On the path to quantum computers: Ultra-strong interaction between light and matter realized

July 30, 2010
This is an impression of the interaction between a superconducting electrical circuit and a microwave photon. Credit: Dr. A. Marx, Technische Universitaet Muenchen

Researchers around the world are working on the development of quantum computers that will be vastly superior to present-day computers. Here, the strong coupling of quantum bits with light quanta plays a pivotal role. Professor Rudolf Gross, a physicist at the Technische Universitaet Muenchen, Germany, and his team of researchers have now realized an extremely strong interaction between light and matter that may represent a first step in this direction.

The interaction between matter and represents one of the most fundamental processes in physics. Whether a car that heats up like an oven in the summer due to the absorption of light quanta or that extract electricity from light or light-emitting diodes that convert electricity into light, we encounter the effects of these processes throughout our daily lives. Understanding the interactions between individual - photons - and atoms is crucial for the development of a quantum computer.

Physicists from the Technische Universitaet Muenchen (TUM), the Walther-Meissner-Institute for Low Temperature Research of the Bavarian Academy of Sciences (WMI) and the Augsburg University have now, in collaboration with partners from Spain, realized an ultrastrong interaction between microwave photons and the atoms of a nano-structured circuit. The realized interaction is ten times stronger than levels previously achieved for such systems.

The simplest system for investigating the interactions between light and is a so-called cavity resonator with exactly one light particle and one atom captured inside (cavity quantum electrodynamics, cavity QED). Yet since the interaction is very weak, these experiments are very elaborate. A much stronger interaction can be obtained with nano-structured circuits in which metals like 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.

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.

This is an electron microscopical picture of the superconducting circuit (red: Aluminum-Qubit, grey: Niob-Resonator, green: Silicon substrate). Credit: Thomasz Niemczyk, Technische Universitaet Muenchen

To facilitate the measurements, Professor Gross and his team captured the photon in a special box, a resonator. This consists of a superconducting niobium conducting path that is 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 twelve percent of the resonator frequency. This makes it ten times stronger than the effects previously measureable 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," says Professor Gross. "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 new experimental inroads into this domain are already providing researchers with a whole array of new experimental options. The targeted manipulation of such atom-photon pairs could hold the key to quanta-based information processing, the so-called quantum computers that would be vastly superior to today's computers.

Explore further: Yale scientists bring quantum optics to a microchip

More information: 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 and R. Gross, Nature Physics, published online July 25, 2010 - DOI:10.1038/NPHYS1730

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5 / 5 (1) Aug 01, 2010
well...this expriement may lead to formation of artificial molecule comprising a photon and an atom....quantum bit is interesting....quantum computer is gonna ROCKKKK...
not rated yet Aug 02, 2010
the coputer the size of sugar cube with enough power to simulate large neural net, overshadpwing our litle brain by faktor of a thousands. The new world is being born right now, just hold tight and survive.
not rated yet Aug 02, 2010
Initial uses of this technology will be in creating uncrackable codes, I doubt we'll have quantum computer PCs within our lifetimes.
not rated yet Aug 04, 2010
Maybe one of you can explain this to me. So I read that it will produce "uncrackable" codes for reasons such as the message no longer being correct once the qubits have been measured. If that is true, then how do we read the message at all? Do we get one failed attempt and then the message is destroyed forever and need to be retransmitted?

Also, how does another party know that it has been measured? This is where I really need a better understanding because it appears to me that a qubit is nothing more than a bit whose value is unknown( or in quantum terms - superposition ). For example schrodinger's cat does not appear to me as having both states of alive and dead. To me it is simply unknown. Now if I observe the cat as in fact being dead and then close the box( and told no one of my observation), how would anyone else know what the state of the cat is? More importantly would it not still retain its quantum state to everyone else with the exception of myself?

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