While small isolated quantum systems undergo well-defined wave packet dynamics, the observables in very large, locally isolated quantum systems generally relax to states of maximum entropy. To explain this, the eigenstate thermalization hypothesis (ETH) holds that the unitary dynamics of arbitrary superpositions yield equilibrium expectation values as a time-average [1, 2]. Thus, in this picture – despite the deterministic nature of the Schro¨dinger equation and the absence of outside perturbations – an arbitrarily prepared isolated quantum system relaxes to a thermal equilibrium that is somehow hardwired in its eigenstates. Indeed, unimolecular rate theory depends on energy randomization, and quantum systems as small as three transmon qubits exhibit ergodic dynamics. [3].

But, theory predicts the existence of certain interacting many-body systems that lack intrinsic decoherence and preserve topological order in highly excited states. These systems exhibit local observables that retain a memory of initial conditions for arbitrarily long times. Such behaviour has important practical and fundamental implications. For this reason, experimental realizations of isolated quantum systems that fail to thermalize have attracted a great deal of interest [4, 5].

Here we describe particular conditions under which an ultracold plasma evolves from a molecular Rydberg gas of nitric oxide, adiabatically sequesters energy in a reservoir of mass transport, and relaxes to form a spatially correlated strongly coupled plasma. Short-time electron spectroscopy provides evidence for complete ionization. The long lifetime of the system, particularly its stability with respect to recombination and neutral dissociation, suggest a robust process of self-organization to reach a state of arrested relaxation, far from thermal equilibrium.

[1] Rigol M, Dunjko V, Olshanii M: **Thermalization and its mechanism for generic isolated quantum systems**. *Natur**e*

2008, **452**(7189):854–858.

[2] Eisert J, Friesdorf M, Gogolin C: **Quantum many-body systems out of equilibrium**. *Natur**e** Physics *2015, **11**(2):124– 130.

[3] Neill C, et al: **Ergodic dynamics and thermalization in an isolated quantum system**. *arXiv: *2016, **1601.00600v2**. [4] Kondov SS, McGehee WR, Xu W, DeMarco B: **Disorder-Induced Localization in a Strongly Correlated Atomic**

**Hubbard Gas**. *Phys Rev Lett *2015, **114**(8):083002.

[5] Schreiber M, Hodgman SS, Bordia P, Lu¨schen HP, Fischer MH, Vosk R, Altman E, Schneider U, Bloch I: **Observation of many-body localization of interacting fermions in a quasi-random optical lattice. ***Science* 2015, **349**:842–845.