Progress in this reporting period can be separated into three tasks: planning for flight operations on the Cold Atom Laboratory (CAL), development of new atom-based inertial sensing methods, and development of new precision measurement techniques based on atom interferometry.
For flight planning, we have developed and analyzed three related experiments. The first is adiabatic cooling and release, in which Bose-condensed atoms are released from a magnetic trap into free space by slowly reducing the magnetic field amplitude. When the trap field is suitably low, the atoms are transferred to a magnetically insensitive state to avoid degradation of subsequent measurements from environmental fields. When performed correctly, this method can produce extremely cold atoms, with temperatures on the order of 100 picoKelvin (pK). This corresponds to extremely low atomic velocities, which prevents the atom sample from expanding or drifting out of the interaction region in subsequent experiments. Adiabatic expansion also provides the minimum possible size expansion for a given amount of cooling, so the final sample is relatively compact.
Successful implementation of adiabatic expansion requires careful control of the magnetic fields. We have imported the CAL field design into a numerical simulation tool and developed a set of control trajectories for the fields. The expansion method is limited by uncontrolled environmental fields and gradients. We developed an expansion to an estimated temperature of 200 pK which is robust against the expected level of stray fields. We plan to continue investigating the field geometry to determine if further cooling is possible.
We have also developed experiments to implement atom interferometry using a Bose-Einstein condensate that has been released from the trap. A set of experiments will be used to optimize parameters and test the performance of atom interferometry. A culminating experiment will be a measurement of the atomic recoil frequency using a contrast interferometer.
We also developed a method to implement simultaneous interferometers using both rubidium and potassium atoms. This requires careful control of the laser beams used to manipulate the atoms to ensure that both species respond correctly. Using this technique we can measure the ratio of the atomic recoil frequencies, which could ultimately provide improved knowledge of the mass ratio of the species.
Finally we have proposed and developed an alternative inertial measurement technique in which the atomic sample is used as a "proof mass" reference for rotation sensing. A set of three atom clouds can be prepared in a line that is aligned to the controlling laser beam. After a delay time the atom clouds can be imaged, and any rotation of the system will appear as a deviation in the apparent orientation of the line. This method is readily sensitive enough to detect the orbital motion of the International Space Station.
These experiments have been presented for our Science Concept Review, and were approved by the advisory board for CAL.
On the ground, we are also developing the "proof mass" rotation sensing technique. This is more challenging since it is not possible to observe the atoms for a long time without supporting them against gravity. However, we have implemented a magnetic trap with excellent cylindrical symmetry such that atoms can be set to oscillating along one axis, and over time the Coriolis force causes the axis to precess. We have observed sensitivity at the level of 1 mrad/s, and expect to be able to reach Earth rate sensitivity with further optimization.
Also on the ground, we have developed a high-precision method for tune-out spectroscopy. This is the determination of a light frequency for which an atom has zero response. Using light at this frequency can be useful for some applications like dual species atom trapping. It also allows a precise characterization of the quantum state of the electrons in an atom. Our technique is based on atom interferometry and is about one hundred times more precise than previous methods. The resulting improvements in our understanding of electrons in atoms will be useful for many applications. A notable example is interpreting parity violation experiments in atoms, where the quantum state is needed in order to relate the measured parity violation in the atom to the fundamental properties of the electron-nucleon interaction. Understanding these properties better will improve our understanding of fundamental particle physics.
These types of experiments would benefit greatly from a microgravity environment, since that would allow long interaction times without needing to support the atoms against gravity. The magnetic fields we use to support the atoms introduce a number of perturbations that must be controlled for and limit our precision.
Articles have been submitted to the following peer-reviewed journals:
Fallon AG, Sackett CA. "Obtaining atomic matrix elements from vector tune-out wavelengths using atom interferometry." Atoms, in press, expected publication July 2016.
Oh E, Horne RA, Sackett CA. "Fast phase stabilization of a low frequency beat note for atom interferometry." Rev Sci Instrum, in press, expected publication July 2016.