July 22, 2019.
Matter-wave interferometry can be applied for precision metrology with atoms and small molecules, in particular for the measurements of all kinds of inertial forces for instance the variations of the rotation of the Earth, local gravity acceleration (g) as well as gravity gradients. In additions, the measurement of internal property of atoms and molecules such as polarizability provides several advantages in atomic physics and chemistry. The knowledge of inertial forces is important for many fields and applications: geography, and geology, e.g. the gravity gradients can be caused from Earth quakes, geological deformations, waves (Tsunamis) and also the exploration of natural resources, e.g. crude oil. In physics, the general relativity can be observed and better understood if one can search the gravitational waves, or even the fundamental constants, e.g. Newton’s constant, fine structure constant. The quantum mechanics of atom interferometer can serve for the space applications, engineering, and telecommunication with space satellites, and eventually future space missions.
Matter wave interferometry can be performed into two ranges, i.e. near-field and far-field diffraction. In this research, it will be conducted in the near field range which makes the experimental setup smaller than far field diffraction as well as it provides high sensitivity tool.
In this research project which is supported by the Thailand Center of Excellence in Physics (ThEP Center) during March 2017 – February 2020, we will combine both matter-wave interferometry and cold atoms and realize the first matter-wave interferometer in Thailand. The research project is relied on the strength of atom interferometry that is sensitive to external perturbations or external fields which overcomes a light interferometry. Therefore, it provides an idea of the measurement of local gravitational field with accuracy in the range of at least 10-7g with the condition of using a cold atomic source in phase I. The advantages of this method are that the system is simple and the accuracy is based on the technology of measuring the distance between the grating and the atom detection. The accuracy of the gravity field measurement can be better with the improvement of the given distance measurements. In additions, the method will be performed with the cold atomic source, which makes high visibility interference patterns. The main goal of this research project is to demonstrate our proof-of-principle of the near-field Talbot-Lau atom Interferometer. This will be the base knowledge for producing the novel gravimeter. And this high accuracy gravimeter can be potentially improved for the exploration of oil resources and natural gas better than technology used nowadays. In addition, in 2018 the research group also introduced a new method to probe optical vortices  and has a plan to use this with matter-wave interferometry in order to possibly increase the sensitivity of our gravimeter in the future.
The local Earth gravitational measurement or so-called gravimeter is a geological physics technique that is used to explore natural sources, oil and natural gas . Although, it was not as popular as early seismology, but it is increasing significantly especially in shallow target areas.
Gravimeters have been developing continuously over the past 25 years, especially the capability to use in a mobile environment. Moreover, when they combine with the global positioning systems (GPS), the result will give more accurate positioning including the use of a remote control system in hard areas. This will make gravimeters as a main technique for the exploration of natural resources, oil and natural gas in the near future.
Table 1. Comparison of various gravimeter techniques .
Gravimeter technique has several methods as shown in Table 1. It shows 4 examples of gravimeter. For example, the spring / mass system, which has the advantages of being small, easy to transport but the accuracy is low and the measured value is easily changed from the deterioration of the spring. In terms of using superconductors in the magnetic field and under the gravity, it provides the lowest drift compared to the others. Nevertheless, the system has a lot of restrictions on the environment. The next method is to use a laser interferometer which is vertically reflected from a mirror hanging on soft spring. Therefore, the force that changes the spring displacement coming from the gravitational acceleration. But the limitation of this method is that it requires special springs that eliminate the effects of any high frequency vibrations. The last method is the atom interferometer. This method can provide the best accuracy, if the accumulation time is longer.
The method of measuring the local Earth gravitational acceleration with the first atomic interferometer was based on the method in the far-field regime, i.e. Mach-Zehnder-type atom interferometer as shown in Figure 1. In principle, a cold atomic beam is used in the experiment. By dropping atoms under the gravity, the atomic beam is separated into two beams with the laser pulse. Subsequently, it is combined again with the laser pulse and at the end, the atom is again separated by the laser pulse. Data processing is based on the interference of the atoms behind the last laser, which is the same method as a laser Mach-Zehnder interferometer. This method is the most accurate method to the present day with the resolution of the Earth's gravitation of with the integration time of two days [3, 4]. Figure 2 shows a gravimeter based on this method by shooting atoms in the vertical direction. Therefore, this scheme is called the atomic fountain and Figure 3 shows an example of the gravimeter based on this atomic fountain .
Fig. 1 Mach-Zehnder-type atom interferometer .
Fig. 2 Gravimeter based on atomic fountain .
Fig. 3 Gravimeter based on atomic fountain from Beautemps–Beaupré ship .
2. Near-field Talbot-Lau Atom Interferometer
Near-field Talbot-Lau atom interferometer, which this research project conducted is a method that has never been done before. This interferometer will be performed with mechanical gratings as shown in Figure 4. The method uses cold atoms with laser cooling and trapping of 6 beams configuration and anti-Helmholtz coils. This method is called magneto-optical trap (MOT). After that, the cold atoms are dropped in free-fall motion under the influence of gravity through the first grating to produce an atomic coherent beam. The second grating makes the diffraction of cold atoms. The atom near-field interference pattern can be appeared at the certain distance behind the second grating, called the Talbot length. This interference pattern is also modified by the gravitational acceleration. Therefore, the gravitational acceleration can be extracted by this interference pattern. The precision of the measurement from this method depends on the accuracy of the measurement of grating period  and the measurements of the distance between the two gratings and between the second grating and the detector, which can be precisely measured with a commercial high-precision distance measurement sensor.
Fig. 4 Gravimeter based on near-field Talbot-Lau atom interferometer.
3. Work in Progress
The progress of the research project consists of several parts. The first part is to create an approach theory to estimate the gravitational acceleration from empirical results . In addition, a precision method for measuring the grating period has been introduced . In order to establish the gravimeter, one needs to prepare rubidium (Rb) cold atomic beam first, which is the source of the interferometric experiment. A magneto-optical trap (MOT) is used to prepare Rb-85 cold atoms. An external cavity diode laser (ECDL) is a tunable laser that can adjust the frequency of the laser to the cooling (trap) and repuming frequencies of the Rb-85 atom as shown in Figure 5. The cooling laser can excite atoms from F = 3 to F' = 3,4 and the repumping laser can pump the atoms back to F = 3 for successive cycles. Figure 6 shows our ECDL used to tune the frequency and monitor by our home-made spectroscopy. Figure 7 represents the rubidium Doppler-broadened spectral lines and hyperfine structures. We have finished the first ECDL for trap laser and the second (repumping) laser is in the final stage of construction. When we complete the two ECDL lasers, the next task will be a MOT system integration. Finally, the design and construction of our UHV vacuum system as well as an atom detector are in the process of being completed (Figure 8).
Fig. 5 Energy level diagram of Rubidium-85 with two lasers for MOT, i.e. trap laser and repumping laser .
Fig. 6 Our home-made ECDL.
Fig. 7 Example of Doppler-broadened spectral lines and hyperfine structures from our ECDL and home-made absorption spectroscopy.
Fig. 8 Our UHV vacuum chamber for testing atom detector (P~10-9 mbar).
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Assoc. Prof. Dr. Sarayut Deachapunya
Quantum and nano optics laboratory (http://www.qnolab.org/)
Department of Physics, Faculty of Science, Burapha University, Chonburi - 20131