bilayer graphene and others. (Source: MPSD)

Reference: M. Claassen et al.: Universal optical control of chiral superconductors and Majorana modes, Nat. Phys., online 27. Mai 2019; DOI: 10.1038/s41567-019-0532-6

Link: Theory of Pump-Probe Spectroscopy, Max-Planck-Institute für Structure und Dynamics of Matter, Hamburg, Germany

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