Hybrid Superconductor-Semiconductor Devices for Majorana Fermion Detection

Lead Research Organisation: University College London
Department Name: London Centre for Nanotechnology


Our programme will involve looking for the experimental signature of so-far undetected particles known as Majorana fermions in hybrid semiconductor-superconductor devices that we will develop and fabricate. These will involve a semiconducting nanowire or quantum well heterostructure grown by molecular beam epitaxy (MBE) integrated with a superconducting thin film containing ultrasensitive Josephson-based measurement devices fabricated using electron-beam lithography. The devices will be operated at ultra-low temperatures, down to 15 mK. Once Majorana fermions can be detected, we will design and fabricate further more complicated superconductor-semiconductor structures and circuits to enable us to demonstrate that Majorana fermions can be transported and stored in an analogous way to how conventional electrons or holes are controlled in the operation of all present day electronic circuits such as computer processors.

There is currently huge interest in Majorana fermions because it is likely they will be very imporant for future quantum information technology leading to the development of faster, more powerful parallel computers in the future. At the moment the major bottleneck in the develop of such quantum computers is decoherence, where interference from the neighbouring environment to the device, or defects in the device materials, lead to a critical loss of information over a very short timescale. If quantum computers can be built based on the manipulation of Majorana fermions then the qubits (the quantum bits used to store information) can be made from a pair of widely separated Majorana fermions. These would be insensitive to the effect of localised sources of decoherence and this would allow the realisation of a robust quantum computer.

The existence of Majorana fermions was first proposed in 1937 by Ettore Majorana who showed by modifying Dirac's existing theory of conventional fermions (particles with half-integer spin such as electrons) that there could exist a class of particles that are their own antiparticles, very unlike conventional fermions. Despite the fact that they are widely believed to exist, no-one has so far experimentally proved this. The reason is that unlike conventional fermions they do not satisfy the usual rules of charge conservation and cannot be detected by simple electrical means. Our programme will involve investigating systems where the interaction between the magnetic properties of certain semiconductors in contact with a superconductor has been theoretically shown to provide the necessary conditions for Majorana fermions to be created as elementary excitations of the system. In addition to being a key component for producing Majorana fermions, the superconductor can also be used to fabricate Josephson-based sensors which display macroscopically observable quantum effects that can be specifically attributed to Majorana fermions. So by combining ultraclean semiconductor structures with superconducting devices we will have the ideal combination of factors to detect Majorana fermions for the first time.

Planned Impact

Successful implementation of the technology we will develop benefit society and the economy in many important ways:

It will enable faster, more power computers to be developed. Large companies and government will benefit from improved security from more sophisticated cryptography and from more sophisticated intelligence gathering and processing capabilities. Healthcare will benefit from more powerful computers able to faster search larger databases and analyse medical records and data. Planetcare will benefit from a better understanding of climate-change and weather forecasting arising with the advent of more powerful, faster parallel computers. The protection of our environment will also benefit from the development of more powerful computer models used in for instance nuclear reactor design, in modelling oil spills and in modelling airborne flows of pollution and contaminants.

The UK economy will benefit in several ways. In the long term there will be a benefit to the scientific/industrial community interested in developing products and applications based on quantum information processing. There will also be a benefit to commercial end-users of such products such as companies offering security or cryptography services. Manufacturers and users of scientific equipment related to our programme will also benefit. This includes commercial suppliers and users of thin film deposition, clean room, cryogenic systems and Josephson-based sensors. Our work will contribute to strengthening the recent upward sales trends for all kinds of nanofabrication and deposition tools including e-beam lithography and MBE deposition systems. The PDRAs trained by the project will be a valuable future resource for the UK, since the UK currently has a shortage of well trained materials scientists entering the UK job market, and there would be dire implications for the UK's future economic and manufacturing capability without the kind of training that programmes like ours provide.


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Description n this project we developed new types of nanoscale weak links (Josephson junctions) made with semiconductor nanowires (InAs or InSb) grown by molecular beam epitaxy as the "barrier" region between superconducting electrodes. To get these to carry a supercurrent we have developed special processing techniques to achieve clean transparent interfaces. It predicted that above a certain applied magnetic along the nanowire axis, these superconductor-semiconductor weak links could display unusual physics at ultralow temperatures including a non-sinusoidal Josephson current-phase relation. To very precisely measure the current-phase relation in an applied magnetic field we have developed novel 2-loop Andreev interferometer circuits where one loop incorporates the weak link under test.
Exploitation Route The devices/techniques developed could be used by both academic and non-academic researchers interested in measuring exotic quantum behaviour in a variety of nanoscale weak links at ultralow temperatures.
Sectors Electronics