Who has the power to grant immunity to someone who has been convicted?

In the United States, the power to grant immunity to someone who has been convicted typically resides with the president or a governor, depending on whether the conviction is at the federal or state level. The president can issue a pardon, which can nullify the conviction and restore rights, while governors have similar authority within their respective states. Additionally, courts may grant immunity during legal proceedings under certain circumstances, but this is usually related to testimony rather than a conviction.