Researchers report success with complex quantum states

July 23, 2018, Niels Bohr Institute
Researchers report success with complex quantum states
Scanning electron microscope micrograph of a semiconductor nanowire, made from Indium Arsenide, connected electrically to a superconductor and a normal metal. The location on the nanowire of the two spins - the microscopic magnets - are illustrated by the arrows. In this case the microscopic magnets are created by electron spins. Credit: Niels Bohr Institute

Scientists from the Niels Bohr Institute at the University of Copenhagen have, for the first time, succeeded in producing, controlling and understanding complex quantum states based on two electron spins connected to a superconductor. The result has been published in Nature Communications, and has come about in a collaboration between the scientists of the Niels Bohr Institute, a scientist from abroad and last, but not least, a Master's thesis student.

Quantum technology is based on understanding and controlling states in e.g. nanoelectronic devices with components at the nanoscale. The control could be via electrical signals, like in the components of a computer. The devices are just significantly more complex, when we are dealing with quantum components at nanoscale, and the scientists are still examining and attempting to understand the phenomena that arise on this tiny scale. In this case it is about the quantum states in nanoelectronic devices made from semiconductor nanowires and superconducting material. This requires understanding two fundamental phenomena in modern physics, magnetism and superconductivity.

Accumulating new knowledge is like playing with building blocks

The scientists have defined microscopic magnets electrically along a semiconductor nanowire. This is done by placing an electron spin close to a superconductor and then observing how it changes the quantum states. By placing two microscopic magnets rather than one, as has been done before, the possibilities for observing new quantum states arise. In this way the scientists accumulate knowledge by adding more and more complexity to the systems. "It is a bit like playing with building blocks. Initially we control one single electron spin, then we expand to two, we can modify the coupling between them, tune the magnetic properties etc. Somewhat like building a house with each additional brick increasing our knowledge of these quantum states.", says Kasper Grove-Rasmussen, who has been in charge of the experimental part of the work.

Researchers report success with complex quantum states
3-D model of the Yu-Shiba-Rusinov device. Two electron spins are defined along the nanowire, by placing appropriate voltages on the tiny electrodes under the nanowire. By coupling the spins to the superconductor Yu-Shiba-Rusinov states can be realized. Observation of these states are achieved by analyzing the current through the device from the normal metal to the superconductor. Credit: Niels Bohr Institute
Quantum theory from 1960 revitalized in nano devices

It is all about categorizing the different quantum states and their relations to one another, in order to achieve an overview of how the individual parts interact. During the 1960s, the theoretical foundation for this work was done, as three physicists, L. Yu, H. Shiba and A.I. Rusinov published three independent theoretical works on how magnetic impurities on the surface of the superconductor can cause new types of quantum states. The states, now achieved experimentally by the scientists at the Niels Bohr Institute, are named after the physicists: Yu-Shiba-Rusinov states. But they are significantly more complex than the Yu-Shiba-Rusinov states with a single spin previously achieved. This could be a step on the way to more complex structures that would enhance our understanding of potential quantum computer components, based on semiconductor-superconductor materials. Kasper Grove-Rasmussen emphasizes that what they are doing now is basic research.

Gorm Steffensen, now a Ph.D. student at the Niels Bohr Institute, was writing his Master's thesis at the time of the article, and has played an important role for the result. He was studying theoretical physics and has collaborated with his supervisor, Jens Paaske, on describing the quantum phenomena theoretically. So the article also demonstrates that collaboration on a scientific result at the Niels Bohr Institute can include the students. The task for Gorm Steffensen was to develop a theoretical model that encompassed all the phenomena in the experiments in collaboration with his supervisor and the Slovenian scientist, Rok Žitko, on. The nanowires in the experiment were developed by Ph.D. students in the research group of Professor Jesper Nygaard. It is a common modus operandi for scientists at the Niels Bohr Institute to work together, applying many different competences across all scientific levels, from student to professor.

Explore further: Theoretical quantum spin liquid prepared for the first time

More information: K. Grove-Rasmussen et al. Yu–Shiba–Rusinov screening of spins in double quantum dots, Nature Communications (2018). DOI: 10.1038/s41467-018-04683-x

Related Stories

Theoretical quantum spin liquid prepared for the first time

March 15, 2018

In 1987, Paul W. Anderson, a Nobel Prize winner in physics, proposed that high-temperature superconductivity, or loss of electrical resistance, is related to an exotic quantum state now known as quantum spin liquid. Magnetic ...

Protected Majorana states for quantum information

March 9, 2016

Quantum technology has the potential to revolutionize computation, cryptography, and simulation of quantum systems. However, quantum physics places a new demand on information processing hardware: quantum states are fragile, ...

Photons open the gateway for quantum networks

October 23, 2015

There is tremendous potential for new information technology based on light (photons). Photons (light particles) are very well suited for carrying information and quantum technology based on photons—called quantum photonics, ...

A new kind of quantum computer

November 6, 2017

Quantum mechanics incorporates some very non-intuitive properties of matter. Quantum superposition, for example, allows an atom to be simultaneously in two different states with its spin axis pointed both up and down, or ...

Recommended for you

Coffee-based colloids for direct solar absorption

March 22, 2019

Solar energy is one of the most promising resources to help reduce fossil fuel consumption and mitigate greenhouse gas emissions to power a sustainable future. Devices presently in use to convert solar energy into thermal ...

Physicists reveal why matter dominates universe

March 21, 2019

Physicists in the College of Arts and Sciences at Syracuse University have confirmed that matter and antimatter decay differently for elementary particles containing charmed quarks.

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.