Can a police officer who poses as an undercover officer to obtain a false confession be allowed to testify in a criminal case?

It was early established in this state that 'That fact that a confession was procured by the employment of falsehood by a police officer, detective, or other person does not alone exclude it; nor does the employment of any artifice, deception, or fraud exclude it, if the artifice or fraud employed was not calculated to procure an untrue statement. Neither does the fact that a confession was obtained by a promise of secrecy render it incompetent, if there was no motive to produce a false statement.' (People v. Castello, 194 Cal. 595, 602, 229 P. 855, 857.) Also, in People v. Santo, 43 Cal.2d 319, at page 325, 273 P.2d 249, at page 252, a policeman who had posed as one seeking to help a defendant in jail awaiting trial account for her whereabouts on the night of the crime, was permitted to testify regarding statements made by the defendant which tended to show her consciousness of guilt and which amounted to admissions against interest.