Can MongoDB use an index when checking for existence of a field with $exists operator?

If I have data in my users collection that looks like:

{ name: '...', 
  email: '...', 
  ...,
  photos: {
     123: { url: '...', title: '...', ... },
     456: { url: '...', title: '...', ... },
     ...
  }
} 

And I want to find which user owns photo id 127, then I am using the query:

db.users.find( {'photos.127': {'$exists' => true} } );

I've tried, but it doesn't seem possible to get MongoDB to use an index for this query. The index I tried was: db.users.ensureIndex({photos:1});. And when I used explain() mongo told me it was using a BasicCursor (i.e., no index was used).

Is it possible to create an index that mongo will use for this query?