Quantum Mechanics is the fundamental of many of the physical dynamics sorrounding us. The quantum effect, however, is hardly observed in our daily lives. Similarly, a standard electronics circuit does not require the quantum mechanics, eventhough the dynamics of an electon inside a tansistor cannot be explained without the quantum mechanics.
When an elecronic circuit is made of a superconducting material, due to its minimal energy dispersion, the quantum effect start to become visible. A micrometer-scale to even a millimeter-scale superconducting circuit, well-designed and operated at an ultra-low temperature, can become an artificial macroscopic quantum device.
A superconducting circuit equipped with Josephson junctions can effectively become a two-level quantum system, a Qubit. It is possible to coherently manipulate the quantum state and to observe the dynamics using the microwave technique.
The superconducting circuit as an artifical atom can have an enormous electric or magnetic-dipole moment compared to an atomic system, simply due to its large physical size, and couples strongly with an electromagnetic wave. Additional circuits, such as a superconducting resonant circuit and a superconducting transmission line, that has a strong spatial confinement of the electromagnetic mode can furthur enhance the couping.
With these devices, the coupling becomes so strong that even the effect of zero-point fluctuations of the electromagnetic field on the superconducting qubit can become visible.
We are conducting a series of researches, studying the quantum optical effects on the superconducting qubits in a microwave region.
It has become possible to routinely manipulate and to measure superconducting
qubits with high fidelity. Expansion of the system to multiple qubits and
also complex integratation including a use of cavities are developed.
Currently, we are expanding the utility of the superconducting quantum circuits to control other quantum systems. Coherent quantum state transfer between different systems can enrich the possible resources available for the quantum information processing and quantum communication. We are aiming toward devlopment of quantum memory with long lifetime and the quantum interface between the superconducting system and the photonic system.
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