Research content

Project content

Outline

In this project, we will research and develop a quantum communication network essential to a large-scale distributed quantum computer by quantum-coupling the hardware of that computer. Specifically, we have developed a quantum interface technology that connects a microwave photon to a quantum memory, and a quantum memory to a communication photon by quantum connection. Finally, this will establish a quantum interface technology between computing qubits and communication photons by integrating these two technologies.

If you want more information, you can view the presentation given by the project manager Hideo Kosaka here.

Milestones

By 2030, we will build the foundation of the quantum relay network, which is a major step toward building the quantum Internet. By 2025, we will develop a next-generation hybrid quantum interface that fuses diamond quantum memory and optomechanical crystals, enabling a quantum connection between quantum memories.

By 2023, it will be possible to realize a hybrid quantum interface by developing technologies such as optimal quantum light sources and quantum media conversion.

Project R&D scope

① Diamond Quantum Memory

•Diamond Quantum Memory
We will use the color centers of diamonds to build a quantum memory that can perform quantum logic operations and is robust against quantum errors.

•Diamond Quantum Structure
Our goal is to develop diamond nanofabrication technologies for stable charge states, photonic crystals, and phononic crystals. Our methodology will allow us to build quantum structures that will serve as a foundation for diamond quantum memories.

•  Diamond Quantum Crystal
We will grow high-purity diamonds with the desirable impurity composition by using the CVD (chemical vapor deposition) approach to produce diamond quantum crystals with color centers suitable for quantum memories.

• Diamond Color Center
When building the quantum interface, it is essential to achieve an efficient quantum entanglement by emission or scattering. Therefore, we will investigate the conditions that generate color centers to prolong the duration of the quantum states.

②     Optomechanical Crystal

•  Photonic Crystal Cavity
Our photonic crystal resonators will use diamonds to amplify the photon field.

•  Photonic Integrated Circuit
We will investigate the coupling of diamond optomechanical crystals with optical circuits.

•  Phononic Crystal Cavity
Our diamond phononic crystal resonators will enhance the phonon field.

③     Piezo Microwave Resonator

•  Piezo Microwave Cavity
We aim to achieve a very efficient interconversion of microwaves and phonons by coupling the piezo resonator with the microwave resonator.

•  Qubit Control Integrated Circuit
We will fabricate electronic integrated circuits allowing fast and high-fidelity quantum control in the diamond quantum memory and the piezo resonant circuit.

•  Quantum Interface Theory
Our theoretical study will push forward the development of quantum interfaces. These interfaces will be capable of quantum mechanical coupling to ensure high efficiency and fidelity between superconducting qubits and the photonic qubits on the communication wavelength band.



Technical explanation

Quantum teleportation

Figure: Image of quantum teleportation. Teleport to a place away from the quantum state called Schrodinger's cat (provided by the scientific magazine Newton)
Figure: Image of quantum teleportation. Teleport to a place away from the quantum state called Schrodinger's cat (provided by the scientific magazine Newton)
Teleportation means teleporting matter in science fiction, which is impossible. However, this is possible in the quantum world. Quantum is a fundamental particle that behaves like a wave, such as an electron, an electric current component, and a photon, a light component. Quantum teleportation is the technology that teleports this wave-like property to a distant place. We have realized such quantum teleportation using diamonds.

The microcosm in the diamond

Diamond is composed of carbon (12C). A composite defect in which one of these carbons is missing (V in the left figure) and the carbon next to it is replaced with nitrogen (N in the left figure) is called the NV center. Around 1% of carbon isotopes (13C), rich in neutrons, are naturally present. In such a quantum microcosm, we conducted an experiment on quantum teleportation from photons to neutrons by putting photons and neutrons in a state of "entanglement." Quantum entanglement is simply a state in which two quanta at distant points are interrelated.

Transfer state by quantum entanglement and photon absorption

図：ダイヤモンドの中で行われる量子テレポーテーションの原理図
Figure: Principle diagram of quantum teleportation performed in diamond

I will explain the principle of quantum teleportation. First, the electrons captured in the defect and the neutrons of the carbon isotope are prepared in a quantum entangled state. Then, when an electron absorbs a photon from outside and transitions to a specific state, this photon's state is transcribed into the carbon isotope neutron. At this time, neutrons do not interact directly with photons, so they are compared to teleportation.

To the world of the quantum internet

図：量子インターネットのイメージ（科学雑誌Newton様ご提供）
Figure: Image of quantum internet (provided by scientific magazine Newton)

With quantum computers, it will be possible to answer problems that would take a long time to complete the universe with conventional digital computers in a few days. We are trying to build a quantum internet that connects these quantum computers by quantum communication by repeating quantum teleportation (right figure). It is believed that the realization of the quantum Internet will enable the safe transmission and reception of large amounts of data, which will significantly change society. I am very much looking forward to the moment when the quantum Internet is in the world.