Creating Unique Index with Entity Framework 6.1 fluent API

NOTE: Relevant to EF 6

You can use IndexAttribute as mentioned but with Fluent API instead of DataAnnotations which will do the trick:

modelBuilder 
    .Entity<Person>() 
    .Property(t => t.Name) 
    .HasColumnAnnotation( 
        "Index",  
        new IndexAnnotation(new IndexAttribute("IX_Name") { IsUnique = true }));

Unfortunately there is no other way to create unique indexes using Fluent API. There is an open issue regarding this feature: Unique Constraints (Unique Indexes)

UPDATE: Entity Framework Core

In the latest EF Core release you can rely on Fluent API to specify indexes without additional tricks.

HasIndex allows to define it:

modelBuilder 
    .Entity<Person>()
    .HasIndex(x => x.Name);

Hence it returs IndexBuilder object you can use it for further index configurations (i.e uniqueness):

modelBuilder 
    .Entity<Person>()
    .HasIndex(x => x.Name)
    .IsUnique();