Atsushi Noguchi

We have fabricated high-performance superconducting quantum circuits using titanium nitride. The maximum energy relaxation time of the superconducting qubits was 450 us, which is one of the longest value of the world. We also discovered a new type of a resonance between superconducting qubits using couple qubit and succeeded in performing high-fidelity quantum gate. This method can reduce the number of wires in a superconducting quantum computer and we propose the scalable construction of it. An electron trap is one candidate of the highest-performance quantum systems. Trapped electron in the vacuum has a long coherence and wideband quantum manipulations, which realize an ultra-precise quantum system. We developed a method to detect low-energy electrons at cryogenic temperatures for the electron trap experiment. The behavior of electrons trapped at cryogenic temperatures is studied by numerical simulation.

1. Superconducting qubit
1.1 Fabrication of a high performance qubit
Superconducting qubits, called transmon, are widely used around the world in superconducting quantum computers because of their circuit simplicity and scalability. Since the improvement of their performance directly leads to higher fidelity quantum gates, much research has been conducted in decades. In this study, we focused on TiN thin films epitaxially grown on intrinsic silicon substrates and succeeded in fabricating high quality transmon: energy relaxation times of up to 450 us, and coherence times of up to 150 us, were achieved, respectively. Recently epitaxial grown tantalum thin film on a sapphire substrate has been known as the good material for state-of-the-art superconducting circuit. Our transmon with TiN film on Si substrate is reached to the performance of tantalum transmon, and as silicon is much easier to fabricate than sapphire, our TiN qubit has promising candidate for high performance superconducting circuits.

1.2 High fidelity gate with nonlinear coupler and its integration method
A two qubit gate is often limit the performance of the quantum computer. Three main methods of implementing two-qubit gates in superconducting quantum computers have been implemented, respectively: resonance using frequency-variable qubits, crossed resonance using fixed-frequency qubits, and parametric resonance using magnetic field modulation of SQUID couplers. Although these methods have their advantages and disadvantages, we have proposed and realized a new resonance-based gate using fixed-frequency qubits to overcome the disadvantages of these gating methods. This method can eliminate residual interactions during idle time that cause errors, requires less microwave power for gating, and can be extended to reduce the number of wires to the qubits. The new resonance, which they call NCAR (Nonlinear Coupler Assisted Rabi) resonance, is produced by adding another coupler qubit between the qubits and driving the coupler qubits with microwaves. We have actually fabricated a sample and verified the operation of a two-qubit gate based on this scheme. This new gate method can also be used to realize large-scale superconducting quantum circuits with a small number of wires.

2. Electron trap quantum system
2.1. Feasibility study on ground-state cooling and single-phonon readout
Unlike ion traps, laser cooling techniques are not applicable to electron traps, and it is difficult to achieve a quantum system with trapped electrons. Therefore, we discussed the feasibility of a method to cool the electrons trapped in the vacuum to the vibrational ground state and detect their vibrational quantum. We proposed three hybrid system with trapped electron: high-Q superconducting resonator, superconducting quantum bit, and laser-cooled ions. The first two methods are achieved by trapping electrons at

With its beginnings at the dawn of the 20th century, quantum mechanics has made significant progress over the past 100 years, exerting a major impact on our perception of the world in a wide range of scientific fields. Predictions using quantum mechanics have been verified on every scale, from elemental particles to the whole universe, thus consolidating its presence as a basic theory of physics. At the same time, quantum mechanics serves as a foundation for technologies that form the core of today’s
information-oriented society, including those for integrated electronic circuits and optical communications. Though one may not be aware of it on a daily basis, quantum mechanics has become an integral part of our lifestyle.
 
Meanwhile, discussions on the approach of quantum information science that applies basic principles of quantum mechanics, such as superposition of quantum states, to research and development of information processing began as recently as the start of this century. Research efforts have accelerated globally, and operation tests of small-scale quantum computing units have already started. More recently, demonstration of quantum supremacy, whose performance surpasses that of existing supercomputers, has been a burgeoning topic. To do justice to the potential of quantum computers, however, it is believed to be
essential to realize fault-torelant quantum computation that implements error-resilient architecture using quantum control of a higher level of precision.
 
Dr. Noguchi’s research proposal addresses this challenge directly. He is ambitiously seeking to carve out the future of quantum information technologies, such as quantum computing and quantum sensing, by realizing advanced quantum control in a quantum system with higher degrees of freedom, as he pursues greater precision in the control of quantum freedom. Realization of fault-torelant quantum computation, which will not be possible without high-precision quantum control of a system with a high degree of freedom, is not only an overarching goal that would set a major milestone in quantum information science but also one of the peaks of humankind’s scientific and technological prowess in a world governed by quantum mechanics.
 
Dr. Noguchi has conducted a variety of physical experiments to achieve one original outcome after another, which ranges from those dealing with atomic-scale quantum systems, such as ions that are laser-cooled and trapped in a vacuum, to those analyzing millimeter-scale quantum systems, such as qubit elements realized on superconducting circuits and mechanical vibrations of semiconductor nanomechanical elements. He is one of the few young researchers in the world who has a superb command of a variety of quantum control technologies from radio waves and microwaves to infrared light and visible light over a wide range of frequencies and energy scales. In this proposed project, too, Dr. Noguchi not only aims to realize a novel quantum control technology using superconducting circuits but also plans to build a new quantum system, such as for electrons trapped in an electric field in a vacuum, and then establish a technique for controlling the quantum state with a high degree of precision.
 
Dr. Noguchi is a promising leader in the research of quantum control technology for quantum computation. With support from the InaRIS Fellowship Program, it is expected that he will be more productive than ever in furthering his elaborate research based on his novel ideas over the coming decade.