Quantum Guards are special robotic cyborgs mass-produced by the Krone regime for use as advanced super-soldiers. All Quantum Guards possess special time-bending powers that make them significantly more dangerous than standard human infantry soldiers.
Quantum Guards come in two varieties
Flash Guards: Appearing to move very quickly these guards use E.M.F cannons and make a distinctive reverberating sound went moving.
Warp Guards: these guards appear to teleport in combat and use Surge guns. These are the toughest guards and can take two to three crossbow bolts to bring down.
They essentially serve as "rival" enemies that possess the same special abilities as you do. On the plus side, Quantum Guards are quite rare; you won't encounter them until halfway through the game, and even then you'll only fight them on a few occasions. Although all Quantum Guards wear robot-like helmets, these helmets don't seem to protect against head shots, unlike the helmets worn by standard human infantry soldiers. Thus, all Quantum Guards are vulnerable to head shots, and can be killed with a single shot to the head from any weapon.Unlike regular soldiers, Quantum Guards cannot be disarmed of their weapons, as their mechanical hands "lock" onto their weapons too tightly.
All Quantum Guards are resistant to the incendiary rounds of the Hell-Fire sub-machine gun. They cannot be set on fire, and thus cannot be killed with only a few hits from the Hell-Fire (for example, it takes 12 hits to kill a Flash Guard, whereas a Krone Guard with the same amount of health will burst into flames and die after only 3 hits).
On a side note, according to posters found in the Quantum Guard assembly area in Mission 17: Sabotage, these guys are armored with7mm, 15mm, and 30mm of composite armor for the Flash Guards, Storm Guards, and Warp Guards respectively.