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Efficient and coherent frequency conversions and nonlinear interference in optical parametric and atomic Raman processes

Yu Ding

Published Jan 1, 2014• 0 citations
OpenAlex

Abstract

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.

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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.
By using a parametric downconversion process with a strong signal field injection, we demonstrate coherent frequency downconversion from a pump photon to an idler photon. Contrary to a common misunderstanding, we show that the process can be free of quantum noise. With an interference experiment, we demonstrate that the coherence is preserved in the conversion process. This may lead to a high-fidelity quantum state transfer from a high-frequency photon to a low-frequency photon and connects a missing link in a quantum network. With this scheme of coherent frequency downconversion of photons, we propose a method of single-photon WDM.
By using a parametric downconversion process with a strong signal field injection, we demonstrate coherent frequency downconversion from a pump photon to an idler photon. Contrary to a common misunderstanding, we show that the process can be free of quantum noise. With an interference experiment, we demonstrate that the coherence is preserved in the conversion process. This may lead to a high-fidelity quantum state transfer from a high-frequency photon to a low-frequency photon and connects a missing link in a quantum network. With this scheme of coherent frequency downconversion of photons, we propose a method of single-photon WDM.
We experimentally demonstrate efficient Raman conversion to respective Stokes and anti-Stokes fields in both pulsed and continuous modes with a Rb-87 atomic vapor cell. The conversion efficiency is about 40-50% for the Stokes field and 20-30% for the anti-Stokes field, respectively. This efficient conversion process is realized with coherent feedback of both the Raman pump and the frequency-converted fields (Stokes or anti-Stokes). The experimental setup is simple and can be applied easily to produce light sources with larger frequency shifts using other Raman media with long coherence time. They may have potential applications in nonlinear optics, Raman spectroscopy and precision measurement.
Multi-photon nonlinear processes in atoms have served as important tools for quantum metrology, quantum communications, and quantum sensing. In this thesis, we experimentally address the interplay of various multi-photon Raman processes in hot Rb vapor, with the four-wave mixing (FWM) process being a central theme. FWM is the nonlinear response of a medium to a strong optical pump field inelastically scattering off atomic resonances and resulting in the generation of additional photons in different modes. FWM is a detrimental, but inherent part of electromagnetically induced transparency (EIT) and Raman based quantum memories. However, we were able to weaken the four-photon resonance by utilizing two-photon absorption to remove the additional photons without interfering with the signal beam. We also demonstrate the ability to tailor FWM to generate new photons in a controlled fashion for mode conversion. With this, we showed the conversion of 795 nm light to 420 nm light. While FWM is a source of noise in quantum memories, it can also be used for the generation squeezed twin-beams. Such beams have relative intensity noise reduced below the classical shot noise limit and share mode dependence based on the phase-matching conditions. Using this, we demonstrated that twin-beams can be generated with largely different spatial structure (optical angular momentum) and still share strong correlations, so long as the phase-matching conditions are satisfied. We then constructed and demonstrated the operation of a polarization-based quantum interferometer using squeezed twin-beams and showed that our beams were entangled under the inseparability condition. Using this interferometer, we were also able to achieve squeezing at low detection frequencies, which is necessary for things like quantum imaging and gravitational wave detection. We also demonstrated that squeezed twin-beams can be utilized to enhance the sensitivity of two-photon absorption spectroscopy. This research has touched on many different subjects related to quantum information science and improved upon some of the tools needed for the implementation of such technologies.
This thesis presents a study of photon generation and conversion processes in nonlinear optics. Our results extend beyond the first order perturbative regime, in which only pairs of photons are generated in spontaneous parametric down-conversion or four wave mixing. They also allow us to identify the limiting factors for achieving unit efficiency frequency conversion, and to correctly account for the propagation of bright nonclassical photon states generated in the nonlinear material. This is done using the Magnus expansion, which allows us to incorporate corrections due to time-ordering effects in the time evolution of the state, while also respecting the quantum statistics associated with squeezed states (in photon generation) and single photons or coherent states (in photon conversion). We show more generally that this expansion should be the preferred strategy when dealing with any type of time-dependent Hamiltonian that is a quadratic form in the creation and annihilation operators of the fields involved. Using the Magnus expansion, simple figures of merit to estimate the relevance of these time-ordering corrections are obtained. These quantities depend on the group velocities of the modes involved in the nonlinear process and provide a very simple physical picture of the interactions between the photons at different times. These time-ordering effects are shown to be important only when the photons and pump beam overlap within the nonlinear material for a significant amount of time. We also discuss the possibility of preparing Fock states with many photons using parametric down-conversion sources and realistic photon-number-resolving detectors. After discussing photon generation processes, we study photon conversion processes in which a nonlinear material and a classical strong pump field are used modify the frequency of a second incoming low-intensity field. We show that time-ordering effects limit the efficiency of this conversion process, re-examine strategies to get around this problem, and show that it should be possible to have a clear observation of modified upconversion probabilities due to these effects, and thus have an experimental demonstration of time-ordering effects in nonlinear optics.
Quantum frequency conversion (QFC), a critical technology in photonic quantum information science, requires that the quantum characteristics of the frequency-converted photon must be the same as the input photon except for the color. In nonlinear optics, the wave mixing effect far away from the resonance condition is often used to realize QFC because it can prevent the vacuum field reservoir from destroying the quantum state of the converted photon effectively. Under conditions far away from resonance, experiments typically require strong pump light to generate large nonlinear interactions to achieve high-efficiency QFC. However, strong pump light often generates additional noise photons through spontaneous Raman or parametric conversion processes. Herein, we theoretically study another efficient QFC scheme based on a resonant four-wave mixing system. Due to the effect of electromagnetically induced transparency (EIT), this resonant QFC scheme can greatly suppress vacuum field noise at low light levels; consequently, the converted photon can inherit the quantum state of the input photon with high fidelity. Our research demonstrates that if the conversion efficiency of the EIT-based QFC is close to 100%, the wave function and quadrature variance of the converted photon are almost the same as the input probe photon.