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Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften
Chair for Technical Physics (E23), Technische Universität München

Pushing light-matter interaction to the limits


July 26, 2010


In a collaboration with partners from Spain (University of Bilbao and CSIC Madrid) and Augsburg University, a team of researchers in the group of Prof. Gross at the Walther-Meißner-Institut (WMI), Bayerische Akademie der Wissenschaften and Technische Universität München, managed to enhance the light-matter interaction strength to the so-called “ultra-strong coupling” regime.

NIM NIM The interaction between matter and light is one of the most fundamental processes in physics and has far reaching relevance for our everyday life. In the field of cavity quantum electrodynamics (cavity QED) this process is studied on the most fundamental level in systems consisting of only a single atom and photon. However, the interaction strength between light and natural atoms is very weak, making such experiments very demanding. Typically, the interaction strength is many orders of magnitude smaller than the spacing of the energy levels of the atom. In a forthcoming article in Nature Physics, the group of Prof. Gross at Walther-Meißner-Institut (Bayerische Akademie der Wissenschaften and Technische Universität München) reports on experiments demonstrating a strongly enhanced light-matter interaction. This breakthrough is achieved in systems formed by artificial superconducting atoms and microwave photons. With these ingredients the researchers were able to reach the so-called ultra-strong coupling regime, where the light-matter interaction is of the same order of magnitude as the atom level spacing. In this regime, light and matter are coupled so strong that the coupled system has to be viewed as a new entity, a weird kind of molecule consisting of matter and light. Accessing the ultra-strong coupling regime opens fascinating new possibilities. On the one hand, this regime allows for novel experiments on fundamental quantum physics. On the other hand, it is a key ingredient for applications in solid state quantum information processing, which is considered a promising candidate for the next generation information technology.

Light-matter interaction on a quantum level has been theoretically described already in 1963 within the renowned Jaynes-Cummings model. The predictions of this model have been confirmed in many different and far reaching experiments operating at various energy scales. The experiments reported now provide for the first time clear evidence for the breakdown of this model, opening the avenue to the physics beyond the Jaynes-Cummings model. This novel regime opens a wide range of yet unexplored quantum phenomena, e.g. exotic photon-atom molecules and causality effects in quantum field theory, but also potential applications in future quantum processors. The important achievements are the outcome of the fruitful research environment and financial support provided by the Collaborative Research Center 631 of the German Science Foundation and the cluster of excellence Nanosystems Initiative Munich (NIM). They also profited from the nanotechnology facilities at Walther-Meißner-Institut and theory support from international partners at the University of Bilbao and CSIC Madrid.

The experiments are based on superconducting quantum circuits. When properly designed, such solid state nanostructures behave equivalent to natural atoms. Although these so-called “artificial atoms” consist of many billions of aluminum atoms, they exhibit a discrete level structure and obey the laws of quantum mechanics. In the simplest case, these artificial atoms can be viewed as quantum mechanical two-level systems, commonly called “quantum bits” or briefly “qubits”. The coupling of such qubits to microwave resonators has gained huge interest in recent years, meanwhile forming the prospering research field known as circuit quantum electrodynamics (circuit QED). Solid state qubits and circuit QED are studied intensively within the cluster of excellence Nanosystems Initiative Munich (NIM) and the Collaborative Research Center 631. They are excellent examples how state-of-the-art nanofabrication tools provide researcher broad design flexibility for purposely engineering their own artificial quantum world. To this end, superconducting qubits are particularly promising candidates for a future solid state quantum information technology.

Of course, the researchers have to play some tricks to achieve the ultra-strong coupling between matter and light. First, the photon has to be locked up in a box allowing one to store the photon for a sufficiently long time. This box is realized by a high quality factor superconducting microwave resonator consisting of a quasi-one-dimensional electrical circuit with two highly reflecting “mirrors”. Second, the artificial superconducting atom has to be placed in the same box at a suitable position. In reality, the whole structure is fabricated as a superconducting circuit on a silicon chip using nanotechnology tools. Now, ultra-strong coupling is simply achieved by making the artificial atom very big (much larger than natural atoms) and the photon box very small. In this way the photon can more easily “hit” the big atom and “comes close” to it due to the small box size. Translated to the language of physics, big atom simply means big dipole moment and small cavity small mode volume. The huge coupling achieved by the WMI group has been realized by an additional circuit element, a so-called Josephson junction, which is unique to the world of superconducting circuits. With all these tricks interaction strength could be increased to the astonishing level of 12% of the resonator frequency. This value is an order of magnitude larger than the values obtained previously in circuit QED experiments and many orders of magnitude larger than the coupling achieved in experiments with natural atoms.

Although the researchers are very happy to have achieved the ultra-strong coupling between matter and light, at the same time they have a hard time. Owing to the huge light-matter coupling, the spectra the researchers observe in their experiments can no longer be explained by the renowned but simple Jaynes-Cummings model. The spectra rather belong to a novel complex object which can be considered a light-matter-molecule. The study of its detailed structure will keep both experimentalists and theorist busy and in good mood.

Journal reference

Circuit quantum electrodynamics in the ultrastrong-coupling regime
T. Niemczyk, F. Deppe, H. Huebl, E. P. Menzel, F. Hocke, M. J. Schwarz, J. J. Garcia-Ripoll, D. Zueco, T. Hümmer, E. Solano, A. Marx & R. Gross
Nature Physics, published online: 25 July 2010; | doi:10.1038/nphys1730


This work has been supported by the Deutsche Forschungsgemeinschaft through SFB 631 and the German Excellence Initiative through NIM. E.S. acknowledges financial support from UPV/EHU Grant GIU07/40, Ministerio de Ciencia e Innovación FIS2009-12773-C02-01, Basque Government Grant IT472-10, European Projects EuroSQIP and SOLID. D.Z. acknowledges financial support from FIS2008-01240 and FIS2009-13364-C02-0 (MICINN).
The author also thank G. M. Reuther for discussions and T. Brenninger, C. Probst and K. Uhlig for technical support.


Prof. Dr. Rudolf Gross
Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften
und Physik-Department, TU München
Tel.: +49 (0)89 / 289 – 14201
E-Mail: Rudolf.Gross@wmi.badw.de
Web: http://www.wmi.badw-muenchen.de/

Nature Physics 2010

An artist's view of the superconducting quantum circuit interacting with a microwave photon.

Nature Physics 2010

Scanning electron microscope image of the superconducting flux qubit (aluminium, red) galvanically coupled to a superconducting λ/2 coplanar waveguide resonator (niobium, violet). The width in the overlap regions with the centre conductor of the resonator is 20 μm, and that of the constriction is 1 μm.

Nature Physics 2010

Microwave spectroscopy of the coupled qubit–resonator system. The measured transmission magnitude (colour coded, blue: low; red: high) is plotted as a function of the relative flux bias δΦx and the spectroscopy frequency νrf = ωrf/2π.