Android 5.1 Lollipop is already on its way out to some Nexus devices, and the factory images and OTA download links are available for those waiting on the OTA to hit their Nexus phones and tablets officially which is currently ongoing. Sony devices may not be getting Android 5.1 within the next few weeks, as the Sony Xperia Z3 is just now slated to be getting Android 5.0 next week, but Sony already appears to be working on 5.1 Lollipop development as they have just released the binaries for a handful of different Xperia device lines including the Xperia Z family, the Xperia E3, the Xperia T2 Ultra, and the Xperia T3.
With Android 5.0 waiting in the wings for at least the Xperia Z3 Sony looks to be turning some of their focus on the next version of Lollipop, and perhaps they're simply trying to get an earlier start with this version. Since these are binaries and not factory images or OTA downloads, the binaries don't do average users any good. For developers however, as well as Sony's own development team, the binaries are a great tool for getting the software tested and getting the development process rolling so updates can be pushed out to devices quicker.
While official builds from Sony and stable ROM builds from community developers may be a ways off yet, there appears to be an alpha build of the Xperia Z2 ready to with all but a few features seemingly functional. It would likely be presumptuous and hopeful to think that Sony could end up pushing out Android 5.1 Lollipop really soon, but with many flagships from other OEMs having been on Lollipop for weeks and with 5.1 already pushing out to Nexus phones and tablets, Sony at least has something to work off of so the wait in between 5.0 and 5.1 may not be too long. If you're a developer and want to get your hands on the binaries, you can do so by hitting the source link. If you're a brave individual and want to test out the alpha build working for the Xperia Z2, you can grab it here.