Thai English

Cold Rubidium Atoms Gravimeter

December 27, 2019.

 

       There are two main energy sources which mobilize our world, solar energy and fossil fuel. The solar energy mostly has an influence on earth’s ecosystem as energy source for plants, agriculture and food production. This external unlimited energy source has driven human civilizations long time before beginning of the industrial revolution in 1760. Although the solar energy is an external source which mean it is free, its harvesting efficiency is low and power per unit area of this energy is very low. Another source of energy is the fossil fuel which has been accumulated beneath the earth surface for hundreds of million years. Many sources of fossil fuel were discovered around the globe before the Great War and become the most importance energy source that supplied post-war industries until now. The fossil energy source was estimated to be abundance and long term sufficient for the world.

       

       However, the expectation which was done seem to be not applicable for the current world and its future because of extra-high consume rate due to unexpected rising of human population. The rise is the result of technologies development and breakthrough especially on medical. The fossil fuel or petroleum reserve amount has been recalculated and the result is not favored, it would not last long and will be terminated in 30 years. This give a rise to the research on alternative energy sources such as solar energy and bio-energy which become global interested topic as a sustainable solution of the globe. In the meantime, searching of new petroleum source is still necessary work for lengthen the estimated time and is short to middle term solution for world industries.

 

       There are significant amount of undiscovered petroleum sources around the globe because of high exploration cost and comparably low success rate. The well-known technique which were used on petroleum exploration are such as quantitative map comparison/integration technique, seismic exploration and gravimeters.  By combine those technique properly, petroleum source location can be pinned. However, the success rate is not 100% because limited precision of equipment and tool. Therefore, there are rooms to develop precision of current petroleum exploration technologies. In this case, we would pay attention on the spring pendulum based gravimeter.

 

       Spring pendulum based gravimeters is used to measure value of gravity at certain location. It’s widely used in physics and geology and its precision rely on vibration stability of the ground and spring constant.  Spring based gravimeters in market are heavy and hard to carry. In addition, long settle time of the gravimeters is required in order to obtain no vibrating condition leading to accurate g. Moreover, the gravimeter required calibration before operation because its spring constant is time-dependent. The restrictions can be compensate or even eliminated by a new measurement technique using atomic properties which is assumedly constant and time-independent. A good example of obtaining ultra-high measurement accuracy and precision using the atomic properties is cesium atomic clock which only 1 second tick is expected to alter in 1 million years. In this project, atomic-based system is a good candidate to replace the spring-based system in the gravimeter which will be upgrade to cold atoms gravimeter.

 

       Cold Atoms Gravimeter (AG) employ atomic property, in this case is phase of matter wave function of atoms, to measure the earth gravity with 1 in 108 precision which is at least 100 time better than spring-based gravimeters. In the AG operation, an instance and calibration free measurement is guarantee because AG is less sensitive to the vibration of ground level and it used atomic system instead of spring, respectively.  The measurement process of AG is following: First, cold rubidium atom sample is prepared by mean of magneto optical trap (MOT) which is a combination of magnetic trap and optical trap. The MOT system is comprised of two parts, the first part is 2D MOT where the atoms are loading in the trap in 2D, and are optically transferred along to another dimension to second part of the system. In the second part, the transferred and pre-cool atoms are trapped by 3D MOT and then cool down to 10 micro-Kelvin by polarization gradient cooling technique (PGC). After that, the atom is shot up to 50 cm magnetically sealed tube by unbalancing MOT laser frequency detuning. The moving up atoms are optically pumped and populated to state  , which also lead to narrowing atoms cloud’s velocity distribution width.  Once the atoms reach height for interference, two-photons Raman pulses technique are applied to the atoms in order to generate Mach-Zehnder Atom Interferometer. The pulses sequence begin with  pulse, which is applied to equally populate atoms in to two ground state,  and . These two groups of atoms contain difference momentum which result in path and phase difference. Then a  pulse is applied in order to switch the atomic state of the atoms. This population switching allow recombination of the two separate groups. At the recombination point, another  pulse is applied to generate interference pattern. The atoms number of each state are counted and the phase shift of atomic wavefunction is calculated which finally lead to extraction of g with ultra-high resolution of . The resolution is equivalent to exitance of 108 kg rock salt at 1 km beneath the earth surface.

 

       Portable cold atoms gravimeter project was found in Quantum Atom Optics (QAO) laboratory which is located on the 2nd floor of Central Laboratory, Faculty of Science, Chiang Mai University (CMU) in October 2016. The project funding is supported by Thailand Center of Excellence in Physics (ThEP) and PTT Exploration and Production (PTTEP). In long term, the project is divided into two phases. The first phase involve prototyping of lab-based AG, which will be station in laboratory in order to be project’s proof of principle. The lab-based AG is basically a large cold atom experiment on 2x1 m2, 200 kg optical table and therefore unmovable. We have spent three year on this phase and plan to move to the 2nd phase by the end of 2019. The second phase is prototyping of portable AG. The system in the first phase will be redesigned the system in compact and portable way. The portable AG is expected to be in field tests and finalized design by the end of 2022.

 

       Currently, the experiment is in the first phase. We have success in trapping of cold atom sample in MOT (Figure 1), where the atoms are trapped in UHV vacuum chamber using near infrared (NIR) 780 nm diode lasers (Figure 2). The cold atom sample via MOT is the main part and a mile stone of this project. After that, the atoms will be shot up along 50 cm magnetically sealed tube and then Raman pulses will be applied to the atoms as explained above. The lasers using in this experiment were developed in QAO laboratory under cooperation of QAO researcher team, Faculty of Engineer, CMU and National Astronomical Research Institute of Thailand (NARIT).

 

 

Fig. 1 Picture of cold Rubidium-87 atoms cloud trapped in UHV system by magneto optical trap which comprised of 780 nm diode lasers and magnetic field gradient of 10 G/cm generated from anti-Helmholtz coils.

 

 

 

Fig. 2 780 nm diode laser systems developed by QAO researcher team.

 

       Research team hope that the portable atoms gravimeter project will succeed and practically work in real working environment by 2023. This will be a big step for Thailand petroleum exploration. Moreover, technologies, innovations and know-how which are by-products of the project would be applicable and useful for Thailand’s quantum research in the future and therefore leading to reduction of technological imports.

 

Reported by

 

Asst. Prof. Waranont Anukool

Quantum Atom Optics Laboratory, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

E-mail: waranont@qaocmu.org