Research Subjects

Pursuing the ultimate accuracy and stability with "optical lattice clocks"

Since the proposal of "optical lattice clock" by professor Katori in 2001 and the first demonstration by our laboratory in 2003, lattice clocks now provide 18 digits of time measurement accuracy (corresponding to an error of one second in 30 billion years). Optical lattice clocks have been attracting attention as a promising candidate for the "redefinition of the second". With 18 digits of clock accuracy, a tiny height difference of 1 cm on the ground can be detected through a time dilation due to the Einstein's general relativity. Now, the role of such clocks is changing from a mere tool to share the time to advanced fields like exploring the relativistic space-time distortion induced by gravity.

Currently, our research can be divided into four themes.

① "Demonstration of relativistic geodesy": we perform high-precision comparison of remote clocks located in Hongo and Wako (RIKEN) connected by a noise-cancelled optical fiber link. This measurement yield the height difference between two places with 1 cm accuracy, and has become a pioneering field of research in geodetic techniques utilizing the relativistic time dilation (relativistic geodesy). In future, development of transportable clocks will expand the possibility of clocks' application to novel tools such as a sensor to search underground resources and monitoring of volcanic activity.

② "Development of super-radiant laser": using a cooperative phenomenon of atoms called super-radiance, we develop a laser source with a narrow-linewidth spectrum.

③ "Development of optical lattice clocks with 19-digit accuracy that incorporates the multipolar effect": we challenge the ultimate accuracy of optical lattice clocks towards 10^-19. Basic premise of such a high-precision atomic clock is the assumption that "physical constants is a constant."

④ "Verification of the constancy of the fundamental constants by the atomic clock comparison": a high-precision comparison of the optical lattice clock with Sr, Yb and Hg atoms verifies the constancy of the fine structure constant to give the most stringent upper limit of variation thus far.

Laser cooling and trapping of neutral atoms

By incorporating speed and position dependence cleverly into momentum transfer between atoms and lasers, a damping force along with a centripetal force can be exerted against the atoms. In this manner, the cooled and trapped atomic ensemble with a temperature of around milli-Kelvin or micro-Kelvin is a very clean sample for high-precise spectroscopy free of any Doppler broadening. Moreover, as the de-Broglie wavelength is incomparably larger than that of room-temperature atoms, the wave nature of atoms can be shown in the form of interference. It has become a starting point for the demonstration of quantum-mechanical effects. Currently, we perform laser cooling and trapping of Sr, Hg, Yb and Cd atoms.

Development of atom chip

By coherent control of motion of laser-cooled atoms on a solid substrate, it is aimed to realize quantum information processing system using neutral atoms with a wealth of internal degrees of freedom in comparison to electron or photons. While the other groups in the world use Zeeman effect induced by a magnetic field for manipulation of cold atoms, we utilize the Stark effect induced by an electric field. This allows to avoid problems such as heat generated by the current producing magnetic field and to realize a low power consumption and high input-impedance device.

Quantum computation using neutral atoms

Due to the limitation of the energy loss (heat generation) and the degree of integration, the current generation of computers confront the plateau of its processing capacity. Furthermore, since the treatment method is only a combination of essentially independent elements, even if a plurality of computers are operated in parallel, the calculation speed is limited. Therefore, a quantum computer is expected to have a potential to overcome both of these problems. As the quantum computer utilizes the superposition of the wave function as a basic logic element (qubit), reversibility (unitary transformation) and parallelism (principle of superposition) are inherent and in principle the energy loss is absent in such devices. To configure this qubit, we utilize laser-cooled neutral atoms. Specifically, by irradiating a laser beam to a sufficiently cooled strontium atoms loaded into the optical lattice, we perform the operation corresponding to the unitary transformation and 2-qubit gate operation to generate the entangled state (entanglement) of atoms.

Atomic interferometer

From the beginning of quantum theory, the wave nature of matter particles such as electrons and neutrons has been recognized. The wave nature has motivated the development of electron and neutron interferometers which has established the foundations of quantum mechanics. It has already been known in the 1930s that the atom, which is their composite particles, also exhibits wave nature. However, an atomic interferometer has not been considered for a long time. This is because the wavelength at room temperature is very short (on the order of pico meter) due to their mass and it was impossible to create a device such as a diffraction grating with comparable size to the wavelength of atoms. However, advances in laser cooling techniques have made the atom interferometer a reality by extending the wavelength of the atom to about the wavelength of light. Compared to the light, the laser-cooled atoms pass through the interferometer at a very low speed, giving rise to 10 orders of magnitude greater sensitivity, for sizes similar to light interferometers. Moreover, as the atom has a mass, the effect resulting from gravity can be measured precisely.

Related technology

Our policy is to design and develop our own experimental apparatus in order to explore new paradigms of research

Laser sources

For experiments such as producing ultracold atoms, trapping and manipulation of atoms, high-resolution spectroscopy of atomic transition, etc., we use many different kind of home-made lasers operated at wavelengths ranging from 229 nm to 2.9 µm. All the lasers are controlled by electronic feedback circuits to narrow down their frequency linewidth and kept locked to the wavelength of the atomic transition.

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Vacuum technology

Laser cooling experiment is performed under ultra-high vacuum (UHV) environment. We realize vacuum pressure of around 10^-10 Torr with optical cavities and fibers installed inside the chamber using vacuum sealing techniques (ex. ICF flange and Indium sealing).

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Electronic circuit technology

Electronics serves a key role in our experiments. We control the laser frequencies very accurately to the required frequencies using electronic circuits in order to probe atomic transitions with high resolution. Almost all the components are designed and developed indigenously. One can learn how to construct an electronic circuit with our step by step guidance.

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Machining technique

Machining is an integral skill in our experiment. You can learn machining skills using tools such as drilling machines, cutting screw threads by using a tap etc. with professional support.