ISSN 1068-3356, Bulletin of the Lebedev Physics Institute, 2011, Vol. 38, No. 8, pp. 235–241. c  Allerton Press, Inc., 2011. Original Russian Text c  A.Yu. Samokotin, G.A. Vishnyakova, E.O. Tereshchenko, A.V. Akimov, N.N. Kolachevskii, A.V. Sokolov, V.N. Sorokin, 2011, published in Kratkie Soobshcheniya po Fizike, 2011, Vol. 38, No. 8, pp. 36–46. Coherent Population Trapping Resonances in the Problem of Quantum Filtering of Light Pulses A. Yu. Samokotin a,b,c , G. A. Vishnyakova a,b , E. O. Tereshchenko a,b , A. V. Akimov a,b , N. N. Kolachevskii a,b , A. V. Sokolov a , and V. N. Sorokin a,b a Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia b Moscow Institute of Physics and Technology, Institutskii per. 9, Dolgoprudnyi, Moscow Oblast, 141700 Russia c E-mail: samokotin@gmail.com Received July 4, 2011 Abstract—The conditions necessary to implement a single-photon pulse source using quantum filtering based on the coherent population trapping phenomenon in N -systems of atomic levels are determined. The dependences of dark resonance characteristics on laser field intensities are experimentally measured in Rb vapor. These dependences define optimum intensity ratios and pulse durations of used laser beams, at which the system can efficiently operate as a single-photon quantum filter. DOI: 10.3103/S1068335611080069 Keywords: coherent population trapping, single-photon population pulses, quantum filtration, N - systems. Introduction. Coherent population trapping (CPT) [1, 2] is a subject of basic research and applied developments in the field of precision spectroscopy [2], metrology [3], magnetometry [4], and light pulse storage and conversion using coherent excitations in an atomic medium [5–7]. In [8], generalized dark states (GDSs) of CPT in the (atom + field) system were considered; it was shown that GDSs can arise in both classical (coherent light states) and quantized (n-photon or Fock light states) fields. Of most interest is the consideration of GDSs in atomic level systems forming the so-called N -chains during interactions with light fields (Fig. 1). The N -chain is a sequence of L Λ systems complemented by one resonance transition. Due to this additional transition playing the role of a dark state decay channel, CPT does not occur in such a system in the classical field. Nevertheless, the theoretical consideration [8] shows that GDSs can be formed in such systems as well. In this regard, the quantum filter idea was advanced in [8], which is described in detail in [9]. In an optically thick medium consisting of atoms whose level system forms an N -chain, in the presence of a continuous strong (pumping) light wave with the σ − polarization, photons from the weak signal pulse with the σ + polarization will be scattered until no more than L photons will remain in the pulse. Photons will be scattered by the right transition of the N -chain (Fig. 1). The pump wave is blocked and we obtain, at the medium output, an L-photon pulse with the σ + polarization. It is supposed that the system can be taken as the basis of a new type of Fock light state sources which are necessary in experiments on quantum optics and quantum systems of data transmission [10]. Previously, we showed that a strong magnetic field can be used to implement the single-photon quantum CPT filter in the stable bosonic isotope 87 Rb [9]. In the field with induction B> 70 G, the Zeeman effect becomes nonlinear in m F , and Λ-chains forming at magnetic sublevels 5 2 S 1/2 (F = 2) and 5 2 P 1/2 (F = 1) in 87 Rb in the two- frequency field σ + − σ − are transformed into effective N -chains (Fig. 2). Propagation without absorption of the single-photon pulse in an atomic medium with the N -system of sublevels will be controlled by the efficiency of excitation of dark resonances in the Λ-system entering it in classical laser fields with a large number of photons in mode [8]. Therefore, it is important to study the CPT resonance in the Λ-system existing in 87 Rb in a weak magnetic field. 235