How we defined a kilogram ?

 Primarily we defined kg by a platinum iridium cylinder as a primary standard which was used to define a kilogram, but strangely we noticed a considerable deviation(about 50 microgram) among the standard cylinders. Due to this we were motivated to redefine a kg into something more fundamental, which is more absolute and can be expressed by some natural phenomena like time was defined by speed of light in vacuum. 

So the efforts of redefining a kg was realized by the kibble balance(watt balance), which was used to calculate plank's constant and subsequently used to define a kg, this approach was more related to physics and the other approach which was also complementing this was the Avogadro project which was essentially a chemistry related approach using a pure Silicon-28 crystal, which was polished so well into 'the worlds roundest object' a sphere. 

{Later the scraps of this pure Si-28 crystal were used to make quantum bits(Q-bits)}

     For more insights and detailed explanation you can also check out the 'veritasium' video for greater insights.