Cesium Vapor Demonstrates Bistability Due to Collective Alignment and Orientation under Strong Spin Exchange

The unusual behaviour of alkali atoms, such as cesium, forms the basis of a new investigation into the collective interactions of light and matter. M. V. Petrenko and A. K. Vershovskii, conducting research into these atomic systems, demonstrate that under specific conditions, cesium vapor exhibits a surprising bistability, meaning it can exist in two distinct states depending on subtle changes in its environment. This effect arises from the interplay between atomic alignment and orientation, maintained through a process called spin-exchange relaxation free mode, and reveals a previously unobserved coexistence of these properties. The discovery of this bistability, characterised by a hysteresis effect and storage times measured in hundreds of seconds, opens up exciting possibilities for developing novel memory elements and cryptographic devices.

Cesium Vapor Bistability via Spin Exchange Interaction

Alkali atoms, such as cesium, are fundamental to quantum optics and essential tools for research. This research investigates the bistability of optical properties in cesium vapor, focusing on the collective interaction of atomic alignment and orientation under specific conditions involving optical pumping, strong spin exchange, and ultra-weak magnetic fields. This interaction significantly influences the absorption and emission of light, resulting in a bistable behaviour where the system can exist in two distinct optical states for the same input intensity, expanding understanding of collective atomic effects and offering potential applications in optical switching and information storage. Relaxation-free modes allow ensembles of atoms in gaseous form to demonstrate the absence of spin-exchange relaxation, and also exhibit non-linear collective effects. The team presents experimental evidence that alignment, specifically the quadrupole momentum, can be preserved under these conditions and coexist with, and interact with, orientation, representing the dipole momentum. This interaction leads to bistability, where a small change in conditions causes the medium to transition to a different steady state, characterised by hysteresis.

Enhanced Magnetometry via Vapor Manipulation

This research focuses on improving the sensitivity, stability, and functionality of Spin Exchange Relaxation Free (SERF) magnetometers, which are known for their extremely high sensitivity. The team investigates manipulating the optical properties of alkali metal vapors to enhance performance and create new functionalities, using the polarization of light to control and read out the magnetometer signal, and to create devices that can act as optical switches and memory cells. SERF magnetometry relies on suppressing relaxation processes in the atomic vapor, allowing for highly sensitive detection of magnetic fields. This involves using circularly polarized light to create a spin-polarized atomic vapor through optical pumping and spin exchange.

The core readout mechanism measures the rotation of the polarization of light as it passes through the vapor, affected by changes in the magnetic field. The research demonstrates that careful control of the optical pumping and polarization of light significantly enhances the sensitivity of the SERF magnetometer. The authors successfully implemented a readout scheme based on the rotation of the polarization of light, providing a robust and sensitive way to detect magnetic fields. Most excitingly, the team demonstrated the potential to create optical switches and memory cells using the polarization state of the light, offering possibilities for all-optical computing, optical data storage, and quantum information processing.

Atomic Alignment, Orientation, and Bistability Observed

This research demonstrates the preservation and interaction of atomic alignment and orientation within ensembles of alkali atoms under specific pumping conditions and ultra-weak magnetic fields. Scientists observed that both alignment, representing quadrupole momentum, and orientation, representing dipole momentum, can coexist and influence each other, leading to a phenomenon known as bistability. This bistability manifests as a system that can transition between different steady states in response to small changes in conditions, exhibiting hysteresis, and potentially maintaining these states for hundreds of seconds. The team’s experiments reveal that introducing ellipticity into the pumping light alters the alignment of the atoms, specifically through the influence of atomic orientation.

They confirmed that this effect is distinct from changes induced by a direct magnetic field, and is instead driven by the creation of a coherence signal within the ultra-weak field. The observed broadening of resonance signals is consistent with theoretical predictions, supporting the conclusion that both orientation and alignment operate within the spin-exchange relaxation free (SERF) mode. The authors acknowledge that the observed effects are sensitive to factors such as the degree of ellipticity and the presence of additional magnetic fields, which can reduce the strength of the hysteresis. Future research may focus on further refining the control of these parameters to optimise the bistable effect, opening up potential applications in information storage and cryptography.