One of the outstanding achievements of modern physics is the development of experimental methods for isolating, manipulating and controlling systems that display quantum-mechanical behaviour. Having a practical handle on quantum systems enables fundamental studies to test the often counter-intuitive predictions of quantum theory, but also provides the tools for novel technological applications in areas ranging from computation and communication to sensing. But whereas individual quantum technologies are getting increasingly mature, a current challenge is to construct hybrid devices that integrate several types of quantum systems in one experimental platform. Addressing this challenge, ETH physicists Anna Stockklauser, Pasquale Scarlino and their colleagues in the group of Andreas Wallraff have now teamed up with the fellow ETH groups of Thomas Ihn and Klaus Ensslin and of Werner Wegscheider to construct a device in which single electrical charges in a semiconductor are strongly coupled to a superconducting resonator that hosts photons — the ‘quanta of light’ — in the microwave-frequency range. The coupling between the two quantum systems is stronger than has been realized so far in any other hybrid system of this sort and might pave the way to a wide spectrum of applications.
The range of quantum systems that have become available to experimentalists over the past few decades extends from single atoms, ions and photons to complex engineered systems, including micrometre-size superconducting circuits and semiconductor structures. Combining the expertise of Wallraff’s Quantum Device Lab in superconducting systems and that of the Nanophysics group of Ensslin and Ihn in semiconductor structures, and working with the fabrication experts in Wegscheider’s Advanced Semiconductor Quantum Materials lab, Stockklauser and Scarlino had the pieces in place to design and construct a unique new hybrid device. Coupled semiconductor–superconductor systems have been demonstrated before, but in the new device, the coupling is up to six times stronger than in previous designs. Importantly, the coupling exceeds the limit where energy can be exchanged at a rate that is faster than the losses from either of the sub-systems. In other words, quanta in the two delicate quantum systems can be made to work before their energy is lost due to unavoidable disturbances from outside.
The key to the successful device design is an array of 32 superconducting quantum interference devices, or SQUIDs (see the figure above). This element significantly increases the impedance of the system; that is, it is more difficult for a current to enter the circuit. This in turn implies that the field in the microwave cavity has a strong electric component, which then efficiently couples to the individual electric charges in the semiconductor structure.
This sort of coupling is important, as microwave photons can be relatively easily transmitted from one location to another. The new device should therefore be useful, for example, for coupling distant semiconductor or other solid-state systems. And as the frequency of the cavity is tunable, the hybrid system can also be used for mapping out the properties of the electrons in the material it contains, as Stockklauser and Scarlino demonstrate. The versatility of their approach means that the method could be applied to a broad variety of quantum systems based on electrical charges.
The collaboration between these three groups in the Department of Physics is a powerful example of how expertise in different fields can be successfully brought together to create an innovative approach to an important problem. This effort has been further supported by the Swiss National Science Foundation through the National Centre of Competence in Research (NCCR) Quantum Science and Technology (QSIT), where ETH Zurich is the leading house.
Author: Andreas Trabesinger