Hi Ann,
Windows server supports smart card logon. If you use smart card logon and configure SQL server to use Windows authentication, it should provide high security. As for SQL server itself, I have not heard any information that SQL provides a separate CAC or PKI authentication way besides the existing Windows and SQL authentication. As this question is mainly related to SQL, I suggest that you pos to the following SQL Security forum so that this issue can be answered efficiently.
SQL Server Security
http://forums.microsoft.com/TechNet/ShowForum.aspx?ForumID=92&SiteID=17
Laura Zhang - MSFT