Ask HN: What startups are working on hard, technically challenging problems?

In the article, "Silicon Valley's Youth Problem" [1] the author mentions Meraki (now Cisco-Meraki) as an example of a startup working on advances in technology rather than the latest web app.

Do you know of other companies that fit the description?

Follow up: Are they (you?) hiring?

[1] http://www.nytimes.com/2014/03/16/magazine/silicon-valleys-youth-problem.html