There seems to be a lot of confusion regarding this.

The Nexus 6 replicants don't have implanted memories of the same sort as Rachael. That wouldn't make sense, because of reasons we'll try to explain.

When Deckard arrives in Tyrell's office, he is asked to perform a V-K test on a subject that is, according to Tyrell, human. This subject is Rachael. Deckard's test reveals she is actually a replicant. When Deckard tells this to Tyrell, to his surprise he finds out she doesn't know (although "she is beginning to suspect" according to Tyrell).

Tyrell then explains how it is possible she is a replicant without even knowing it. The answer is: they "provided" (implanted) her with false memories (the memories are real, that is; they're just not her own), giving her the memories of a normal lifetime, a childhood, etc. so she would have no reason to suspect she is anything but human.

Tyrell goes on to explain *why* they did it. The replicants "developed strange obsessions" and exhibited emotional problems because of their lack of experience in dealing with them. So Tyrell devised an experiment, namely: to see what would happen if "they" (speaking generally) would be given an emotional cushion (provided by a lifetime of memories, as illustrated by Rachael).

Providing a replicant with (false) memories seems a fairly new (not to mention unheard of) concept to Deckard, who is absolutely stunned by the idea ("Memories… you're talking about memories!")

Bottom line: Rachael is the only Nexus 6 replicant with implanted memories; that's precisely why she (not "they" or anything, but rather *Rachael* in particular) is considered "an experiment, nothing more" by Tyrell.

Later on, Bryant says the following to Deckard: "That skin job that you V-K-ed at the Tyrell Corporation, Rachael. Disappeared. Vanished. Didn't even know she was a replicant. Something to do with a brain implant says Tyrell." Evidently, this was all new to Bryant as well.

Note: even the people from Westwood who made the Blade Runner game, appear to have assumed all replicants have them. (See section "About the game" for more about this.)