In applying the Katz analysis to this case, it is important to begin by specifying precisely the nature of the state activity that is challenged. The activity here took the form of installing and using a pen register. Since the pen register was installed on telephone company property at the telephone company's central offices, petitioner obviously cannot claim that his "property" was invaded or that police intruded into a "constitutionally protected area." Petitioner's claim, rather, is that, notwithstanding the absence of a trespass, the State, as did the Government in Katz, infringed a "legitimate expectation of privacy" that petitioner held. Yet a pen register differs significantly from the listening device employed in Katz, for pen registers do not acquire the contents of communications. This Court recently noted: "Indeed, a law enforcement official could not even determine from the use of a pen register whether a communication existed. These devices do not hear sound. They disclose only the telephone numbers that have been dialed -- a means of establishing communication. Neither the purport of any communication between the caller and the recipient of the call, their identities, nor whether the call was even completed is disclosed by pen registers." United States v. New York Tel. Co., 434 U. S. 159, 434 U. S. 167 (1977). Page 442 U. S. 742 Given a pen register's limited capabilities, therefore, petitioner's argument that its installation and use constituted a "search" necessarily rests upon a claim that he had a "legitimate expectation of privacy" regarding the numbers he dialed on his phone.
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