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International Conference on Spectral Line Shapes (ICSLS)

Spectroscopy with non-classical light and non-classical atoms

Eugene Simon Polzik, Jan Hald, Jens Lykke Sørensen

Published Jan 1, 1999• 0 citations
Publisher

Abstract

In the last few years a new field, atomic physics and spectroscopy with non-classical, squeezed light, has emerged. The frequency tunable sources of such light utilize tunable lasers and frequency doubling and/or parametric down conversion to create quantum correlations between frequency components of radiation [1–5]. The non-classical features of light result in manifestly non-classical features of spectral lines, namely in the rate of atomic processes, in the line shapes, and in the level of quantum noise in atomic spectroscopy. Moreover, the non-classical features of light can be transferred onto atoms leading to their manifestly non-classical behavior. We will briefly review some theoretical proposals and experimental results in this new area.

DOI
10.1063/1.58368

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In the last few years a new field, atomic physics and spectroscopy with non-classical, squeezed light, has emerged. The frequency tunable sources of such light utilize tunable lasers and frequency doubling and/or parametric down conversion to create quantum correlations between frequency components of radiation [1–5]. The non-classical features of light result in manifestly non-classical features of spectral lines, namely in the rate of atomic processes, in the line shapes, and in the level of quantum noise in atomic spectroscopy. Moreover, the non-classical features of light can be transferred onto atoms leading to their manifestly non-classical behavior. We will briefly review some theoretical proposals and experimental results in this new area.
Parametric downconversion in media has, so far, proved to be the most effective way of generating squeezed light. The development of frequency tunable sources of such non-classical light opens up novel possibilities in atomic physics and spectroscopy. A brief overview of the results in this area of quantum optics is given, and a new experiment aimed on subshot-noise measurement of atomic orientation is described. We report a preliminary 5 dB quantum noise reduction observed directly around the 917 nm Cs line. We introduce a figure of merit for the medium utilized for squeezed light generation in an optical parametric oscillator and evaluate some available nonlinear media from this perspective.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.
Squeezed light is of interest as an example of a non-classical state of the electromagnetic field and because of its applications both in technology and in fundamental quantum physics. This review concentrates on one aspect of squeezed light, namely its application in atomic spectroscopy. The general properties, detection and application of squeezed light are first reviewed. The basic features of the main theoretical methods (master equations, quantum Langevin equations, coupled systems) used to treat squeezed light spectroscopy are then outlined. The physics of squeezed light interactions with atomic systems is dealt with first for the simpler case of two-level atoms and then for the more complex situation of multi-level atoms and multi-atom systems. Finally the specific applications of squeezed light spectroscopy are reviewed.
Squeezed and other nondassical fields can be employed for enhanced spectroscopic sensitivity surpassing the standard quantum limit.1 However, beyond improvements in quantitative analysis where the nondassical field acts only as a passive probe are a wealth of possibilities associated with the active alteration of fundamental atomic radiative processes.2 In this regard, we are currently pursuing two experiments which make use of our frequency tunable source of squeezed light. The first studies two- photon excitation by correlated photon pairs,3 while the second investigates line- width alterations in cavity QED with squeezed light.4
For decades, ultrafast light-matter interactions have provided a powerful means to observe electrons in motion across a wide range of physical systems. This dissertation builds new theoretical frameworks to make sense of the emergent phenomena that arise when light drives matter into the strong-field regime, spanning atomic, molecular, and condensed-matter systems. The resulting dynamics leave their fingerprints on a variety of observables, such as photoionization spectra, high-harmonic generation, and nonlinear spectroscopic signals, each of which provides a different window onto the underlying electronic motion. The work is organized into two parts.The first part focuses on the electric-field observables of nonlinear optical signals. Because both the amplitude and phase of the emitted field are experimentally accessible through techniques such as spectral interferometry and electro-optic sampling, the full field itself serves as a rich observable for characterizing nonlinear responses in atomic, molecular, and condensed-matter systems. Leveraging this, the simulations presented here uncover novel and largely unexplored physical phenomena encoded in these field observables. The second part turns to optical spectroscopy with quantum light. Although nonlinear optical processes are conventionally modeled within a semiclassical framework, treating the matter quantum mechanically while the driving ultrafast fields remain classical, the resulting signals can nonetheless exhibit intrinsically non-classical properties. Building on this insight, a framework for interpreting experimental data is introduced and used to show that four-wave mixing driven by ultrafast femtosecond laser pulses generates bright squeezed light. Beyond squeezed light, strong field-matter interactions can also produce attosecond entangled photons, a distinct form of quantum light. Several strategies are proposed to enhance the efficiency of their generation, with the developed theory predicting rates of energy-time entangled biphoton production orders of magnitude beyond those achievable with conventional spontaneous parametric down-conversion (SPDC) sources. The studies presented here are initial steps toward investigating quantum properties of these systems through quantum phynonlinear spectroscopy, which is not achievable with classical light.
In this talk, I will discuss the growing field of entangled-photon spectroscopy as related to mol. systems. An entangled \nstate is represented by a wavefunction, describing a group of particles, which cannot be factored into sep. states of \nindividual particles. For photons, these correlations lead to non-classical light-matter interactions, potentially creating new opportunities for spectroscopic techniques. For example, second-order processes with two entangled photons, such as two-photon absorption or sum frequency generation, occur at a linear rate with a cross section that should be close to that of one-photon classical processes. However, early results in the field have produced conflicting measurements as to its magnitude. I will discuss the role of excited state dephasing and time-ordering for entangled photon interactions. I will also briefly remark on other interesting prospects of entangled photons, such as their ability to break the temporal-spectral Fourier transform limit. For instance, few-femtosecond spectroscopy is predicted to be possible with entangled light sources of MHz linewidths. Given the entangled photons quantum correlations, this property may lead to interesting opportunities in directing quantum coupled quantum states, although the techniques are still at an early stage.