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Physical Review A

Effect of atomic coherence on the second- and higher-order squeezing in a two-photon three-level cascade atomic system

Nadeem A. Ansari

Published Dec 1, 1993• 60 citations
Publisher

Abstract

A three-level cascade atomic system is considered where atomic coherence can be achieved either by applying an intense pump field or initially preparing the atoms in a coherent superposition of the states. It is predicted that the system, under certain conditions, exhibits almost perfect squeezing outside the cavity. By using the steady-state Q solution it is also shown that certain higher-order squeezing can also be achieved inside the cavity, under a suitable choice of different parameters.

DOI
10.1103/physreva.48.4686

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Physical Review A•Sep 1, 1994•24 citations
A three-level cascade atomic system is considered where atomic coherence can be achieved either by applying an intense pump field or initially preparing the atoms in a coherent superposition of the states. It is predicted that the system, under certain conditions, exhibits almost perfect squeezing outside the cavity. By using the steady-state Q solution it is also shown that certain higher-order squeezing can also be achieved inside the cavity, under a suitable choice of different parameters.
We consider a three-level cascade laser in which a coherently driven non-degenerate three-level atom available in a closed cavity coupled to a twomode vacuum reservoir.Applying the interaction Hamiltonian and Heisenberg equation of motion, we analyze the mean photon number, quadrature squeezing, and entanglement properties of the light produced by this quantum optical system inside the cavity.We find that the light generated by this system is in a two-mode squeezed state under certain conditions and the state of the system is weakly entangled at steady state due to the presence of the two-mode vacuum reservoir noise.
The generation and evolution of entanglement and squeezing for the two-mode fields are discussed in a cascade three-level atomic system. Two photons of a strong external pump field induce atomic coherence between the top and bottom levels. The dynamics of this system in the presence of cavity losses is studied. It is shown that entanglement and squeezing between the two modes increase initially, then decrease and eventually disappear. The periods of them are extended with the increasing of the pump field intensity. The effects of the cavity losses on squeezing and entanglement are also discussed.
Considering two Λ-type three-level atoms initially in the entangled state,one atom is poured into the cavity and interacts with a two-mode cavity.The dipole squeezing of an atom inside the cavity are investigated.The influences of the selective atomic measurement and the intensity of the coherent field which interacts with atom on dipole squeezing of an atom inside the cavity are discussed.The result obtained by numerical calculations shows that the dipole squeezing of an atom inside the cavity can be strengthened through selective atomic measurement for atom outside of the cavity.
Squeezing and entanglement of a two-mode cascade laser, produced by a three-level atom which is initially prepared by a coherent superposition of the top and bottom levels then injected into a cavity coupled to a two-mode squeezed vacuum reservoir is discussed. I obtain stochastic differential equations associated with the normal ordering using the pertinent master equation. Making use of the solutions of the resulting differential equations, we determined the mean photon number for the cavity mode and their correlation, EPR variables, smallest eigenvalue of the symplectic matrix, intensity difference fluctuation, and photon number correlation. It is found that the squeezed vacuum reservoir increases the degree of the statistical and nonclassical features of light produced by the system. Furthermore, using the criteria developed by logarithm negativity and Hillery-Zubairy criteria, the quantum entanglement of the cavity mode is quantified. It is found that the degree of the entanglement for the system under consideration increases with the squeezing parameter of the squeezed vacuum reservoir.
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.