In a paper published this week in Nature Physics, a team of researchers from Microsoft and Copenhagen University demonstrated a novel heterostructure with remarkable properties. A heterostructure is, roughly, a device formed out of a sandwich between different solid materials. When the interfaces between the different materials are clean, the device can have properties that would be difficult, if not impossible to obtain in any single material. But when the interfaces contain impurities, the device may capture the worst, rather than the best properties, of the materials comprising it.
The device described in the new Microsoft-Copenhagen University paper is a heterostructure between a semiconductor, a superconductor, and a ferromagnet. The three materials and the interfaces between them were fabricated within an ultra-high-vacuum molecular beam epitaxy (MBE) machine, made possible by the compatibility between the growth and fabrication conditions for the three materials—europium sulfide (ferromagnet), aluminum (superconductor), and indium arsenide (semiconductor)—leading to extremely flat and clean interfaces.
The authors showed that the device has gate-tunable superconductivity and ferromagnetism induced in and coexisting in the semiconductor. These two phenomena, ordinarily antithetical, are able to peacefully coexist due to a property of indium arsenide called spin-orbit coupling. In fact, when such coexistence occurs in a quantum wire device of the type fabricated and measured by the Microsoft-Copenhagen University team, Majorana zero modes can result, enabling such a wire to be an integral component of a topological quantum computer. The new Nature physics paper shows data that is consistent with the presence of Majorana zero modes in their devices.
Previous devices without a ferromagnetic layer have exhibited similar signatures upon the application of a large magnetic field, in a direction aligned with the wire. But such a large field brings problems of its own, including the need to align all of the wires in a topological quantum computer to fairly high accuracy, as well as the field’s possible effect on other components higher in the stack. In the devices created by the Microsoft-Copenhagen University team, the magnetic moment due to the ferromagnetic layer is highly localized and automatically aligned with a preferred crystal axis.
Microsoft’s Quantum program has made a big bet that new methods for the design, fabrication, and measurement of these types of novel heterostructures will be essential if we are to build a commercial-scale quantum computer. While some might argue that tools invented for classical devices will be sufficient to produce quantum devices, Microsoft and Copenhagen University have already shown in previous work that long-envisioned, but never previously realized, combinations of superconducting and semiconducting elements could be grown and fabricated via MBE and probed by quantum transport, overturning conventional wisdom about what is possible.
Thus, this work, has intrinsic interest as a new device type with a unique mix of features and is also a significant step towards the creation of simpler topological quantum computing systems. It is also another example of how Microsoft and its partners, such as Copenhagen University, are reinventing the science and engineering of quantum devices.