Gravitational waves, the elusive ripples in spacetime caused by massive objects like black holes, have long been detected through the minuscule changes in distance between mirrors separated by kilometres. However, a recent study challenges this conventional approach by suggesting that these waves could leave traces in an unexpected place: the light emitted by cold atoms. This groundbreaking research, published in Physical Review Letters, opens up a new avenue for gravitational-wave detection, offering a unique perspective on the interaction between quantum systems and spacetime itself.
The study, led by Navdeep Arya and colleagues at Stockholm University and Eberhard Karls Universität Tübingen, reveals a hidden signal within the seemingly unaffected emission rate of photons from a single atom. While gravitational waves do not alter the total number of photons emitted, the researchers calculated that they do influence the distribution of these photons in angle and frequency. This subtle effect, when examined through the lens of photon sorting, reveals a characteristic pattern that mirrors the wave's stretch-and-squeeze geometry.
The key to this discovery lies in the interaction between the atom and the quantum field. Arya explains that the field, being a global entity, can carry information about the gravitational wave, even when the atom itself remains unaffected. This phenomenon allows for the detection of lower-frequency gravitational waves, which are currently challenging to capture with ground-based detectors like LIGO. Instead of measuring distance changes between mirrors, a next-generation detector might focus on how passing waves alter the light emitted by atoms, offering a novel approach to gravitational-wave detection.
The experimental setup required to detect these effects is quite different from traditional detectors. It involves exciting a large cloud of atoms, collecting the emitted photons, and meticulously resolving their angles and frequencies. While this is not a standard experiment, the necessary technology already exists in cold-atom experiments, which can trap and control millions of atoms. The challenge lies in combining these capabilities with precise measurements of photon directions and frequencies while managing technical noise.
The researchers' next steps include understanding the survival of the signal under realistic experimental conditions. According to Jerzy Paczos, the Stockholm PhD student who led the study, the critical task is to consider the full range of technical noise in a real experiment, identify the most significant noise sources, and determine the feasibility of their proposal. They are also exploring the potential amplification of the signal through cavities or collective effects in atomic arrays.
This groundbreaking research highlights the potential of using quantum systems to probe spacetime, offering a fresh perspective on gravitational-wave detection. By looking beyond the traditional methods of measuring distance changes, scientists may unlock new insights into the nature of these waves and their impact on our understanding of the universe.
