Even though the United States Constitution gives citizens the right not to incriminate themselves, the New Jersey Supreme Court ruled that police can force you to unlock your iPhone.
From Ars Technica:
On Monday, the New Jersey Supreme Court rejected that Fifth Amendment claim. The Fifth Amendment only protects defendants against self-incriminating testimony, not the production of incriminating documents. While "testimony" usually refers to speech, that's not always the case. Sometimes, a defendant can reveal information by his or her actions. For example, if the government doesn't already know who owns a phone, then forcing a defendant to unlock it amounts to forced testimony that the defendant is the owner.