Quantum Device Lab

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Climbing the Jaynes-Cummings energy ladder
On-chip molecules made of matter and light

While the formation of molecules made of particles of matter constitutes the basis for all of chemistry and therefore for our own existence, the formation of molecules made of photons seems not to occur in nature since light quanta hardly ever interact with each other. In a clever experimental setup however the quantum mechanical laws allow strong interactions between light and matter which in turn can be used to mediate interactions between two otherwise unaffected light particles. In such an experiment both the atom and the photons lose their individual properties entirely and form a new entangled system - half matter half light. Quantum theory predicts the interaction between circuit and cavity enhanced by a factor of the square root of 2 in the presence of a second photon in the resonator, which is the key observation of the reported work.

Berry Phase
Geometry enters quantum computing on a microchip

In quantum information science, the phase of a wavefunction plays an important role in encoding information. While most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance. We demonstrate the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit, manipulating the qubit geometrically using microwave radiation, and observing the accumulated phase in an interference experiment. We find excellent agreement with Berry's predictions, and also observe a geometry dependent contribution to dephasing.

Circuit Quantum Electrodynamics
a Cooper pair box coupled to a transmission line resonator

We have realized an experiment in which we investigate the quantum dynamics of an electrical two-state system coupled to a single photon of a quantized electromagnetic field. We use a Cooper-pair box as a qubit and a superconducting resonator as a cavity.

Single Vortex Quantum Dynamics
Quantum Tunnelling and Level Quantization

We have performed experiments in which we have observed for the first time the quantum dynamics of a single Josephson vortex. Spectroscopically we have shown that the energy of a vortex in a potential well is quantized and that the vortex can tunnel through a potential barrier. These observations are important for the understanding of vortex properties at low temperatures in a wide range of systems such as superconductors, superfluids and atomic Bose condensates. They also have implications for performance of superconducting devices which may be limited by the dynamics of vortices.

Whispering Galleries ...
... from St. Paul's cathedral to annular Josephson junctions

Acoustic whispering gallery modes can be observed in St. Pauls cathedral in London. Josephson vortices moving at high velocities in annular superconducting tunnel junctions may excited electromagnetic whispering gallery modes by means of the Cherenkov effect. We have experimentally observed this effect for the first time.

Multi-photon Processes ...
... in macroscopic quantum systems

We have spectroscopically studied the energy level structure in a macroscopic quantum nonlinear oscillator realized as a current-biased Josephson tunnel junction. Due to the strong nonlinearity of the oscillator, transitions between adjacent levels can be realized absorbing either a single photon or an arbitrary number of photons simultaneously. These effects are important in many solid state qubits.

Andreas Wallraff


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