Superconducting circuits with lateral dimensions between 100 nm and a few micrometers behave in many aspects similar to natural atoms.
Despite the fact that these socalled “artificial atoms” are huge compared to their natural counterparts, they still have a discrete level structure and exhibit properties unique to the world of quantum mechanics. In the simplest case, artificial atoms can be viewed as quantum mechanical twolevel systems, commonly called “quantum bits” or briefly “qubits”. Such qubits are studied intensively within the cluster of excellence Nanosystems Initiative Munich (NIM) and the Collaborative Research Center 631 of the German Science Foundation. They are not only the basic building blocks of solidstate based quantum information processing systems but also allow for the investigation of fundamental quantum phenomena on a macroscopic scale.
The big advantages of solidstate qubits over natural atoms are design flexibility and wide tunability by means of external parameters such as electric or magnetic fields. This tunability has now been deliberately exploited by a team of researchers in the group of Prof. Gross at the WaltherMeißnerInstitut (Bayerische Akademie der Wissenschaften and Technische Universität München) to break the symmetry of a superconducting flux qubit coupled to a microwave resonator in a controlled way. The exciting experiment belongs to a young and prospering research field, which is called circuit quantum electrodynamics (circuitQED) due to the analogy with the well established field of quantumoptical cavity QED. The experiments were performed in collaboration with the NTT Basic Research Laboratories in Japan and supported by theorists of the LMU München.
The results, which have recently been published in Nature Physics, demonstrate that the behavior of the coupled qubitresonator system is governed by a frequency upconversion process for microwave photons. The occurrence of this process strongly depends on the underlying symmetry properties of the system (see figure), which can be chosen on purpose by an external control parameter (magnetic flux in our case). Besides its role in fundamental research, the controlled symmetry breaking in artificial quantum systems has good prospects of use in numerous applications. Particular examples are parametric upconversion, the generation of single microwave photons on demand, and squeezing of quantum states.
Journal reference
Twophoton probe of the Jaynes–Cummings model and controlled symmetry breaking in circuit QED
Frank Deppe, Matteo Mariantoni, E. P. Menzel, A. Marx, S. Saito, K. Kakuyanagi, H. Tanaka, T. Meno, K. Semba, H. Takayanagi, E. Solano & R. Gross
Nature Physics, published online: 29 June 2008;  doi:10.1038/nphys1016
Contact:
Prof. Dr. Rudolf Gross
WaltherMeißnerInstitut, Bayerische Akademie der Wissenschaften
und PhysikDepartment, TU München
Tel.: +49 (0)89 / 289 – 14201
EMail: Rudolf.Gross@wmi.badw.de
Web: http://www.wmi.badwmuenchen.de/

In the symmetric situation (left) the twophoton process is forbidden. Qubit and resonator can be excited only resonantly at frequency ω_{1} = ω_{q}. In contrast, in the case of broken symmetry (right) the qubit can be excited both by one and twophoton processes. Therefore, also for ω_{2} = ½ ω_{q} (twophoton process) the excited qubit state e> can be populated. Due to the coupling g a state transfer to the resonator is achieved. The resonator state, in turn, decays by emission of a photon of twice the frequency (frequency upconversion). The symmetry properties of the system can be changed in a controlled way by varying the applied magnetic flux Φ_{x}.
