A solid basis for quantum simulation

The laws of quantum physics describe the properties of the smallest constituents of matter, such as atoms or electrons. For individual particles, predictions about their behaviour can be made with extremely high precision. But when several quantum entities interact with one another, then the overall behaviour of the system becomes quickly too complex to predict using analytical calculations or computer simulations.

This complexity is a hallmark of quantum physics, and one way to tackle it is to emulate the behaviour of these quantum many-body systems with so-called quantum simulators. These are technological platforms based on accurately controlled elements that themselves display quantum-mechanical behaviour. And developing such a device is precisely what an international collaboration of researchers from Delft (The Netherlands), Maryland (USA) and Zurich has done.

Disorder kept in check

The collaboration, lead by Lieven Vandersypen of the Delft University of Technology, developed semiconductor devices in which single electrons can be spatially confined and made to interact in a well-controlled manner. One of the grand challenges in pursuing this approach to quantum simulation is that the material containing the electrons is never perfectly clean. Even tiny flaws in the host material can have detrimental effects on the electrons, making quantum-level control essentially impossible.

Yet, the team has found a strategy to suppress the disorder. They combined several electrical fields — each of which under control of the experimenter — in a manner tailored to the specific device studied. As a result, the individual electrons and how they interact could be controlled with unprecedented precision and flexibility.

Perfecting the almost perfect

The advance in compensating disorder is a breakthrough in the field. But for the method to work in practice, the semiconductor material used has to be almost perfect, and robust enough that repeated experiments can be performed on it. Such a material was provided by Christian Reichl and Werner Wegscheider of the Advanced Semiconductor Quantum Materials group at ETH Zurich.

Building on their extensive expertise in molecular beam epitaxy, a powerful technique for growing semiconductors, Reichl and Wegscheider provided the sort of stable high-purity samples needed for these demanding experiments. Only a few groups worldwide have the infrastructure and know-how to fabricate samples of such quality, making them important collaborators for groups around the globe.

A solid basis

The work with the Delft group is a prime example of how the unique materials fabricated in Zurich enable novel explorations of intriguing physics. The electrons in the device can be controlled to a degree that the interactions between them can be given a clearly defined form. Specifically, in this work the interactions were chosen to reproduce those of the Fermi–Hubbard model, a theoretical model that describes a wide range of electronic materials, giving it a central role in condensed-matter physics. But even for an idealised setting such as that of the Fermi–Hubbard model it is notoriously difficult to calculate the properties of quantum systems that follow its rules.

With a practical quantum simulator of the sort presented in the work of this collaboration, however, the model can be explored directly in experiments. So far, the team has studied systems of up to twelve electrons, but with the procedures thus established it should be possible in the future to work with larger ensembles. Eventually, quantum simulations might provide unique insights into the electric and magnetic properties of novel materials. This knowledge should in turn be useful for practical applications, for example to find or design materials for electronics components.

Different routes, same goals

The quantum-simulation platform based on electrons in semiconductor materials is one of several approaches that are currently being developed to experimentally study models describing quantum many-body systems. In particular, the ETH group of Tilman Esslinger has achieved in the past few years several seminal results studying atoms trapped in structures created with laser light, which provides a very clean implementation of the Fermi–Hubbard model. Further approaches are explored, experimentally and theoretically, in the framework of the National Center of Competence in Research (NCCR) Quantum Science and Technology (QSIT), in which ETH Zurich is the leading house.

The different experimental methods are not so much in competition with each other rather than complementary. With each having its own advantages and limitations, an intriguing prospect for the future is to either combine several platforms in hybrid devices, or to compare the results obtained on different quantum simulators.

Author: external page Andreas Trabesinger