Researchers Proposes Trap-Quenched Collimation for Dual-Species Atom Interferometry

A new trap-quenched collimation method has been developed to control the expansion of atom clouds, enhancing the precision of atom interferometry for testing fundamental physics. Gabriel Müller and colleagues at Texas A&M University, in collaboration with California Institute of Technology, Hagler Institute, Leibniz University, University of Rochester, Bates College, and Ulm University, achieved free expansion times of up to 700ms with rubidium-87 condensates and a two-dimensional expansion energy of 78 pK. Detailed modelling and theoretical study of potassium-41 and rubidium-87 mixtures suggest this approach could enable tests of the Universality of Free Fall with an accuracy of 10^-15, representing a key advance in precision measurement.

Trap-quenched collimation achieves record low expansion energies for precision tests of gravity

A two-dimensional expansion energy of k_B cdot 78pm 9 ;pK has been achieved, representing a substantial reduction from previous state-of-the-art measurements of approximately k_B cdot 38 ;pK using Delta-Kick Collimation. Such a low energy level surpasses the threshold required for Universality of Free Fall tests demanding 10^-15 accuracy, a level of precision previously unattainable due to limitations in controlling atomic cloud expansion. Researchers at Leibniz University Hannover and collaborating institutions achieved this advance using NASA’s Cold Atom Laboratory (CAL) on the International Space Station, employing trap-quenched collimation to manipulate rubidium-87 atoms. The Universality of Free Fall (UFF) is a fundamental principle of general relativity, stating that all objects fall with the same acceleration in a gravitational field, irrespective of their composition. Testing UFF with increasingly high precision is crucial for probing potential deviations from general relativity and searching for new physics.

Atom interferometry functions by splitting a cloud of atoms into multiple wavepackets, allowing them to follow different paths, and then recombining them to observe interference patterns. These patterns are exquisitely sensitive to accelerations, including those caused by gravity, making it an ideal tool for precision measurements. However, the accuracy of these measurements is fundamentally limited by the initial velocity spread of the atoms. Reducing this spread, and thus the expansion energy of the atomic cloud, is paramount. Traditional methods, such as Delta-Kick Collimation, rely on applying a series of carefully timed laser pulses to reduce the atomic velocities, but are limited in their ability to reach the extremely low energies required for 10^-15 accuracy. Trap-quenched collimation offers a significant improvement by utilising the confining potential of a magnetic trap to initially cool and then release the atoms in a controlled manner.

The trap-quenched collimation technique involves exciting collective modes within the atomic condensate while it is still held in the magnetic trap. These collective modes, analogous to sound waves within the condensate, redistribute the atomic momentum, effectively reducing the initial velocity spread. By carefully controlling the excitation parameters, researchers can ‘tune’ the expansion energy to an unprecedentedly low value. Detailed modelling confirms this method is readily extendable to dual-species potassium-41 and rubidium-87 mixtures, opening new avenues for fundamental physics research in space. Using two different atomic species allows for the cancellation of systematic errors and enhances the sensitivity of UFF tests. Leibniz University Hannover and partner institutions have refined their atom manipulation technique, achieving free expansion times of up to 700 milliseconds within the Cold Atom Laboratory on the International Space Station. This extended duration is vital for performing more complex atom interferometry experiments, and modelling suggests the achieved two-dimensional expansion energy of k_B cdot 78pm 9 ;pK corresponds to k_B cdot 15⁺¹²_-5; pK along two axes, providing insights into the atomic cloud’s behaviour during expansion and informing future optimisation strategies. The longer interrogation time afforded by the space-based experiment, combined with the reduced expansion energy, dramatically increases the sensitivity of the atom interferometer.

Rubidium condensate expansion anomalies observed during space-based atom interferometry

Atom interferometry, a highly sensitive method of measurement relying on the wave-like properties of atoms, is being refined by scientists to test fundamental physics in space. The new trap-quenched collimation method, demonstrated with rubidium atoms aboard the International Space Station, offers a significant improvement in controlling the spread of these atomic clouds. However, a notable discrepancy exists between measured expansion energy and theoretical predictions along the condensate’s primary axes, requiring further investigation to fully understand this anomaly. This discrepancy, while not immediately detrimental to the overall accuracy of the experiment, highlights the complexity of ultra-cold atom dynamics and the need for continued refinement of theoretical models.

The observed anomaly could be attributed to several factors, including residual magnetic field gradients, imperfections in the trap geometry, or previously unaccounted-for interactions between the atoms. Detailed analysis of the experimental data, coupled with advanced simulations, is underway to pinpoint the source of this deviation. Understanding this behaviour is crucial not only for improving the accuracy of future UFF tests but also for advancing our fundamental understanding of condensed matter physics. The Cold Atom Laboratory provides a unique environment for studying ultra-cold atoms, free from the disturbances of Earth-based laboratories, such as vibrations and gravity gradients.

The scientists are now modelling dual-species atom interferometry, which could potentially enable even more precise tests of fundamental physics. The use of potassium-41 and rubidium-87 offers complementary advantages, including different mass ratios and magnetic properties, which can be exploited to further reduce systematic errors. Achieving 700 milliseconds of free expansion with rubidium atoms aboard the International Space Station significantly enhances the capabilities of atom interferometry, allowing for more detailed observation of the atoms’ wave-like properties and improving the accuracy of measurements. This demonstration of controlling the spread of ultra-cold atoms by manipulating the magnetic trap represents a key advance for precision measurements in space, paving the way for future experiments. Future research will focus on extending this technique to dual-species experiments and exploring the potential for even longer interrogation times, ultimately pushing the boundaries of precision measurement and our understanding of the fundamental laws of physics.

Researchers successfully demonstrated a technique to control the expansion of ultra-cold rubidium-87 atoms for up to 700 milliseconds aboard the International Space Station. This level of control is vital because precise measurements of fundamental physics, such as tests of the Universality of Free Fall, require minimising the spread of these atomic clouds. The team achieved a two-dimensional expansion energy of k_B cdot 78pm 9 ;pK and are now modelling how to apply this method to mixtures of potassium-41 and rubidium-87 to potentially reach an accuracy of 10^-15 in future experiments.