From the blog post:
    “Let’s Encrypt is committed to a post-quantum-safe Web PKI. The path we’re planning to take is Merkle Tree Certificates (“MTCs”), a new approach that adds post-quantum authentication to the web without sacrificing the speed and reliability that have made TLS universal.

    This post is about these plans and why we believe MTCs are worth pursuing as a key to a post-quantum future. […]”