While small isolated quantum systems undergo well-defined wave packet dynamics\, the observables in very large\, locally isolated quantum systems generally rel ax to states of maximum entropy. To explain this\, the eigenstate thermali zation hypothesis (ETH) holds that the unitary dynamics of arbitrary super positions yield equilibrium expectation values as a time-average [1\, 2]. Thus\, in this picture &ndash\; despite the deterministic nature of the Schro¨\;dinger equati on and the absence of outside perturbations &ndash\; an arbitrarily prepar ed isolated quantum system relaxes to a thermal equilibrium that is someho w hardwired in its eigenstates. Indeed\, unimolecular rate theory depends on energy randomization\, and quantum systems as small as three transmon q ubits exhibit ergodic dynamics. [3].

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\n\nBut\, theory predicts the existence of certain interactin g many-body systems that lack intrinsic decoherence and preserve topologic al order in highly excited states. These systems exhibit local observables that retain a memory of initial conditions for arbitrarily long times. Su ch behaviour has important practical and fundamental implications. For thi s reason\, experimental realizations of isolated quantum systems that fail to thermalize have attracted a great deal of interest [4\, 5].

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\n\nHere we describe particular conditions under which an ultracold plasma evolves from a molecular Rydberg gas of nitric oxide\, adiabatically sequesters e nergy in a reservoir of mass transport\, and relaxes to form a spatially c orrelated strongly coupled plasma. Short-time electron spectroscopy provid es evidence for complete ionization. The long lifetime of the system\, par ticularly its stability with respect to recombination and neutral dissocia tion\, suggest a robust process of self-organization to reach a state of < a name="References">arrested relaxation\, far from thermal equilibrium .

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2008\, \;**452**(7189):854&ndash\;858.

[2] Eisert J\, Friesdorf M\, Gogolin C: **Quantum many-bo
dy systems out of equilibrium**. *Natur**e** Physi
cs *2015\, **11**(2):124&ndash\; 130.

[3] Neill C\, et al: **Ergo
dic dynamics and thermalization in an isolated quantum system**. **1601.00600v2**. [4] Kondov SS\, McGehe
e WR\, Xu W\, DeMarco B: **Disorder-Induced Localization in a Strong
ly Correlated Atomic**

**H
ubbard 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: **Obse
rvation of many-body localization of interacting fermions in a quasi-rando
m optical lattice. ***Science* 2015\, **349**:8
42&ndash\;845.