Superconducting qubits do not work as they are. It is necessary to implement readout circuits as well as coupling circuits among qubits, and importantly, to protect the superconducting qubit from the surrounding noisy environment. To realize these objectives, interesting elements have been actively developed in various countries around the world. Small circuit devices using quantum mechanics are called superconducting quantum circuits.
Microwaves are overflowing around us, such as communications represented by mobile phones, microwave ovens for heating, or in-vehicle radar to protect people’s safety. Microwaves are a member of the same electromagnetic waves as light. Its wavelength is several centimeters. For this reason microwave devices are often palm-sized, and it is easy to realize interesting elements using boundary conditions for electromagnetic waves.
Let’s think about the room temperature surrounded by walls on all sides. Since the temperature of the wall is normal temperature, blackbody radiation occurs from this wall, and there is a lot of microwave thermal noise in the space. In order to protect qubits and microwave quanta from this noise, we have to cool the wall temperature down to 0.01 Kelvin (= -273.13 °C) and try to minimize the radiation from the wall. The dilution refrigerator is a big instrument that creates this cryogenic environment. Under this environment, qubit and microwaves can behave quantum mechanically without being disturbed by thermal noise.
Let’s consider an opposing mirror a few centimeters wide. Microwaves propagating towards the mirror will hit the mirror and will not be able to escape into the outer space as it goes back and forth. At this time the microwave is isolated from the outside space and is confined in this space. This structure is called a resonator. Microwaves that behave like quantum mechanics can also be trapped in this resonator and can be used as a memory element to store quantum states. Placing a superconducting qubit in the resonator also protects the qubit from external noise. The qubit lives in a resonator that is secure, and it talks with the world outside through the resonator.
The microwave signal acting quantum mechanically is a very small signal, for example, a few nanovolts. In order to observe this signal, we need an excellent amplifier that is not buried in thermal noise. In the limitations of no thermal noise, the noise caused by the uncertainty principle in quantum mechanics, so-called quantum noise, becomes dominant. The amplifier operating at this limit is called a standard quantum limited amplifier and we can realize it with a superconducting circuit. With this amplifier it is possible to obtain microwave signals without being affected by thermal noise.
In our project we will create a large macroscopic quantum machine centered on superconducting qubits. The qubits confined in resonators are protected from the external environment and are controlled through resonators. Coupling between qubits is also induced through dedicated resonators or superconducting circuits. Quantum state measurement is performed by introducing weak microwaves into the resonator, and reading is performed with a standard quantum limit amplifier.
We aim to expand to 18 qubits based on a 3 × 3 qubit array. This basic 3 × 3 qubit array becomes one block, and we design architecture so that the blocks can be expanded to a larger system. We realize vertical three-dimensional mounting in which the control line is introduced from either the top side or back side of the substrate, ensuring scalability. We also develop a high-speed digital signal processor, aiming for complete coherent control of qubits. Once quantum error detection is performed on this block, it becomes possible to have resistance against the collapse of the quantum state.
Microwaves at extremely low temperatures behave quantum mechanically because thermal noise is sufficiently small. Microwaves can perform signal processing by themselves as well as be good carriers that carry quantum states far away. Using superconducting qubits and superconducting devices, such as quantum limited amplifiers, it is possible to perform quantum information processing with flying quantum microwaves. In this project we aim to realize such signal processors on propagating microwaves.