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Physical Review Letters

Resonant Enhancement of Parametric Processes via Radiative Interference and Induced Coherence

Mikhail D. Lukin, Philip Hemmer, Markus Löffler, Marlan O. Scully

Published Sep 28, 1998• 182 citations
Publisher

Abstract

Nonlinear optical parametric processes are studied in a resonantly driven multilevel system displaying quantum interference effects. It is shown that in such systems a new regime of nonlinear amplification is possible, in which a pair of correlated Stokes and anti-Stokes fields can be generated from infinitesimally small initial values. An atomic coherence grating emerges during this process of efficient nonlinear amplification. The present analysis explains the results of recent optical phase conjugation experiments involving atomic phase coherence.

DOI
10.1103/physrevlett.81.2675

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Nonlinear optical parametric processes are studied in a resonantly driven multilevel system displaying quantum interference effects. It is shown that in such systems a new regime of nonlinear amplification is possible, in which a pair of correlated Stokes and anti-Stokes fields can be generated from infinitesimally small initial values. An atomic coherence grating emerges during this process of efficient nonlinear amplification. The present analysis explains the results of recent optical phase conjugation experiments involving atomic phase coherence.
By implementing a parametric down-conversion process with a strong signal field injection, we demonstrate that frequency down-conversion from pump photons to idler photons can be a coherent process. Contrary to a common misconception, we show that the process can be free of quantum noise. With an interference experiment, we demonstrate that coherence is preserved in the conversion process. This technique could lead to a high-fidelity quantum state transfer from a high-frequency photon to a low-frequency photon and connect a missing link in quantum networks. Coherent and efficient nonlinear interaction and frequency conversion are of great interest in many areas of quantum optics. Traditionally, the low efficiency of Raman scattering is improved by a high-finesse optical resonator or stimulated Raman conversion. It was recently found that the atomic spin wave initially built through electromagnetically induced transparency or a weak Raman process can actively enhance the Raman frequency conversion. An experimental demonstration of an efficient Raman conversion scheme with coherent feedback of both pump and Stokes fields is presented. The temporal profile of the generated Raman pulse shows that the coherence time of the atomic spin wave is $\sim$1.8 ms. A laser-like power threshold is observed and its low threshold is attributed to the long coherence time of the atomic spin wave. The mechanism of the conversion enhancement process is discussed and the conversion efficiency of a single pass of the beams is compared with that of double passes. Finally, a beat signal is observed between the two Stokes fields and its Fourier transform shows that the frequency difference is caused by the AC Stark effect. Precision phase measurement is traditionally restricted by the standard quantum limit. However, this limit is not as fundamental as the Heisenberg limit and can be circumvented by use of nonclassical quantum states and structure modification of the interferometers. Several examples of nonlinear interferometers are proposed and implemented. The wave propagation equations for the nonlinear interferometers are solved. The interference fringes are measured and compared with that of linear interferometers. The first nonlinear interferometer presented is based on second harmonic generation and degenerate parametric down-conversion. The second nonlinear interferometer is composed of two parametric amplifiers. The idea of nonlinear beam splitters is introduced as an analogy to traditional beam splitters. We show that a nonlinear interferometer can be built alternatively by using only one parametric amplifier. Type II phase-matched crystals can be used to increase the amplification factor. An interferometer based on a Raman amplifier is analyzed for its application to sensitive magnetic field measurement.
Summary form only given. Nonlinear optical properties of an atomic medium can be modified by coherence and quantum interference that utilizes electromagnetically induced transparency (EIT). Because of the vanishing linear absorption and large enhancement of nonlinearities in an EIT medium, nonlinear optics may be studied at low light levels. We report studies of light absorption and parametric amplification based on the EIT enhanced third-order nonlinearity in a four-level atomic system. We have observed the enhanced nonlinear absorption when two coupling lasers and one probe laser are present. We are in the process of preparing the experiments for the double EIT demonstration and observation of the efficient parametric amplification in the four-level system. Because the experimental system is essentially free of Doppler broadening, quantitative comparisons between the experimental measurements and the theoretical calculations can readily be achieved.
We investigate the quantum properties of fields generated by resonantly enhanced wave mixing based on atomic coherence in Raman systems. We show that such a process can be used for generation of pairs of Stokes and anti-Stokes fields with nearly perfect quantum correlations, yielding almost complete (i.e., 100100%100) squeezing without the use of a cavity. We discuss the extension of the wave mixing interactions into the domain of a few interacting light quanta.
We study the effect of nonlinear interference processes on quantum transitions in intense resonant electromagnetic fields with allowance for changes in level populations, relaxation processes, incoherent excitation, and Doppler broadening of the transitions, on the absorption, amplification, and refraction of the interacting fields. The theory is generalized to the case of nonlinear interference interaction of two intense fields in open and closed three-level quantum systems. Using the density matrix, we derive general expressions in the case of stationary interaction that make it possible to analyze the optical characteristics for various configurations of the interfering transitions by a simple substitution of parameters. The possibility of amplifying light without saturated population inversion in a resonant transition is discussed. We formulate the conditions for such amplification and use examples to show that under appropriate changes in the initial level populations and the intensity of the auxiliary light, the inversionless amplification coefficient does not decrease with increasing intensity of the amplified radiation. We also show that allowance for these accompanying processes greatly affects the choice of optimal conditions for interference in optical transitions. As an illustration we list the results of a numerical analysis of possible experiments.
By investigating the third-order nonlinearity of a four-level ladder-type atomic system, it is found that, with spontaneously generated coherence (SGC) present, the nonlinear absorption or refraction can be significantly enhanced with vanishing linear absorption. We attribute the enhancement of nonlinearity mainly to quantum interference in the two decay pathways from the two upper closely lying levels. With a standing-wave trigger field, absorption or phase grating, which effectively diffracts a weak probe into high-order direction, can be induced by the SGC-enhanced absorptive or refractive nonlinear modulation. In contrast to the schemes for enhancing nonlinearity, no additional coupling field is required. Moreover, the present gratings result from the nonlinear modulation, which differs from the recent investigations based on linear modulation.