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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Generation of sub-Poisson light in a negative feedback and cascade three-level system

Jin-Hua Chai, Guang-Can Guo

Published Oct 28, 1992• 0 citations
Publisher

Abstract

The aim of this paper is to try to find out the possibility of reducing the photon number noise in an optically pumped three-level atomic system. Consider a three-level atomic system. The atomic transition between level 1 and level 3 is forbidden. Each atom is incoherently excited to upper level 3 from level 1, transits to level 1 through intermediate level 2, and emits photons at frequency (omega) 1 and (omega) 2. We place the atoms with the above feature into an oscillator and may obtain two coherent light beams, whose frequency are (omega) 1 and (omega) 2, respectively. There may be some correlation between these two light beams. We make one beam to control the pump source by a feedback loop and expect to reduce the noise of photon number of the other beam.

DOI
10.1117/12.140331

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The aim of this paper is to try to find out the possibility of reducing the photon number noise in an optically pumped three-level atomic system. Consider a three-level atomic system. The atomic transition between level 1 and level 3 is forbidden. Each atom is incoherently excited to upper level 3 from level 1, transits to level 1 through intermediate level 2, and emits photons at frequency (omega) 1 and (omega) 2. We place the atoms with the above feature into an oscillator and may obtain two coherent light beams, whose frequency are (omega) 1 and (omega) 2, respectively. There may be some correlation between these two light beams. We make one beam to control the pump source by a feedback loop and expect to reduce the noise of photon number of the other beam.
A scheme for generating sub-Poisson light in a negative feedback and cascade three-level system is proposed in this paper. By using Langevin quantum theory, it is shown that there is a partial correlation between the two output light beams, one of which is fed back to control pump source so that the pump noise may be suppressed. When the pump noise is perfectly suppressed, the photon noise level of the internal field is 50% below the standard quantum limit (SQL), but the reduction of the output photon noise is related with the atomic number N. When N is very large, the output noise level is SQL. When N is very small, the sub-Poisson light can be produced within the cavity bandwidth in the external field, and the fluctuation of output photon flow approaches zero when N approaches 1. The conclusion in this paper is of generality.
We theoretically consider a three-level laser with an incoherent one-way pump from a ground state into an intermediate excited state and a coherent drive into the upper laser state. The possibilities of inversionless lasing are studied. It is demonstrated that, in the case of a strong coupling with the drive laser (when its generation depends essentially on the three-level medium), the quantum properties of laser radiation can be improved by using m-quantum coherent excitation or by applying a sub-Poissonian coherent pump.
Expressions are derived for the intensity correlations or degrees of second-order coherence within and between two light beams of different frequency generated by cascade emission from a three-level atom. The atoms are assumed to be excited by two monochromatic coherent light beams in resonance with the upper and lower pairs of levels that constitute the triplet. The two emitted beams individually show photon antibunching and the correlation between the beams shows another complementary kind of non-classical effect. It is suggested that measurements on the system described here would provide more detailed information on quantum effects in cascade emission than previous observations with direct excitation to the uppermost atomic level by electron bombardment.
A single three-level atom with the ground state strongly coupled to an excited state and weakly coupled to a third intermediate state exhibits a telegraph signal of fluorescent emission when driven by laser light at the strong coupling frequency. The ``on'' periods correspond to normal resonance fluorescence, whereas the ``off'' periods correspond to the atom being ``shelved'' in the intermediate state. Such a system has interesting statistical properties. The photon statistics during the ``on'' periods correspond to two-level behavior and are close to Poisson in nature. However, the photon number distribution of a succession of ``on'' periods exhibits Bose-Einstein statistics. Thus the three-level system can exhibit both ``quiet'' and ``noisy'' behavior. The mathematical connection between the two cases is established, and an atomic beam experiment is described which records a mixture of the two statistical extremes. The composition of the mixture is varied, and the data are shown to be well represented by an analysis based upon the model developed.
Light emitted from laser-driven atoms often has squeezed quadrature fluctuations reflecting the phase dependence of the atomic source excited into a coherent superposition by the driving field. In this paper, we study the maximum squeezing in the emitted light fields from three-level atoms, paying particular attention to the role of atomic coherences in governing the optimum squeezing which is possible. Some phase dependent nonlinear optical processes have the potential for substantial quantum noise reduction at useful intensities: for example, four-wave mixing involving atomic systems has already been used to generate quadrature-squeezed light. The threoretical description of such a source is generally complicated with damping processes, coherent and incoherent pumping, and so on, all needing to be taken into account. Fully quantum treatments are available, but the basic physics of the process is often difficult to grasp in the necessarily complicated analysis. Although less important from the point of view of efficient practical sources of squeezed light, studying the generation of squeezed light in simpler systems such as two- or three-level atoms interacting with one or two modes (as in Jaynes-Cummings models) or in spontaneous emission or resonance fluorescence gives greater insight into the fundamentals of the squeezing process. In particular, the relationship between the atomic coherences and the degree of squeezing in the generated fields can be explored. We study the maximum (optimized) squeezing that can be obtained in the emitted fields from the irreversible decay from three-level atoms for any choice of initial conditions, excitations and decay process, for all quadrature components and for all possible configurations (V, \ensuremath{\Lambda}, and ``ladder'' systems).We show that for V and \ensuremath{\Lambda} systems, optimum squeezing is actually associated with a two-level state (in a suitable basis) involving a single one-photon coherence, but for ladder systems the state for maximum noise reduction is not equivalent to a two-level system and involves a single two-photon coherence and no intermediate state population. We pay particular attention to whether the total field is in a minimum uncertainty state, and the nature of the atomic state associated with maximum squeezing, especially whether it is a mixed or pure state. In the case where the initial free field has a zero amplitude at the detector and the source atoms are confined to a region small compared to the wavelength (Dicke source) we show that the squeezing in the total field is given in terms of the squeezing in the source field, and hence related to atomic populations and atomic one-photon and two-photon coherences for the case of three level atom. Previously unconsidered source-free field interferences terms are shown to be zero. The choice of quadrature phase and frequency is optimized to minimize the source field normally ordered variance. We then minimize further with respect to the choice of atomic density matrix elements subject to the constraints that the density matrix is Hermitean, positive, and has a trace equal to unity. In all cases we find that the optimum squeezing is produced when the source field is in a minimum uncertainty state.
The low-frequency intensity noise of high-quantum-efficiency lasers or light-emitting diodes (LED's) is dominated by the pump noise. By coupling two or more semiconductor lasers (or LED's) in series with a current source, the pump noise of the two lasers will be equal, and the intensity noise of the lasers will be highly correlated. It is shown that one may generate sub-Poissonian light by measuring the intensity fluctuations of one of the beams and feeding back (or forward) the information to regulate the other photon beam. The generation scheme is shown to have much in common with intensity-correlated twin beam generation from nondegenerate optical parametric oscillators.