Keyboard shortcuts offer the fastest access to content on your Google+. They are generally more efficient because the shortcut starts you at the right spot for new posts instead of scrolling halfway through them. They are also a bit easier on your hands and wrist since you won't have to switch back and forth as often (especially useful for those with joint issues).
With keyboard shortcuts available for the Google+ stream, it only makes sense that there should be a set of them to work on notifications, too.
Without further delay, here are the shortcuts:
Up and down arrow keys -- These allow you to scroll through the list of notifications. The active notification is marked with a thin blue bar on the right-hand side of its tile.
Scroll to bottom with down arrow and then press Enter -- These actions will open the previously read notification area.
Enter and o -- Either of these keys will expand/open the currently selected notification.
Right and left arrow keys -- These will open the next or previous notification. You must have one notification open already to use these.
d - Mark your currently selected notification as "Read."
u - This will return you to the list of notifications.
Esc - Close the entire notification panel.
Unfortunately, Google has not written in a specific keyboard shortcut for opening the notifications panel. If you have a tip for opening it quickly without the mouse, please leave it in the comments below.
(Via Google Operating System)