Implants add volume to breasts, and properly-chosen implant profile (and size) will give the best cleavage a woman's anatomy will allow. However, a woman's anatomy does not always cooperate with ANY specific implant to "automatically" create cleavage--that requires a good push-up bra (and having adequate breast volume to be pushed-up).
It is often (incorrectly) said that high(er) profile implants give the best cleavage. Actually, for a chosen size, high profile implants have the narrowest diameter (and this includes vertical height, so hold that thought also), so when two narrow implants are centered under the nipples, there is more gap than the same size moderate profile implants will show. Plus, because they are shorter in vertical height, there will be LESS upper pole fullness. And you hear all the time that HP implants give "more" upper pole fullness. (They do, until they drop, but more projection only sticks out a bit more, and the width never changes.)
What this means is that once size is decided upon, choose the widest profile that fits your specific anatomy--THIS will give not only the best-possible cleavage, but also the most upper pole fullness! (Of course, if you are very petite with a narrow breast base who chooses large implants, you may require high(er) profile implants, but avoid HP unless you fit the above scenario).
What you really need is an experienced, ABPS-certified, ASAPS-member plastic surgeon who does lots of breast surgery and has both the skill and expertise to explain all of this to your satisfaction. You may want to read my Comprehensive Guide to Breast Augmentation by clicking on the web reference link below. Best wishes! Dr. Tholen