The study of Bose–Einstein condensates (BECs) — the exotic form of matter that emerges when the atoms or molecules in an ultracold cloud of gas condense into a quantum state in which they behave as a coherent whole — provides unique insight into the wave-like behaviour of matter. This in itself is reason enough to study them — but, writing in Physics Review A, David Harber and colleagues provide yet another1. By taking advantage of the sensitivity of a BEC’s behaviour to subtle changes in its environment, they conduct the most precise measurement made so far of a quantum-mechanical phenomenon known as the Casimir–Polder effect.

Since ancient times, philosophers have figuratively asserted that nature abhors a vacuum; according to quantum mechanics, this is indeed so. Even in the depths of space, the Heisenberg uncertainty principle requires that an otherwise near-perfect vacuum will churn with a constant bubbling of virtual particles popping in and out of existence. A consequence of this is that when two neutral parallel conducting plates are brought close enough together, the bubbling vacuum between and on either side of them will generate a weak attractive force — a phenomenon known as the Casimir effect2. A similar effect, known as the Casimir–Polder effect3, generates a force between a neutral atom (or collection of atoms) near a conducting surface. Although both effects were posited theoretically in 1948, neither was experimentally verified until almost a half a century later4,5, owing to the painstaking nature of the experiment needed to measure the very small forces they generate.

The limited sensitivity of experiments that have been designed to confirm the Casimir and Casimir–Polder effects meant that they could only be tested over relatively small distances. For the Casimir–Polder effect in particular, it has not been possible to study its expected changes at larger distances. The advent of BECs, however, could solve this shortcoming.

BECs can only exist at temperatures within a fraction of a degree of absolute zero, and their properties and behaviour are extremely sensitive to changes in their environment and other external influences. This sensitivity makes them ideal for performing ultra-precise measurements — from detecting changes in the Earth’s gravitational field caused by density variations in the planet’s crust, to monitoring the movement of an airplane fitted with a BEC-based gyroscope. Harber et al.1 use that sensitivity to measure the forces acting on a magnetically confined rubidium BEC placed near a fused-silica surface. By making the BEC oscillate and measuring the frequency of those oscillations, the forces acting on the BEC can be calculated and the contribution resulting from its proximity to the silica surface deduced.

The authors’ approach enabled them to measure the Casimir–Polder force with unprecedented precision, but also at atom–surface separations of 5–10 μm, several times larger than in previous studies. At the same time, they were able to place more stringent limits on the occurrence of other non-newtonian forces that have been suggested theoretically (so-called Yukawa-type forces). In future, the authors hope to use their approach to study thermally induced deviations from the Casimir–Polder effect that occur at similar distances but higher surface temperatures than those investigated in the present work.