Competition with “superconducting system” over first place

As literally stated, “ion trap” means a state where the positively charged ion arising from removal of one electron from an atom is trapped by voltage within a vacuum.

At room temperature, ions repeat random thermal motion. If a state not allowing thermal motion is created by laser cooling, ions remain almost completely still within the “trap.” Complex calculation through quantum-based manipulation of the ions in a highly stable state has been attempted.

The beginning of quantum information processing with ion trapping dates back to a quarter of a century ago. In 1995, a 2-quantum bit (2-qubit) “quantum operation gate” was realized by making use of an ion trap. Researchers are continuing their studies with perspectives for its social implementation. On the other hand, the superconducting system began to distinguish itself around the start of the 21st century. Google organized its development team in 2014. The paper made public by this team 5 years later declared: “Quantum supremacy has been verified.” This triggered close attention of society to quantum computers.

Will the superconducting system continue to run at the top from now on? Or will any other system, such as ion trap, catch on and overtake it soon?

Excellent characteristics of “ion trap”

A US business giant “Honeywell”, well known in the field of aerospace, announced in 2020 its ongoing development of ion trap-based quantum computers. In December 2020, the research team involving the University of Science and Technology of China and others made public the achievement of quantum supremacy with the use of a photonic quantum computer. The competition over development of quantum computers based on multiple systems is becoming increasingly more intense.

The superconducting system is a “solid-state device” making use of the electrodes made of superconducting materials. This is advantageous in that its development can be proceeded as an extension of “classical computers” such as supercomputers composed of electronic circuits. It, however, involves a technical problem of how to eliminate or correct errors through removing the “noise” generated from surrounding materials. To induce the superconducting phenomenon, the materials must also be cooled to a low temperature near absolute zero.

With the ion trap, however, quantum information can be obtained at high precision because tens of ions can be kept isolated within a room temperature vacuum. Associate Prof. Toyoda explains: “Ion trapping involves less factors that can disturb the quantum state and thus has a favorable characteristic.” Although the period for which the quantum state can be preserved is said to be about 100 microseconds with the superconducting system, the ion trap can preserve it for 10 minutes or longer (more than 1 hour according to some reports).

However, the ion trap, which is a state independent from the electronic circuit, requires a preparatory step of initializing the quantum state by trapping the ions before each session of calculation. In addition, despite the longer preservation time than the superconducting system, ions require continuous application of wavelength-accurately-regulated laser, and even talking or a slight vibration can cause a shift in the laser wavelength, thus requiring delicate manipulation under the ion-trapped setting. The awareness of the current status by Associate Prof. Toyoda is: “To conduct complex calculation, the system made of a combination of many quantum bits needs to be operated repeatedly in a reliable manner. There are technical hurdles, although none of them is deep-rooted.”

A “mecha-loving” researcher

The laboratory of Associate Prof. Toyoda is full of computers, lasers and many lenses for laser wavelength adjustment. This is a scene that we can call an “experimental workshop.”

Associate Prof. Toyoda belongs to the generation “at the startup of personal computers.” When he was an elementary school student, personal computers made in Japan began to be introduced to the market. He was a “mecha-loving” kid, fond of preparing electronic circuits, etc. as a hobby. This hobby led to the “desire of learning physics serving as the base for the mechanics” and he entered the Kyoto University School of Science.

At Kyoto University, he studied neutral atoms. After moving to Osaka University in 1999, he began to handle ions. Both fields allow clear-cut observation of the behavior of “quantum”, which has two aspects (“particle” and “wave”). He came to think: “If the fundamental nature of things is explored, we reach a quantum-like phenomenon as a basic principle.” In this way, he set quantum at the core of his research.

Associate Prof. Toyoda calls himself “an experimentalist.” His experience of electronic handcrafts during childhood guided him into the microworld, where he began to handle atoms close to the smallest unit of matter. He says: “I am conducting research with the sense of “craftmanship” as if I were making devices with the use of atoms.” As quantum computing is a field that is going to become mature from now on, “the close connection between basic interest and the application and breakthrough is a highly attractive feature of this field,” he says so with shining eyes.

Challenges on a new path

To enable a larger scale quantum computer making use of the ion trap characteristics, two new techniques, i.e. “quantum CCD technique” and “photonic interconnect technique,” are now under investigation.

The quantum CCD technique adopted by Honeywell was designed to achieve complex calculation through connecting the ion trap to many electrodes, followed by repetition of voltage-stimulated movement of ions. Photonic interconnect is a technique also used for “quantum communication.” It takes out the photons (light particles) generated by multiple ions and induces mutual interference among the photons, resulting in the connection of ions from different ion traps.

It is also possible to combine these two techniques. One possible approach is to construct the system with the quantum CCD technique to some level and then to combine it with the photonic interconnect technique.

On the other hand, the team of Associate Prof. Toyoda has begun to challenge a new way of constructing quantum bits. In November 2015, the research group including Associate Prof. Toyoda (Assistant Professor in those days), Professor emeritus Shinji Urabe, Osaka University and others published their research outcome in Nature in a paper titled “First Success in Observation of Quantum Interference between Phonons.” Phonon is a fundamental particle expressing the vibrational energy of matter. When phonons were generated at different locations using two ions within the ion trap, the phonons moved due to the interaction between the ions, resulting in detection at the same location of each ion. Thus, the phenomenon previously confirmed with photons was demonstrated to occur also with phonons.

Towards the goal of realizing quantum bits making use of the nature of phonons, Associate Prof. Toyoda has been conducting full-scale research since 2020. As there are a limitless number of energy states for phonons, a single quantum bit can reflect numerous quantum states, also enabling an error-correcting function to be incorporated. “Simplicity compared with the other techniques” is an advantage of this technique. He intends to advance this research to enable the combination of a larger number of quantum bits.

What is the significance of existence for scientists? Associate Prof. Toyoda is strongly motivated for “exploring the uncultivated fields of mankind.”

In this universe, there are still many tasks that cannot be resolved even with “Fugaku” (the supercomputer with the highest calculation speed in the world) installed at the Institute of Physical and Chemical Research (RIKEN). “If quantum information processing is realized, it will be possible to conduct simulation on the properties of matter and chemical reactions. Exploration of new superconducting materials currently viewed as difficult will also be possible.”

The small ion trap, several centimeters in length, bears limitless dreams.

● Specially Appointed Associate Professor (Full-time) Kenji Toyoda
Institute for Open and Transdisciplinary Research Initiatives/ Center for Quantum Information and Quantum Biology, Osaka University

• 1994: Graduation from Kyoto University School of Science.
• 1999: Course work completed without a degree at Kyoto University Graduate School of Science.
• 2002: Ph.D. (Science).
• 1999: Research Associate at Osaka University Graduate School of Engineering Science.
• 2007: Assistant Professor at the same school.
• Since April 2019: Current position. Majors: Laser cooling/trapping, ion trap, quantum information processing, atomic physics, laser spectroscopy.

(Interviewed in November 2020)