The shortest poem may actually be by Aram Saroyan and runs as follows:
lighght
Lighght was first published in 1967. For the inclusion of this poem in a 1970 anthology Saroyan controversially received an award from the National Endowment of the Arts, the example of which is still used by opponents of government arts subsidies in US Congress.
Lighght is an example of what Geof Huth calls a pwoermd, a one-word poem. These poems, by virtue of their minimalism, of course concern themselves with what language is, and play games with the difference between spoken and written language, generating new kinds of meaning because of this tension. Lighght, for instance, doubles the (near-)silent gh of the word light, which we all seem to recognize in lighght anyway. The doubling of the silent gh draws out the length of the word, conveying a sense of space. Light itself illuminates, makes it possible to read lighght, is a major concern in visual arts (which lighght approaches because of its affinities with concrete poetry).
Saroyan has written more poems like lighght some of which are "shorter", like:
eyeye
(which can be pronounced, like eye, as i, but also as aye aye)
and
blod