Our research focus is on physics and engineering regarding superconducting quantum information electronics. Superconducting circuits allow us to manipulate quantum information on qubits, resonators, and waveguides, carried by the excitations at the energy scale of microwaves. We are developing new technologies for precisely controlling and measuring quantum states in such artificially-designed quantum systems on electric circuits. Applications in quantum information science, such as quantum computing, quantum simulation, quantum communication, and quantum sensing, are expected.

### Integrated superconducting quantum circuits

We are developing integrated superconducting quantum circuits and surrounding control hardware toward realization of a medium-scale quantum computing unit. It requires precise control and readout of a large number of qubits integrated on a chip. More concretely, we need qubits with a long coherence time, accurate and well-synchronized quantum gates, nearly quantum-limited low-noise fast qubit readouts, etc. Packaging, wiring, and electronics for microwave control and measurement also constitute important parts of our development target.

Schematic image of integrated superconducting qubits and their packaging

### Microwave quantum optics in superconducting circuits

There is almost no energy dissipation in superconducting electrical circuits. Therefore, alternating current flows in a microwave resonator for a long period of time, and microwave signal travels along a transmission line without damping. Quantum mechanically, these can be described as storage and transmission of microwave ‘photons’ confined in the circuits. Similarly, superconducting quantum-bit (qubit) circuits consist of elements such as capacitors and inductors. However, due to strong nonlinearity induced by a Josephson junction, an additional inductive element, they can store at most a single quantum of microwave.

We combine those components, i.e., qubits, resonators, and transmission lines, for manipulation of quantum states of the microwave modes. Differently from the case in free space, electromagnetic field in the circuits is confined effectively either in zero-dimension (qubit and resonator) and one-dimension (transmission line), allowing efficient generation and detection of single photons and other nonclassical states of microwave. We have proposed and demonstrated a quantum node consisting of a driven superconducting qubit, which works as a microwave single-photon detector, for example.

Microwave single-photon detector using a superconducting qubit. (a) Schematic of the device, (b) Schematic of the circuit, (c) Energy-level diagram and the pulse sequence, (d) Detection quantum efficiency as a function of the qubit drive power and the signal frequency, (e) Quantum efficiency vs. qubit drive power.