There are several protocols considered for quantum radar, such as interferometric quantum radar , quantum illumination (QI) , hybrid quantum radar [136, 137] or Maccone-Ren quantum radar . None of the mentioned protocols is perfect. Interferometric quantum radar, for example, is too sensitive to noise and requires quantum entanglement preservation. QI is an ideal protocol for a noisy environment and is even laboratory-verified for microwave spectrum , but it requires knowledge of the distance to the target, and such as it has no ranging function. Nevertheless, the QI-based approach to quantum target ranging is under development . This ranging problem is also solved by the hybrid quantum radar, but at the expense of sensitivity. The Maccone-Ren protocol has QI properties and ranging function, but it is only a theoretical concept so far.
NP is a complexity class characterised by the fact that it cannot be solved in polynomial time but can be verified in polynomial time. Specifically, the NP-hard problems are not only hard to solve but are difficult to verify as well. Examples of NP-hard problems are the Travelling Salesman Problem and Graph Colouring problems. 2b1af7f3a8