Doors are some seriously ancient tech but they have this problem where they can only keep people out if they're closed. This bizarre but clever take on the "revolving door" by a company called Barbecan, on the other hand, aims to get you inside, but simultaneously keep any ne'er-do-wells out. Specifically ne'er-do-wells with bombs and guns.
This system, which Barbecan calls the "Linear Revolving Door" works by encasing every entrant to a building within his or her own little private cell on the way in. While they're in there, sensors check for guns or bombs or whatever bad news you might be able to detect, and if the results are positive, the linear revolving door can either trap the target in there, or reverse to spit him out. It helps to see what it would look like in action.
While it seems like the Linear Revolving Door could be effective, it's also got none of the simplicity of traditional doors. And man would it suck if the mechanical doors that are designed to contain bombers and gunman break down and won't open. And that's to say nothing of expense. Still, a system like this could be pretty useful in particularly high risk areas. Don't expect to see it any time soon though; Barbecan is just in the patenting stage of this one. But if crazy spurts of violence keep up, then you can imagine something like this is going to show up eventually. [Barbecan via Discovery]