Atoms Reveal Surface Forces with New Precision Method

Researchers have developed a new technique to probe the subtle interactions between ultracold atoms and material surfaces. J-B. Gerent from the Department of Physics and Astronomy at Bates College, alongside R. Veyron and V. Mancois, and colleagues demonstrate a method combining optical and magnetic trapping with surface rotation to precisely control and measure atom-surface distances. This innovative approach allows for the adiabatic transport of a rubidium Bose-Einstein condensate to within a few hundred nanometres of a surface, where interactions significantly influence trapping potential and increase tunnelling rates. By measuring cloud lifetimes and comparing them with a refined tunnelling model, incorporating factors such as noise-induced heating and experimental biases, the team estimates a relative uncertainty of 10% in determining the Casimir-Polder force coefficient in the retarded regime. This work, conducted in collaboration between Bates College, represents a significant step towards understanding fundamental atom-surface interactions and is applicable to a wide range of magnetically and optically trapped species.

Scientists are developing increasingly precise ways to examine the forces acting on individual atoms, fundamental to advances in quantum technologies and materials science. A new technique promises to measure these subtle forces with greater accuracy by manipulating atoms with light, magnetism, and a rotating surface. This work introduces a method that combines magnetic and optical trapping with precise surface rotation to transport a cloud of rubidium atoms from micrometers to a few hundred nanometers from a surface.

At these extremely short distances, the interaction between the atoms and the surface significantly alters the trapping potential, increasing the rate at which atoms tunnel towards the surface. Consequently, by carefully measuring the lifetime of the atomic cloud and comparing it to a theoretical tunneling model, researchers can now extract the Casimir-Polder force coefficient, a measure of the attractive force between an atom and a surface, in the retarded regime.

The proposed method relies on a unique setup integrating an optical dipole trap reflected from the surface alongside a magnetic trap created by embedded wires. This allows for adiabatic transport of a rubidium Bose-Einstein condensate (BEC), a state of matter formed when bosons are cooled to near absolute zero, bringing it exceptionally close to the surface.

Once at these distances, the atom-surface interaction becomes dominant, influencing the trapping potential and accelerating the tunneling process. The team’s model accounts for factors like noise-induced heating, calibration errors, and measurement accuracy, estimating a relative uncertainty of just 10% for the Casimir-Polder coefficient. This approach is broadly applicable and can be adapted to work with any atomic species that can be both magnetically and optically trapped, opening possibilities for studying a wider range of atom-surface interactions.

The research details a method for precisely controlling atomic states near surfaces, a capability essential for advancing the field of atomtronics, the manipulation of atoms as information carriers. Achieving this level of control demands a thorough understanding of the forces governing atom-surface interactions, a challenge this work directly addresses.

The team’s simulations demonstrate that the measurement of cloud lifetime, coupled with a detailed tunneling model, provides a sensitive probe of the Casimir-Polder force. By rotating the surface, atoms are effectively ‘transported’ towards it, allowing researchers to tune the atom-surface distance and observe the resulting changes in tunneling rate.

The attractive Casimir-Polder force diminishes the barrier height, leading to a shorter tunneling time constant towards the surface. The precision of the measurement hinges on carefully controlling experimental parameters such as the surface angle and optical power. The researchers have developed a comprehensive model that incorporates potential sources of error, including heating from background noise and inaccuracies in calibrating the trapping parameters, establishing a pathway to achieve a 10% relative uncertainty in determining the Casimir-Polder coefficient.

Unlike previous static measurements limited to larger distances, this dynamic method allows for probing interactions in the crucial Casimir-Polder regime, where retardation effects become significant. At distances approaching a few hundred nanometers, the interaction potential is strongly modified, and the tunneling rate increases, making precise measurements possible.

Optical and magnetic trapping combine with surface rotation to achieve nanometre condensate proximity

A combination of optical and magnetic trapping techniques underpins this work, designed to precisely control the distance between ultracold rubidium-87 atoms and a reflecting surface. Initially, atoms are confined using an optical dipole trap, created by focusing laser light, reflected from the surface itself. Complementing this, a magnetic trap is formed by carefully arranged current-carrying wires positioned beneath the surface, providing additional confinement and control.

These two trapping potentials work in concert to initially hold the atomic cloud before it is moved closer to the surface. The core methodological advancement lies in the controlled rotation of the surface, which adiabatically transports the rubidium Bose-Einstein condensate (BEC) from initial distances of a few micrometers down to a few hundred nanometers from the surface, ensuring the atoms remain trapped throughout the process.

At these extremely short distances, the interaction between the atoms and the surface becomes significant, altering the trapping potential and increasing the rate at which atoms tunnel towards the surface. To accurately measure this tunneling rate, researchers monitor the lifetime of the atomic cloud. By comparing the observed lifetime with predictions from a detailed tunneling model, the Casimir-Polder (CP) force coefficient, a measure of the atom-surface interaction in the retarded regime, can be extracted.

The model accounts for several factors that could influence the measurement, including heating caused by background noise, potential biases in the calibration of experimental parameters, and the precision with which the cloud’s lifetime is determined. Numerical estimations reveal a relative uncertainty of 10% for the CP force coefficient, considering typical trapping parameters and experimental limitations. This methodology is not limited to rubidium; it can be adapted to study interactions with any atomic species amenable to both magnetic and optical trapping, broadening the scope of future investigations.

Casimir-Polder forces modulate atom trapping and reduce minimum fringe position

Initial simulations, conducted without accounting for Casimir-Polder (CP) forces, revealed a clear dependency between the rotation angle and the position of the first interference fringe, establishing the principle of atom transport via surface rotation. However, these initial models lacked the influence of surface interactions, a critical factor at nanometre distances.

Further numerical work incorporated the effects of CP forces, demonstrating a substantial impact on the trapping potential. The inclusion of these forces, specifically using a CP coefficient, c4, of 2.1x 10−55 J⋅m4, reduced the minimum achievable fringe position because the CP force lowers the potential barrier towards the surface, accelerating tunneling.

Simulations showed that for a given optical power, the presence of CP forces limited the lowest attainable trapping angle, effectively defining a lower bound on the atom-surface distance. The research team focused on quantifying the achievable precision in measuring the c4 coefficient, estimating a relative uncertainty of 10% in the determination of c4, stemming from the sensitivity of the cloud lifetime to the barrier height.

A shorter tunneling time constant, τt, towards the surface was observed with increasing surface-dependent CP coefficient and well-controlled experimental parameters. The study highlighted the method’s ability to extract the CP force coefficient in the retarded regime, demonstrating that by carefully measuring the lifetime of the atomic cloud and comparing it to a tunneling model, the c4 coefficient could be determined with reasonable accuracy. Furthermore, the method is adaptable, being applicable to any species that can be both magnetically and optically trapped, broadening its potential use in precision measurements of atom-surface interactions.

Measuring weak atomic interactions using rotating surfaces and Bose-Einstein condensates

Scientists have long struggled to precisely measure the subtle forces governing the interaction between atoms and surfaces, particularly difficult to quantify at the nanoscale, where quantum effects dominate and traditional measurement techniques falter. Yet, understanding these forces is essential for controlling ultracold atoms near surfaces, a key requirement for developing new quantum technologies and exploring fundamental physics.

A new technique offers a pathway towards resolving this challenge, combining magnetic and optical trapping with a rotating surface to measure these forces with unprecedented accuracy. Probing the Casimir-Polder force, a specific manifestation of these van der Waals interactions, is now within experimental reach. By carefully controlling a Bose-Einstein condensate (BEC) and observing its lifetime near a surface, researchers can deduce the strength of this force.

The method’s ingenuity lies in its ability to bring the BEC incredibly close to the surface, a feat previously hampered by sticking and loss of atoms. A 10% uncertainty remains, highlighting the continuing difficulty of isolating the delicate signal from background disturbances. Beyond this, the technique currently relies on relatively simple surfaces; extending it to more complex, nanostructured materials will be a significant undertaking.

This work represents a step towards building more precise atom-surface interaction models, which are vital for designing future quantum devices. The ability to manipulate atoms at surfaces opens doors to novel sensors and quantum simulators. Future research might focus on adapting this method to study different atomic species or exploring the impact of surface roughness on the observed forces, pushing the boundaries of our understanding even further.