&quot;Permanent&quot; has a very different meaning from &quot;infinite amount of time.&quot;<p>Permanent implies that it&#x27;s irreversible, and this is definitely not the case.<p>Given that the gain element must be employed for the storage of the light, and the removal of the gain element causes the release of the light, it is not &quot;permanent&quot; - it is &quot;potentially infinite, as long as the gain element is in place.&quot;<p>That makes it useful for research into all-optical computation.