Annual report FY2021

Experimental Quantum Information Physics Unit
Assistant Professor Hiroki Takahashi

From left to right: Ezra, Kei, Chitose, Shigeo, Makoto, Tatsuki, Hiroki and Joel. Ayano and Soon are missing in this photo.


After the low-key start in 2020 amid the pandemic, the unit started to expand in AY2021 in terms of personnel as well as research activities . Two new postdocs, Dr. Joel Morley (initially remote in UK, on site since Sep. 2121) and Dr. Diptaranjan Das (from Nov. 2021) joined the team. We had rotation students, Bolu Feng and Tatsuki Hamamoto in the term 3 of AY2020, Dyon van Dinter and Mathieu Couillard in the term 2 of AY2021. In addition we had two intern students Makoto Endo (Apr.-Sep.2021) and Chitose Maruko (Jun.-Sep. 2021). We also welcomed unit technicians Shigeo Sugitani (May 2021-) and Tatsuya Yamazaki (Sep. 2021-).

We started "working group (WG) system" to manage different ongoing projects efficiently. WG1 is in charge of developing a linear ion trap integrating a micro fiber cavity. WG2 works towards a cavity QED experiment using Ba+ ions instead of Ca+. WG3 is in charge of CO2 laser shooting for production of fiber cavities. Besides there have been independent studies conducted by rotation students. Bolu worked on programming the ARTIQ control electronics. Dyon worked on COMSOL simulation of a linear trap to study the effects of dielectrics. Mathieu experimented a laser ablation of calcium in vacuum. Hiroki supervised all the projects

1. Members


  • Dr. Ezra Kassa, Postdoctoral Scholar (WG1 & WG2)
  • Dr. Joel Morley, Postdoctoral Scholar (WG1 & WG3)
  • Dr. Diptaranjan Das, Postdoctoral Scholar (WG2)


  • Shigeo Sugitani
  • Tatsuya Yamazaki

Graduate students

  • Soon Teh (WG1)

Rotation students

  • Bolu Feng (ARTIQ development)
  • Tatsuki Hamamoto (WG3)
  • Dyon van Dinter (COMSOL simulation)
  • Mathieu Couillard (Laser ablation of Calcium)

Intern students

  • Makoto Endo (WG2)
  • Chitose Maruko (WG1)

2. Collaborations

2.1 Fiber-based Fabry-Perot cavities for opto-mecanical NMR

  • Description: This collabration aims to detect nuclear magnetic resonance signals with optical means. The device is an electro-mechanical circuit coupled with a Fabry-Perot optical cavity. Our mission in this collaboration is to provide a miniaturized fiber-based Fabry-Perot cavity optimized for this purpose.
  • Type of collaboration: Joint research
  • Researchers:
    • Professor Koji Usami, University of Tokyo
    • Professor Kazuyuki Takeda, Kyoto University

3. Activities and Findings

3.1 Fabrication of linear ion traps (WG1)

In the cotinuation of the last FY's effort to fabricate substrates for linear ion traps using selective laser etching (SLE), we became able to produce such trap substrates with the in-house SLE machine at OIST. At the same time we ordered the same trap design to an external company FemtoPrint who specialises in SLE machining. The in-house substrates and one from FemtoPrint both implemented the necssary electrode structure successfully. Only difference is the roughness of the surfaces, with FemtoPrint ones being slightly better (Ra~ 120 nm). Therefore we chose to use the FemtoPrint substrates. They are subsequently coated with gold using an electron-beam evaporator and electroplating. We verified that the trap electrodes have low enough resistance as a result of the coating.    

A linear Paul trap fabricated in-house at OIST.

3.2 Trap assembly and installation in vacuum (WG1)

The ion trap assembly consists of three components: ion trap, routing circuit, and optical cavity. The ion trap and routing circuit are put together by mechanically clamping them to each other. In the first run of the experiment the optical cavity is not introduced to the system but machined glass substrates to imitate the housing that would host cavity fibers are introduced. They are mounted on nano-positioner stacks controlling the x-y alignement of the substrates. The alignment and calibration of the nano-positioners were conducted to ensure smooth insertion of the cavity substrate. All the ion trap components were assembled and wired within the vacuum chamber. After solving minor leak issues, the vacuum chamber undertook a prolonged bake. After the bake the base pressure at around 10-9 mbar was achieved. Even though a basic experiment can be run at this pressure level, it needs to be further improved in the coming months .

3.4 Home-made laser and Fabry-Perot cavity at 493 nm (WG2)

WG2 aims at a cavity QED experiment using Barium ions. Barium ions host a strong S-P dipole transition at 493 nm. By coupling the ion to the cavity resonant on this transition, we can expect a significant enhancement of the coupling strength in comparison to the traditional use of the P-D transition of Ca+. However whether a good quality optical cavity can be constructed at this wavelength is an open question. In particular, it was experimentally observed that cavities at violet to ultraviolet wavelength suffered increasing optical losses when they were placed in vacuum. We decided to answer this question first. We started from building an ECDL at 493 nm. The laser has a monolithic design derived from this paper. A test Fabry-Perot cavity was constructed outside vacuum. A ringdown measurement scheme was established to measure the cavity finesse, and the finesse was confirmed to be consistent with the mirror specification.

3.5 CO2 laser shooting of optical fiber facets (WG3)

The micro cavity mirrors to be integrated in the trap are fabricated on the facets of optical fibers. In order to produce an appropriate surface curvature on the facet and make the surface ultra-smooth, CO2 laser ablation is used. From the beginning of this year, we started to build the optical set up for the CO2 laser ablation. The main components such as the laser, AOM, 3D positioner and laser mirrors were ready but needed to be assembled. The set up includes a Michelson interferometer to measure the surface topography. The Michelson interferometer can be activated or deactivated by mechanically inserting a beam splitter into the beam path. We established a numerical algorithm to reconstruct the surface topography from multiple interferrograms taken at different sample positions. Matlab software was built to automate the shooting and reconstruction processes. Optical characterization of the laser beam was performed. Optimal beam parameters such as the beam power, pulse length, beam spot size were sought after by actually shooting fibers and glass plates. This effort is still ongoing. 

3.6 Laser ablation of Calcium (Rotation project)

The traditional loading scheme of ion trap is by thermal effusive ovens. This scheme is relatively straight foward: one only needs to run an electric current in the oven to heat it up. On the other hand, thermal ovens could cause several issues. Control over atomic flux is poor, allowing excessive exposure of the trap to the contamination by atoms. The heat causes mechanical instability especially for the integrated optical cavity. An  alternative scheme that can mitigate these issues is the laser ablation loading. We tested a laser ablation scheme for Calcium. Pulses from a Q-switched ns laser were shone on a sample in a vacuum chamber which was monitored with a residual gas analyzer (RGA). Two different samples were used, CaTiO3 and Ca itself. In the case of CaTiO3 no calcium peak was observed with the RGA irrespective of laser pulse energy. In the case of the Ca sample, we were able to detect an increase in partial pressure corresponding to 40Ca down to 150 uJ of the pulse energy. We could also see the deterioration of the ablation signal which decays after a few 1000 pulses. 

4. Publications

4.1 Journals

4.2 Books and other one-time publications

Nothing to report

4.3 Oral and Poster Presentations

  1. Takahashi, H. Fault-tolerant Quantum Computing with Photonically Interconnected Ion Traps (online invited talk), Moonshot Goal 6 Public Symposium, Japan, Mar. 11 (2022).
  2. Takahashi, H. Quantum computing with trapped ions (online invited talk),  Physics Frontiers with Quantum Science and Technology, Mar. 9 (2022).
  3. Takahashi, H. Ion-photon interface for quantum networks (online invited talk), Quantum Innovation 2021, Dec. 8 (2021).

4.4 Seminars

  1. Takahashi, H. Strong coupling of a single ion to an optical cavity: Towards quantum photonic interconnects between ion traps (online seminar), Atomの会, Japan, June. 21 (2021).
  2. Kassa, E. Ion-Photon Interfaces: Scaling quantum Information processing with optical cavities, OIST Internal Seminars, July 30 (2021).
  3. Takahashi, H. Quantum photonic interconnects for ion trap quantum computers (online seminar), OIST-KEIO Showcase Talks, Oct. 29 (2021).

5. Intellectual Property Rights and Other Specific Achievements

Nothing to report

6. Meetings and Events

Nothing to report

7. Other

Nothing to report.