Quantum trions in equilibrium.

Excitons—neutral quasiparticles of a negatively charged electron bound to a positively charged hole—are ubiquitous in semiconductors and optoelectronic devices. Incorporating an extra charge carrier (positive or negative) into an exciton produces additional charge and spin degrees of freedom, converting excitons into fermions with increased effective mass. Joining three fermions creates a trion, which often requires optical excitation. However, these quasiparticles are short-lived, which limits the study of their effects on materials used in optoelectronic devices or their application in quantum optics (1). On pages 299 and 304 of this issue, Qi et al. (2) and Nguyen et al. (3), respectively, report experimental evidence of trions in a state of balance, in which these quasiparticles are formed and then recombined at an equal rate, extending their existence. This state of equilibrium may allow further exploration of trion behavior and new quantum states that are composed of many particles. [ABSTRACT FROM AUTHOR]