Is it possible to store arrays in a custom field?

If I understand correctly, custom fields work like this:

key(string/int) => value(string/int)

Is it possible to define a custom field like this?

key(string/int) => value(array (string/int, string/int))

I want to use such a structure for storing a series of quotations in [quote, source] format, thinking that it would minimize potential input error compared with, say, str_splitting a single value.

Yes you can, either by code:

$demo = array('value1','value2','value3');
update_post_meta($post_id,'meta_key',$demo);

Or simply using the custom fields UI, add ass many values as you want one at a time and make sure they all have the same meta_key.