Atom-Photon Pairs Key to Quantum Computers

In the quest for developing quantum computers that will be vastly superior to present-day computers, physicists have found that strong coupling of quantum bits with light quanta play a pivotal role.

An electron microscopical picture of the superconducting circuit (red: Aluminum-Qubit, gray: Niob-Resonator, green: Silicon substrate). Image: Thomasz Niemczyk/TUM)

Physicists from Technical University of Munich (TUM), including professor Rudolf Gross, and from Walther-Meissner-Institute for Low Temperature Research of the Bavarian Academy of Sciences (WMI), Augsburg University and partners from Spain have now realized an ultrastrong interaction between microwave photons and the atoms of a nanostructured circuit. The interaction is 10 times stronger than levels previously achieved for such systems.

The simplest system for investigating the interactions between light and matter is a so-called cavity resonator with exactly one light particle and one atom captured inside (cavity quantum electrodynamics, cavity QED). Yet because the interaction is very weak, these experiments are very elaborate. A much stronger interaction can be obtained with nanostructured circuits in which metalssuch as aluminum become superconducting at temperatures just above absolute zero (circuit QED). Properly configured, the billions of atoms in the merely nanometer-thick conductors behave like a single artificial atom and obey the laws of quantum mechanics. In the simplest case, one obtains a system with two energy states, a so-called quantum bit, or qubit.

Artist's impression of the interaction between a superconducting electrical circuit and a microwave photon. (Image: Dr. A. Marx/TUM)

Coupling these kinds of systems with microwave resonators has opened a rapidly growing new research domain in which the TUM physics, the WMI and the cluster of excellence Nanosystems Initiative Munich (NIM) are leading the field. In contrast to cavity QED systems, the researchers can custom-tailor the circuitry in many areas.

To facilitate the measurements, Gross and his team captured the photon in a special box, a resonator. This consists of a superconducting niobium-conducting path configured with strongly reflective "mirrors" for microwaves at both ends. In this resonator, the artificial atom made of an aluminum circuit is positioned so that it can optimally interact with the photon. The researchers achieved the ultrastrong interactions by adding another superconducting component into their circuit, a so-called Josephson junction.

The measured interaction strength was up to 12 percent of the resonator frequency. This makes it 10 times stronger than the effects previously measurable in circuit QED systems and thousands of times stronger than in a true cavity resonator. However, along with their success, the researchers also created a new problem: Up to now, the Jaynes-Cummings theory developed in 1963 was able to describe all observed effects very well. Yet it does not seem to apply to the domain of ultrastrong interactions.

"The spectra look like those of a completely new kind of object," Gross said. "The coupling is so strong that the atom-photon pairs must be viewed as a new unit, a kind of molecule comprising one atom and one photon.”

Experimental and theoretical physicists will need some time to examine this more closely. However, the inroads into this domain are already providing researchers with a whole array of experimental options. The targeted manipulation of such atom-photon pairs could hold the key to quanta-based information processing.

The research was funded by the Deutsche Forschungsgemeinschaft (Cluster of Excellence Nanosystems Initiative Munich and SFB 631), the European Community (EuroSQIP, SOLID) and the Spanish Ministry for Science and Innovation.

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