An X-ray laser works in the same general fashion as a ruby laser, but at much higher energy levels. The main problem in producing such a device is that the probability of any given transition between energy states depends on the cube of the energy. Comparing a ruby laser that operates at 694.3nm to a hypothetical soft X-ray laser that might operate at 1nm, this means the X-ray transition is 694, or a little over 334million times less likely. To provide the same total output energy, one needs a similar increase in input energy.