What is the relevance of "physical layer security" to cryptography? I found that there are quite some active research activities in a field named "physical layer security", with in particular some types of codes that apparently are different from those commonly treated in the conventional textbooks on coding theory. 

Could someone kindly explain the relevance, if any, of that field to cryptography, i.e. to issues of interest to our community (and eventually give a little bit of introduction to that field)?

edit: 

Though having no prior knowledge in that field, I did attempt to read a bit of the corresponding literatures but found it too hard to really proceed with my humble knowledge (among other causes: lots of math). A perhaps understandable "reaction" of mine was that I asked myself:

"The physical layer needs 'merely' to transmit the bytes from the sender to the receiver error-free and that's obviously a type of task already successfully achieved via the (conventional) error-correcting codes etc. long before that new field came up. Now, since modern cryptography (as we commonly understand the term) has already provided lots of sufficiently fine techniques for achieving good security for the users, why does one need (at all) any additional theory to be applied, as well as additional work to be done, elsewhere (i.e. in the physical layer) in the entire processing of the messages of the users?"

In other words, I thought that the status-quo of cryptography is good enough and hence the raison-d'etre of the said field would possibly be questionable. 

However, it is on the other hand entirely obvious to me that this kind of "personal" (emotional) thinking of mine can be no scientific argumention at all to "devaluate" that apparently fairly active new scientific research field, with lots of papers to be easily found via Google and even some published books written by univeristy professors. 

I found subsequently in our library two books: 


M. Bloch and J. Barros, Physical-Layer Security (sub-title: From Information Theory to Security Engineering), Cambridge, 2011, 
X. Zhou et al. (Ed.), Physical Layer Security in Wireless Communications, CRC Press, 2014. 


So currently I believe the real situation is as follows: Both (a) the (conventional) cryptography as treated e.g. in HAC of Menezes et al. and underlying the discussions of our community up till now and (b) the said new field of research are at the foundation based on (the same) Shannon's theory and they have merely different realms of applications in practice. This means that, for the benefit of the general readers of our community, it would be highly desirable that an expert who has good knowledge of that new field and who happens to be also in our community kindly volunteer to spend some of his time to post a short but helpful review of the research results of the new field that are presumably yet unknown to the majority of us.
Driven by the saying "secure by design", scientist start thinking about how they gonna secure the network while they are designing it. Cryptography is happening in the network layer. Scientists wishing that, by adding some security features in the physical layer, this will improve the security of the information in the upper layer and overall. In the future, millions of devices will be connected together and crossing each other… securing them by cryptography alone is a scary idea.