A High Precision Mobile Atom Interferometer
M. Schmidt, A. Senger, M. Hauth, S. Grede, C. Freier, T. Kröker, A.
Peters
In recent years, matter wave interferometry has developed into a powerful tool for the ultra precise measurement of accelerations and rotations. It is used in various laboratories for experiments in the fields of fundamental physics and metrology. However, up to now these measurements have been limited by the fact that laboratory-based atom interferometers are anything but easily transportable which prevented this technology from being employed for on-site high precision measurements. Making this new method of gravity measurement available outside of laboratories opens up new possibilities in the fields of fundamental physics, navigation, seismology and geology, amongst others.
The FINAQS and EuroQUASAR/IQS projects are collaborations of various European workgroups which aim at developing mobile high precision quantum sensors. Here at the QOM workgroup in Berlin we are working on a high-precision gravimeter based on atom interferometry that will be used for on-site measurements of local gravity.
On this page, you will find an overview of atom interferometry in general and our Berlin gravimeter. For new developments, status updates and latest measurements, click here.
A short introduction to atom interferometry
A moving cloud of ultracold atoms in the ground state is subjected to three laser beams. These light beams are prepared in such way that they induce raman transitions between the two hyperfine ground states of the atoms. A Pi/2 pulse will transfer half of the atoms into the upper hyperfine ground state. Because these atoms absorb two photons in the process, they additionally receive a recoil impulse and seperate from the atoms that are left in the ground state. A Pi pulse mirrors the states and a second Pi/2 pulse recombines the two clouds. The number of atoms remaining in the excited state depends on the overall phase difference of the matter wave packets that has accumulated between the two paths.
The phase difference depends on two external factors: One term results from rotation and a second term is caused by acceleration.
Since we cannot determine two unknown factors (rotation Omega, acceleration g) in the same measurement, there are two distinct setups of atom interometers, each designed to eliminate one of these factors in order to measure the other. In a gyroscope setup, two atomic cloudes are launched simultaneously in opposing directions in order to eliminate the acceleration contribution, whereas in a gravimeter setup the enclosed area between the paths of the two matter waves is zero which cancels out the rotational contribution.
Our Berlin setup is a gravimeter using rubidium atoms, designed to measure the local gravity g at highest precision.
Our gravimeter consists of three principal parts: Firstly, there is the vacuum chamber and its suspension in which the trapping, manipulation and detection of the rubidium atoms takes place. The atoms themselves are prepared in a 2D MOT module which is connected to the main chamber. The third part is the laser system which delivers all the light that is necessary for trapping, cooling, preparing and detecting the atoms and that of course also provides the Raman beams for the interferometry sequence as described above.
The vacuum chamber consists of three regions. At the bottom, the atoms are trapped and cooled in a somewhat spherical 3D MOT chamber. Light is inserted by specially designed fiber launchers that are connected to the chamber. The atoms are then launched upwards and pass through the preparation/detection chamber in which all atoms are transferred into the low hyperfine ground state. The top of the vacuum chamber is a long tube where the actual interferometry takes place. During the atoms' parabolic flight, the three raman pulses are applied from optics at the top and the bottom of the chamber. Afterwards, the populations of the two hyperfine states is detected in the preparation/detection region while the atoms are on their way down.
Because our apparatus is mobile, special care had to be taken in the design of its suspension, which you can see in the image above. The chamber is mounted in a rotatable frame on a heavy base plate. The interferometry region is magnetically shielded. The overall height of the apparatus is no more than that of a human being, since it has to fit through standard laboratory doors.
Seven distinct laser frequencies in three principal frequency classes are needed for our experiment, as you can see below. In order to deliver these to the main apparatus and still keep the complete system mobile, four seperate laser modules are required.
Each of the four laser modules is mountable in a 19" rack together with the control electronics. On the road, the laser system will consist of two standard electronics racks of 180 cm height. Inside the laser modules, all optics mounts are selfmade, as you can see below on the example of the reference laser module.
In order to achieve faster loading times in the 3D MOT, it is injected by a beam of precooled atoms coming from a 2D MOT. Here rubidium atoms are trapped and cooled in two dimensions and accelerated towards the 3D MOT in the third. A stacked sequence of three cooling stages is used for very high atom flux.
Click here for new developments, status updates and latest measurements!
In recent years, matter wave interferometry has developed into a powerful tool for the ultra precise measurement of accelerations and rotations. It is used in various laboratories for experiments in the fields of fundamental physics and metrology. However, up to now these measurements have been limited by the fact that laboratory-based atom interferometers are anything but easily transportable which prevented this technology from being employed for on-site high precision measurements. Making this new method of gravity measurement available outside of laboratories opens up new possibilities in the fields of fundamental physics, navigation, seismology and geology, amongst others.
The FINAQS and EuroQUASAR/IQS projects are collaborations of various European workgroups which aim at developing mobile high precision quantum sensors. Here at the QOM workgroup in Berlin we are working on a high-precision gravimeter based on atom interferometry that will be used for on-site measurements of local gravity.
On this page, you will find an overview of atom interferometry in general and our Berlin gravimeter. For new developments, status updates and latest measurements, click here.
A short introduction to atom interferometry
A moving cloud of ultracold atoms in the ground state is subjected to three laser beams. These light beams are prepared in such way that they induce raman transitions between the two hyperfine ground states of the atoms. A Pi/2 pulse will transfer half of the atoms into the upper hyperfine ground state. Because these atoms absorb two photons in the process, they additionally receive a recoil impulse and seperate from the atoms that are left in the ground state. A Pi pulse mirrors the states and a second Pi/2 pulse recombines the two clouds. The number of atoms remaining in the excited state depends on the overall phase difference of the matter wave packets that has accumulated between the two paths.
The phase difference depends on two external factors: One term results from rotation and a second term is caused by acceleration.
Since we cannot determine two unknown factors (rotation Omega, acceleration g) in the same measurement, there are two distinct setups of atom interometers, each designed to eliminate one of these factors in order to measure the other. In a gyroscope setup, two atomic cloudes are launched simultaneously in opposing directions in order to eliminate the acceleration contribution, whereas in a gravimeter setup the enclosed area between the paths of the two matter waves is zero which cancels out the rotational contribution.
Our Berlin setup is a gravimeter using rubidium atoms, designed to measure the local gravity g at highest precision.
Our setup
Our gravimeter consists of three principal parts: Firstly, there is the vacuum chamber and its suspension in which the trapping, manipulation and detection of the rubidium atoms takes place. The atoms themselves are prepared in a 2D MOT module which is connected to the main chamber. The third part is the laser system which delivers all the light that is necessary for trapping, cooling, preparing and detecting the atoms and that of course also provides the Raman beams for the interferometry sequence as described above.
Mechanical setup
The vacuum chamber consists of three regions. At the bottom, the atoms are trapped and cooled in a somewhat spherical 3D MOT chamber. Light is inserted by specially designed fiber launchers that are connected to the chamber. The atoms are then launched upwards and pass through the preparation/detection chamber in which all atoms are transferred into the low hyperfine ground state. The top of the vacuum chamber is a long tube where the actual interferometry takes place. During the atoms' parabolic flight, the three raman pulses are applied from optics at the top and the bottom of the chamber. Afterwards, the populations of the two hyperfine states is detected in the preparation/detection region while the atoms are on their way down.
Because our apparatus is mobile, special care had to be taken in the design of its suspension, which you can see in the image above. The chamber is mounted in a rotatable frame on a heavy base plate. The interferometry region is magnetically shielded. The overall height of the apparatus is no more than that of a human being, since it has to fit through standard laboratory doors.
Laser system
Seven distinct laser frequencies in three principal frequency classes are needed for our experiment, as you can see below. In order to deliver these to the main apparatus and still keep the complete system mobile, four seperate laser modules are required.
Each of the four laser modules is mountable in a 19" rack together with the control electronics. On the road, the laser system will consist of two standard electronics racks of 180 cm height. Inside the laser modules, all optics mounts are selfmade, as you can see below on the example of the reference laser module.
2D MOT
In order to achieve faster loading times in the 3D MOT, it is injected by a beam of precooled atoms coming from a 2D MOT. Here rubidium atoms are trapped and cooled in two dimensions and accelerated towards the 3D MOT in the third. A stacked sequence of three cooling stages is used for very high atom flux.
Click here for new developments, status updates and latest measurements!
