Rails 4 allow subdomain to share authentication using devise

I have a very simple set up where I have a rails application at mydomain.com And recently we have implemented a front end framework (ember.js) that is served up independently (through heroku) to admin.mydomain.com

I want to have people log in on mydomain.com but share the same authentication when using the api (accessible from www.mydomain.com/api) So basically users should be able to use admin.mydomain.com seamlessly.