Letters m Mathematical Physics 34: 297-3()7, 1995. 297 @ 1995 Kluwer Academic Put~h'slwr.~. Printed in the Netherhmd,~. Theoretical Basis for a New Subnatural Spectroscopy Via Correlation Interferometry Dedicated to the tnem<>rv oJProfess<. Julian Schwinget; a role ntodelJor us all ULRICH W. RATHE t'e'" and MARLAN O. SCULLY j'2' ~" I Max_t~lam.k_hlstitutfiir Quantenoptik, $574,~ Garching. Germany : l)cpartmcnt of Physics, Texa,~ A & M l lniversity. Colle~,e Station. TX 77S43. U.S.A. (Received: 16 March 1995) Abstract. A new line-narrowing effect in coincidence intefferometry yielding subnatural resolution of atomic transition frequencies is proposed and analyzed. The approach utilizes second-order pho- ton correlation properties of the radiation lield. This is in contrast to the lirst-order measurements associated with time delay spectroscopy, which is known to yield subnatural resolution. Connections between the two techniques are investigated. Mathematics Subject Classification ( 1991 ): 81VSO. 1. Introduction Thanks in large measure to Julian Schwinger, modem theoretical physicists have Green's function and correlation function technology bred into their bones. Nowhere is this more evident than in modem quantum optics, where we are constantly dealing with one- and two-photon detection events described by the correlation functions (,'(~)(r./) = (qJlf.'(-)(r,t)f.'(+)(r,t)lqJ), (1) (;(e)(rt,t~;re,t2) = (* Il?(-)(r,, t))/~'(-)(r2, 12) x [.'(+)(r2.12)hi(+)( rl. t, )1'). (2) with the electric lield annihilation operator li(+)( r, t ) = s exp( ik. r - ickt ) and the adjoint creation operator [,'(-)(r,t) = [,:{+)(r,t) t. We here show that it is possible to achieve a line narrowing in spontaneously emitted radiation via two-photon correlation measurements on atomic cascade radiation. " Also at Sektion Physik, Ludwig-Maximilians-Universitat, 80333 Munich, Germany. "" Also at Texas Laser Laboratory. IIARC, The Woodlands, TX 77381, U.S.A.