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Physical Review A

Efficient conversion between photons and between photon and atom by stimulated emission

Z. Y. Ou

Published Aug 12, 2008• 57 citations
Publisher

Abstract

Quantum-state conversion among different information-carrying media is essential in a quantum network. It is found that the many schemes of quantum-state conversion can be unified under one simple process---that is, the parametric process of three-wave mixing. These schemes include quantum-state storage in atomic media, where a photonic state is converted to an atomic Raman state and vice versa, the frequency up- and down-conversion in photonic states, and photonic-state conversion to Brillouin phononic states and vice versa. The efficiency of conversion can in principle reach 100%, and the conversion process is noiseless. When the same idea is applied to a four-wave mixing process, unit conversion efficiency between a two-photon state and a single-photon state can be achieved. When applied to a two-photon entangled state in parametric down-conversion, entangled states of multiple photons can be generated.

DOI
10.1103/physreva.78.023819

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Quantum-state conversion among different information-carrying media is essential in a quantum network. It is found that the many schemes of quantum-state conversion can be unified under one simple process---that is, the parametric process of three-wave mixing. These schemes include quantum-state storage in atomic media, where a photonic state is converted to an atomic Raman state and vice versa, the frequency up- and down-conversion in photonic states, and photonic-state conversion to Brillouin phononic states and vice versa. The efficiency of conversion can in principle reach 100%, and the conversion process is noiseless. When the same idea is applied to a four-wave mixing process, unit conversion efficiency between a two-photon state and a single-photon state can be achieved. When applied to a two-photon entangled state in parametric down-conversion, entangled states of multiple photons can be generated.
Photons as flying qubits are ideal carriers of quantum information. However, efficient quantum interfaces which transfer flying into stationary qubits and vice versa are needed to realize complex quantum networks. Stationary qubit systems (atoms, ions, or solid state implementations) couple to photons via characteristic optical transitions. In order to establish key quantum operations, such as coherent photon storage, entanglement swapping, etc., high-quality photons with a tailored spectral, spatial, and temporal mode are needed. One way to generate these photons is to use the stationary qubit systems themselves. Another approach which does not impose limitations in terms of the photon’s wavelength or bandwidth relies on all-optical single photon sources. An example is spontaneous parametric down conversion (SPDC) inside an optical cavity with well-defined linewidth. In SPDC, a pump photon incident on a nonlinear medium is split into two photons of lower energy that are often referred to as signal and idler. Resonating the fluorescent fields, the setup becomes an optical parametric oscillator (OPO) far below the threshold. The usually THz broad emission spectrum of SPDC then follows the Airy function of the resonator. The spectral bandwidth of the generated photons can easily reach values of several MHz, which is comparable to atom resonance linewidths. By introducing additional losses or simply changing the cavity mirror reflectivities the bandwidth is tunable up to several 100 MHz. This is comparable to the bandwidth of single photons generated by e.g. quantum dots.
Significant progress has been achieved in leveraging atomic systems for the effective operation of quantum networks, which are essential for secure and long-distance quantum communication protocols. The key elements of such networks are quantum nodes that can store or generate both single and entangled photon pairs. The primary mechanisms leading to the production of single and entangled photon pairs revolve around established techniques such as parametric down-conversion, four-wave mixing, and stimulated Raman scattering. In contrast to solid-state platforms, atomic platforms offer a more controlled approach to the generation of single and entangled photon pairs, owing to the progress made in atom manipulation techniques such as trapping, cooling, and precise excitation schemes facilitated by the use of lasers. This review article delves into the techniques implemented for generating single and entangled photon pairs in atomic platforms, starting with a detailed discussion of the fundamental concepts associated with single and entangled photons and their characterization techniques. The aim is to evaluate the strengths and limitations of these methodologies and offer insights into potential applications. Additionally, the article will review the extent to which these atomic-based systems have been integrated into operational quantum communication networks.
We propose a method that enables efficient frequency conversion of quantum information based on recently demonstrated strong parametric coupling between two single-photon pulses propagating in a slow-light atomic medium at different group velocities. We show that an incoming single-photon state is efficiently converted into another optical mode in a lossless and shape-conserving manner. The persistence of initial quantum coherence and entanglement within frequency conversion is also demonstrated. We first illustrate this result for the case of small frequency difference of converted photons, and then discuss the modified scheme for conversion of photon wavelengths in different spectral ranges. Finally we analyze the generation of a narrow-band single-photon frequency-entangled state.
Efficient frequency conversion is crucial for interfacing photons with a variety of quantum systems ranging from matter qubits (color centers, quantum dots, atoms), optical fibers, detectors, that often operate at widely different wavelengths. Frequency conversion is also a resource to process quantum information.1, 2 Parametric processes such as sum frequency generation (SFG) and Bragg scattering four wave mixing (BS-FWM) offer a good versatility for such frequency conversion. It is well understood that those nonlinear processes have to be strong enough and satisfy phase matching to achieve a high conversion efficiency. While it is enough to consider those two aspects for moderate conversion efficiency up to 50%, the analysis is a bit more complicated when targeting efficiencies closely approaching unity. We are reviewing the pitfall that must be avoided to indeed reach near unity frequency transduction.
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.
Atomic-ensemble spin waves carrying single-photon Fock states exhibit nonclassical many-body correlations in between atoms. The same correlations are inherently associated with single-photon superradiance, forming the basis of a plethora of quantum light-matter interfaces. We devise a scheme allowing the preparation of spatially structured superradiant states in the atomic two-photon cascade using spin-wave light storage. We thus show that long-lived atomic ground-state spin waves can be converted to photon pairs, opening the way towards nonlinear optics of spin waves via multi-wave-mixing processes.