Nano-implant
From Tardis Wiki, the free Doctor Who reference
In order to thwart the Master's Dalek Plan, the Thirteenth Doctor planted "sonically triggered" hologram nano-implants on Yasmin Khan, Tegan Jovanka and Ace, allowing the Holo-Doctor to appear in close proximity to any one of them (TV: The Power of the Doctor [+]Loading...["The Power of the Doctor (TV story)"])
