Hybrid quantum systems using collective degrees of freedom in solids
Yasunobu Nakamura, The University of Tokyo
In the course of the development of superconducting qubits, we learned that we can fully control quantum states of selected collective degrees of freedom in superconducting circuits. Such collective modes, rigidly extending in a macroscopic scale, strongly couple to electromagnetic fields via their large dipole moments. Moreover, Josephson junctions bring large nonlinearity into the system without adding dissipation. Those properties are favorably exploited in the recent progress of quantum information processing and microwave quantum optics in superconducting circuits. Now we expand the concept to other collective excitations in solids such as magnons in ferromagnets and phonons in nanomechanical devices. We construct hybrid quantum systems by combining them with superconducting qubits and resonators. In the microwave domain, the electromagnonic and electromechanical hybrid systems are brought to the quantum regime [1,2]. For example, we demonstrate coherent control of a single-magnon excitation in a millimeter-scale ferromagnetic sphere. While superconducting circuits are vulnerable to irradiation of photons in the optical domain, magnons and phonons can coherently interact with both microwave and light and may work as a quantum transducer between the two largely different energy domains. We also discuss our approaches to optomagnonics [2,3] and optomechanics.