Section 53. Such special state police officers shall be sworn before a justice of the peace or notary public who shall make a return thereof to the department upon a form provided by said department. Upon receipt of said return, said department shall issue a license to each person so designated in accordance with the provisions of section fifty-one to act as a police officer. Such license shall, in any court of the commonwealth, be competent evidence of the validity of the appointment of such person named therein and of his authority to act as such police officer. Such special state police officers shall hold their offices until their appointment is revoked by the colonel, or until the petitioner corporation or company, upon ceasing to require the service of any such officers, shall file a notice to that effect with the department and thereupon the power of such special state police officer shall cease.