Lets try to put QKD (Quantum Key Distribution) in simple words 

1. Its a method for sharing secret keys, in QKD the information bits will be converted to  Quantum states or to Ket (|>) or Bra (<|) notation logically. 

This states can be seen as vecors with different angle of rotation in the Bloch Sphere. 
  
2. OK, Now you have your data like a  quantum state string but it is still not encoded. 

3. States will be encoded by photon polarizations  using laser pulses or Single Photon sources and will be send across optical fiber Links  up to large distances. 

So how large is large , lets do a country wise analysis 

The following is the current information 

1.) Indian Space Research Organisation proved a 300 m QKD link in 2021

2.) China proved a 4500KM QKD Link (But they used a satellite called Micius as a relay to enhance the range) 

Without the Satellite they have achieved 511 KM in 2021. 

3.) Switzerland proved a 420KM fiber optic Link in 2018 

If Eve tries to  measure the photon polarzation ,due to the state collapse on measurement he dont get any information. 

4. Bob receives the photons and also gets the base which Alice used to encode them.  
 
5. A quantum state collapses to a known state after measurement with a probability which is square of the probabilty amplitude of the states. 
   The measurement result depends on the base chosen and so Bob gets the exact informaton which Alice has  

Looks cool isnt it , now let us see the challenges 

1.) How Alice will send the information about the base to Bob without Eve getting involved (Is that possible without involving a classical link,If not then does that matter?) 

2.) What if the photons are duplicated ( Say a beam splitter is used ) and then different basis are tried out by Eve while the photon transfer  data to Bob is kept as such. It is called a Photon Number Split (PNS) attack. 

So the real challenge is to have good single photon sources and optical amplifiers and also logical hacks  which can avoid PNS attacks like mentioned in the below paper to secure QKD. 

https://icfo.net/images/publications/C03-001.pdf