NOTE: Extended to 9/30/2022 per U. Israelsson/JPL (Ed., 3/9/21)

NOTE: Extended to 10/28/2020 per PI (Ed., 2/28/2020)

NOTE: Extended to 10/30/2019 per U. Israelsson/JPL (Ed., 12/14/17)

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

Unfortunately, the apparatus began suffering a series of problems while this study was underway. Initially, the atom sourcing of the CAL 3 apparatus failed, so that cold atom samples could not be reliably produced. The team decided to suspend operations until the new CAL 3B apparatus could be installed as a replacement. However, the 3B apparatus was discovered to have a vacuum leak, rendering it unusable for science. We are grateful to NASA and the Jet Propulsion Laboratory (JPL) for expediting the launch of the SM-1 system so that there is a chance to resume science operations in 2024. The SM-1 system will permit dual species operation, so we will be able to continue our adiabatic expansion studies.

We have also developed a collaboration with Mark Edwards of Georgia Southern University to further analyze data from our early CAL 2B work. In that work, we observed the trajectory of atoms released from an adiabatically released trap. The trajectory provided information about the temperature of the atoms as well as the background magnetic field. With Edwards, we have developed a new technique to also analyze the changing size of the released cloud, which provides additional information about the temperature and field. Preliminary results support our original conclusions, but provide reduced uncertainties on the quantities of interest.

In our ground-based work, we continued working on our atom-interferometer gyroscope project. After spending the previous year investigating the possibility of using a compact atom-chip system for the interferometer measurements, we have this year transitioned back to our lab-scale system. We completed several characterizations of the orientation and anharmonicity of the magnetic trapping potential that are needed to understand the interferometer performance. We also implemented an improved technique to compensate for tilting of the trap with respect to gravity. Unfortunately, we also discovered that a new noise source has appeared, causing the position of the atoms in the trap to fluctuate unacceptably from run to run. We have recently been investigating the source of this noise and working to eliminate it.

NOTE: Extended to 9/30/2022 per U. Israelsson/JPL (Ed., 3/9/21)

NOTE: Extended to 10/28/2020 per PI (Ed., 2/28/2020)

NOTE: Extended to 10/30/2019 per U. Israelsson/JPL (Ed., 12/14/17)

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

A particular goal of our efforts is to prepare a very cold sample of Rb atoms in a non-magnetic state to use as a proof mass for acceleration and rotation sensing. For acceleration sensing, the idea is to release the atom cloud nearly at rest and use non-destructive imaging to determine their location. At time T later, the atoms would be imaged again, and finally a third time at time 2T. This would give the cloud center positions at three distinct times, from which the apparent acceleration could be obtained by direct comparison. For rotation sensing, the release atoms would be subject to a splitting pulse from CAL’s atom interferometry Bragg beam, producing two packets with opposite velocities. After time T, the Bragg beam would be applied again to bring a fraction of each packet to rest. Non-destructive imaging would be applied to measure the packet centers. The atoms would then freely evolve for time T, after which the packets would be imaged again. The rotation vector can then be obtained from the change in orientation of the two clouds.

In order for this method to succeed, it is important that the atom cloud centers not drift out of imaging range during the observation time T, and that the clouds not expand so much that the absorption imaging signal becomes too weak. These can both be achieved by adiabatically expanding the clouds into a trap with confinement frequencies of a few Hz, similar to what was achieved in SM2. So far, the results in SM3 have been unfavorable, as we have run into difficulties reaching trap frequencies below about 15 Hz. We are, however, continuing to investigate this problem and hope to resolve it soon.

In our ground-based work, we continued working on our atom-interferometer gyroscope project. During the previous year, we had obtained a compact atom-chip system from ColdQuanta. It is designed to allow interferometer operation similar to what we achieved in our laboratory-scale apparatus, but with a much smaller footprint, lower power consumption, and faster repetition rate. We spent most of the performance period learning how to work with the new apparatus and resolving some problems with it. Due to its compact design, any modifications are challenging and time consuming. Although we made considerable progress improving its operation, we came to the conclusion that there were still many issues that could be addressed more quickly using the larger lab-scale apparatus. So we have for now set the compact apparatus aside. We hope to return to it in the future when we can be more certain exactly what configuration is required to operate with the atom chip.

NOTE: Extended to 9/30/2022 per U. Israelsson/JPL (Ed., 3/9/21)

NOTE: Extended to 10/28/2020 per PI (Ed., 2/28/2020)

NOTE: Extended to 10/30/2019 per U. Israelsson/JPL (Ed., 12/14/17)

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

CAL suffered technical issues later in the year, and after these were repaired, the facility was focused on implementing cooling of potassium atoms. This limited time available for other experiments. However, the adiabatic expansion techniques that we previously developed proved highly useful in bringing the CAL system back online with a new trapping configuration, and for working with potassium after cooling was successful.

In our ground-based work, we made substantial progress in our gyroscope progress, where we rebuilt the apparatus to improve reliability and stability. In the new apparatus, we demonstrated a Sagnac interferometer with an enclose area about ten times larger than our previous results, and we were able to maintain the interference signal after the atoms completed two orbits through the Sagnac path. Furthermore, with the improved stability we demonstrated continuous operation for approximately 36 hours, which was a sufficient time to obtain quantitative data on the stability of the Sagnac signal. Although the signal exhibited more noise than expected, it did not exhibit long term drifts. We believe that the anomalous noise is due to instability of our trap magnetic fields and we have developed a remediation plan.

Our other ground experiment uses atom interferometry to characterize tune-out wavelengths in rubidium, optical wavelengths where the dynamic polarizability of the atom is zero. Last year a new student started on this apparatus, and was able to complete an analysis of earlier data. Those results were written up and published in January 2022.

NOTE: Extended to 9/30/2022 per U. Israelsson/JPL (Ed., 3/9/21)

NOTE: Extended to 10/28/2020 per PI (Ed., 2/28/2020)

NOTE: Extended to 10/30/2019 per U. Israelsson/JPL (Ed., 12/14/17)

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

Some additional work remains to be done with the SM1 dataset. In our time of flight expansions, the Bose condensates can be observed to expand and twist under the influence of the background field. It should be possible to use this data to confirm our field model, but the calculations are challenging. We have initiated a theoretical collaboration with the group of Mark Edwards at Georgia Southern University, with whom we hope to develop tools for this type of analysis. These tools will be useful for many of the experiments to be performed on CAL and eventually Bose-Einstein Condensate/Cold Atom Laboratory (BECAL).

The SM3 system was installed in December 2019 and the first Bose-Einstein condensates were quickly achieved in February 2020. The new system features a more complex atom chip geometry than SM1. This allows for atom interferometry experiments, but also makes sample preparation more complex. We have developed and implemented an adiabatic expansion sequence which positions atoms near the center of the atom interferometry beam. The confinement strength is reduced enough to avoid rapid expansion of the released atoms. The residual ‘sloshing’ motion of the atoms is small. The performance obtained so far, in terms of both confinement and sloshing, is not yet as good as we achieved in SM1. This can be improved with further work but we expect the initial results are sufficient for the atom interferometry experiments being performed.

We have also carried out atom interferometry experiments using atoms prepared with a sequence developed by the Jet Propulsion Laboratory (JPL) team. (This preparation leads to larger release velocities than ours, but so far this is not a limitation.) As a demonstration of the atom interferometer capability, we have demonstrated a simple low-precision measurement of the atomic recoil velocity. We expect to publish the results in 2021.

We also continue work on improving our ground-based trapped-atom Sagnac interferometer. Although the COVID shutdowns had some impact on our progress, we have rebuilt our condensate apparatus to use an atom chip which was developed by our collaborators at Air Force Research Laboratory (AFRL). We have achieved Bose-Einstein condensation in the new apparatus and, significantly, we have demonstrated that the atom trapping potential is at least an order of magnitude more stable than in our previous trap. We will take advantage of this stability to improve our rotation measurements.

NOTE: Extended to 10/30/2019 per U. Israelsson/JPL (Ed., 12/14/17)

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

In the flight effort, we took data through 2019 and demonstrated adiabatic cooling. As mentioned in the previous report, we observed unexpected heating and loss of the atoms during the cooling process. The effect seemed intermittent, making it difficult to study systematically. We ultimately found an experimental procedure that usually provided enough atoms to continue cooling, but we were not able to conclusively explain the source of the losses. We were able to successfully expand atoms into a trap with a mean frequency of about 3 Hz, corresponding to a temperature of about 1 nK. While this remains above our ultimate goal of 0.1 nK, it is an important milestone because it corresponds to residual velocities low enough to enable atom interferometry.

We were unable to reach lower temperatures because the apparatus featured a background magnetic field gradient that was somewhat larger than expected. We measured the field gradient to be about 50 mG/cm, compared to a specification of 10 mG/cm. This field distorts the trap and causes the atoms to be lost when the trap is too weak.

We were able to release the atoms from the 3 Hz trap and observe their subsequent behavior. Because of the large background gradient, the atoms were accelerated relatively quickly, but they could be observed for about 0.5 s. To avoid this acceleration, we hope to transfer the atoms the m = 0 Zeeman state where they do not interact with the magnetic field. This has been previously demonstrated in the CAL apparatus.

At the end of 2019, the apparatus stopped functioning well, but at the same time the new SM2 module was launched as replacement unit. SM2 has now been installed and is operating well. It offers the capability to perform atom interferometry, which will be the focus of the next year’s operation.

In our ground efforts, we have successfully implemented an atom interferometer gyroscope with Earth-rate sensitivity. A Bose-Einstein condensate is produced in our specially-designed magnetic trap. Off-resonant lasers are used to coherently split the condensate into wave packets, which are then driven into circular trajectories orbiting the trap with a diameter of about 0.5 mm. When the packets are subsequently recombined they exhibit interference, and the phase of the interference is related to the rotation rate of the experimental platform. We observe interference signals with a visibility of about 60%. We have verified the rotation sensitivity by slightly rotating the optical table holding the apparatus, and the measured interference shift agrees with expectations. This work has been written up and submitted to Physical Review Letters.

We have also carried out a theoretical study of the interferometer performance and limitations. This turns out to depend sensitively on the anharmonicity of the trapping potential. We developed a method to experimentally characterize the anharmonicity and were able to determine the anharmonic terms with an accuracy of 10% to 30%.

A major effort over the past year has been upgrading the apparatus to improve its stability and speed of operation. This will enable more careful studies of its performance and extension to higher sensitivities. The new trap features additional shimming coils to allow adjustment of the anharmonicity. The new apparatus is nearly complete. We have collaborated with our partners at Air Force Research Laboratory (AFRL) to produce a unique atom chip that the trap will use.

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

In the flight effort, the CAL system launched in spring 2018 and PI operations commenced in November 2018. We have run about 500 successful sequences on the apparatus, pursuant to our effort to implement adiabatic cooling to very low temperatures, on the order of 100 pK or lower. This will be achieved by gradually relaxing the magnetic trap in which the atoms are confined. Initially, the atoms are held about 150 um from the atom chip that produces the trap magnetic fields, with an atom oscillation frequency of about 1200 Hz. Our goal is to move the atoms to about 1 mm from the chip, and reduce the trap frequency to 1 Hz or lower, while minimizing any motional excitation or sloshing of the atoms.

We have successfully implemented a protocol for the first stage of this process, displacing the atom to the desired final location and reducing the trap frequency to 100 Hz, with no measureable motion excitation. We are presently working on reducing the trap frequency to about 2 Hz. We have observed atoms in a trap with this frequency, but accompanied by an unexpected source of heating or atom loss. We are investigating this loss process. If this can be resolved, it should be possible to attain temperatures below 1 nK in this trap. We expect to resolve this issue within the next few weeks. The final stage of expansion will require careful adjustment of the trapping fields to produce a stable but very weak trap. The effort required is not yet easy to predict, but there is a good chance this project will be completed by the end of the current period. Follow-up projects include testing the use of supercooled atoms as an inertial reference mass, and support of the CAL upgrade to permit atom interferometry.

In our ground efforts, we have successfully implemented an atom interferometer gyroscope with Earth-rate sensitivity. A Bose-Einstein condensate is produced in our specially-designed magnetic trap. Off-resonant lasers are used to coherently split the condensate into wave packets, which are then driven into circular trajectories orbiting the trap with a diameter of about 0.5 mm. When the packets are subsequently recombined they exhibit interference, and the phase of the interference is related to the rotation rate of the experimental platform. We observe interference signals with a visibility of about 60%. We have verified the rotation sensitivity by slightly rotating the optical table holding the apparatus, and the measured interference shift agrees with expectations.

We are presently preparing a manuscript on these results for publication, which will certainly be completed by the end of the reporting period. Future efforts include increasing the size of the atom orbit, and allowing the atoms to make multiple orbits before measurement. Both of these techniques will enable further improvements to the rotation sensitivity.

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station (ISS).

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

We have concluded our initial preparations for our proposed adiabatic cooling experiments. We had previously implemented a model of the original CAL atom chip, but we were required to revise this calculation to reflect the final chip geometry that was implemented. The new geometry is slightly less effective for these experiments, but should still permit cooling to a three-dimensional temperature below 150 pK. We re-optimized our dynamical expansion model, and also corrected a small source of heating we discovered in the way that we had originally transitioned between the various current ramps involved. This work has now been published in Microgravity Science and Technology.

We have also assisted the Jet Propulsion Laboratory (JPL) staff with their own efforts to model cooling methods, in particular delta-kick cooling. It seems at this time that the adiabatic expansion technique will be favorable for reaching ultralow three-dimensional temperatures.

II. Ground Investigations

Our ground efforts are directed at developing a rotation-sensing atom interferometer using atoms confined in a cylindrically symmetric magnetic trap. We have made considerable progress in this regard. We can now drive atoms into a circular trajectory with a diameter of about 0.6 mm. A total of four packets of atoms undergo this trajectory, forming two independent interferometer measurements. Most technical noise sources will be common to both interferometers, but the rotation phase from the Sagnac effect will be differential. For this, it is necessary for two pairs of trajectories to close upon themselves at the same time, since the atomic wave packets must overlap with each other to exhibit interference. So far we have achieved a closed trajectory for one pair, and we expect the tools we have developed will allow us to close the second pair as well. The interferometer configuration we are using should have sufficient rotational sensitivity to detect the rotation of the Earth.

We have also made several technical improvements to the apparatus that have improved the reliability of its operation.

Finally, we have initiated planning for the next stage of this experiment, where we hope to transition a working interferometer to a more compact chip-based apparatus. We have designed an atom chip that will provide magnetic field comparable to our current trap, and the chip is now being microfabricated by our collaborators at AFRL.

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station.

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

We have performed extensive modelling and analysis for the adiabatic cooling experiments. Using the original CAL1 chip trap, we developed a set of parameters that produces a very weak trap as an endpoint for expansion. We maintain a relatively large bias magnetic field in the trap to help reduce sensitivity to stray background fields. We expect that stray backgrounds will likely be the limiting factor in the expansion, but we have also modeled techniques to compensate for backgrounds using available current elements in the apparatus. The trap configuration we found will allow adiabatic cooling to a temperature of about 100 pK.

We also investigated the dynamics of adiabatic expansion. We find that an expansion time of 10 s should be sufficient to maintain non-adiabatic heating effects below the 100 pK level. The dominant limitation here will again probably be background field gradients, since these cause the trap to shift in an uncontrolled way. This generally leads to motional excitation.

We have documented these efforts in a paper recently submitted to Microgravity Science and Technology.

We have also performed analysis relevant to our proposed inertial sensing flight experiments. The removal of the Bragg beam from the apparatus has required some changes to this project. We find that an accelerometer measurement should still be possible, with a measurement sensitivity of about 0.1 micro-g. We are exploring methods to recover a rotation measurement, at the 1 micro-rad/s level of accuracy.

For our ground experiments, we continue development of an atom-interferometer gyroscope. It is essential to have a well-characterized trapping potential for the atoms to move in. We have developed a powerful and rapid method to measure the potential energy function, and we are exploring a new method to impose small adjustments to the potential in order to correct for any observed imperfections.

Ground and flight based efforts are proceeding in three broad areas. First, we are performing ground studies and developing a flight mission for the Cold Atom Laboratory (CAL) to study atomic techniques for inertial sensing in microgravity. Ground efforts include development of new rotation-sensing techniques and implementation of an optically suspended atom source for gravimetry. Flight efforts involve implementation and characterization of atom interferometry techniques using the CAL apparatus on the International Space Station.

Second, we are investigating methods to produce an ultra-low temperature atom source in free space using the CAL apparatus. The apparatus produces atoms confined in a magnetic trap, but inertial measurements require free atoms. We will investigate releasing the atoms by gradually turning off the trapping fields, allowing the atoms to adiabatically expand and cool off. This can produce a relatively dense and very low-velocity sample that is ideal for atom interferometry methods.

Third, we will continue ground-based studies to develop novel precision measurement techniques for use with atom interferometry, such as tune-out spectroscopy. Techniques like this are useful for advancing scientific knowledge and would be good candidates for future flight studies.

These techniques also have many applications in geophysics. Gravity sensing can be used for oil and mineral exploration, while rotation sensing can detect dynamics in the Earth's core. Gravity sensing also has defense applications such as locating underground tunnels and potential screening cargo for high-density contraband or weapons.

Other precision measurement applications have less direct impact, but advance scientific knowledge. For instance, precision tune-out spectroscopy measurements of atomic matrix elements can be used to improve the interpretation of atomic parity violation experiments. These in turn impact our understanding of the standard model of particle physics and thus the nature of our universe. Direct benefits of such understanding can be hard to trace, but in general the continued advance of technological applications builds on advances in our fundamental knowledge.

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.

The interferometer will use a low-density Bose-Einstein condensate, as this form of matter has the lowest possible velocity spread and thus allows for the longest possible measurement times. Many of the required components have already been demonstrated in our terrestrial experiments, including long-duration interferometry, high fidelity control of atomic motion using optical pulses, expansion of condensates to extremely low density, and rotation-sensitive interferometer geometries. We will combine these components and investigate their optimization for microgravity performance.

We have also demonstrated an optical levitation technique that permits the table-top simulation of microgravity, up to times of about one second. We will use this approach to test the interferometer performance in a terrestrial experiment without the expense and complexity of a drop tower apparatus. The work will be carried in collaboration between the University of Virginia and the AFRL Space Vehicles Directorate at Kirtland Air Force Base.

This work is highly relevant to the objectives of the solicitation, which specifically calls for the development of atom interferometer experiments. The precise measurement of rotations is of immediate utility for space-based navigation systems and testing of general relativistic predictions. However, the techniques developed would be readily applicable to other interferometric measurements, such as acceleration.

The program solicitation indicates that atom interferometry experiments would be projected for future upgrades to the CAL facility. Besides ground based preparatory work, we propose a preliminary flight experiment to investigate the adiabatic release of a trapped condensate. This will test the critical ability to attain samples with very low relative and mean velocities. It also would likely produce the lowest-temperature matter yet attained. This experiment would be important for future interferometry development, and also other potential experiments that require ultra-low atomic velocity.