[Paper Review] Mollow triplet in cold atoms

Luis Ortiz-Gutiérrez, R Teixeira+5 more|arXiv (Cornell University)|Jan 22, 2019

Quantum optics and atomic interactions42 references33 citations

TL;DR

This paper reports the first quantitative observation of the Mollow triplet in a cold atomic cloud using heterodyne spectroscopy and intensity correlation measurements. By driving a ⁸⁵Rb cloud with a strong, circularly polarized laser beam and minimizing inhomogeneous broadening, the authors achieve excellent agreement between experiment and theory, confirming the triplet's spectral structure and revealing the influence of temperature and beam size on the spectrum.

ABSTRACT

In this paper, we measure the spectrum of light scattered by a cold atomic cloud driven by a strong laser beam. The experimental technique is based on heterodyne spectroscopy coupled to single-photon detectors and intensity correlations. At resonance, we observe the Mollow triplet. This spectrum is quantitatively compared to the theoretical one, emphasizing the influence of the temperature of the cloud and the finite-size of the laser beam. Off resonance measurements are also done showing a very good agreement with theory.

Motivation & Objective

• To observe the Mollow triplet in a cold atomic cloud, a system with low Doppler broadening and high optical density.
• To achieve quantitative agreement between experiment and theory by accounting for inhomogeneous broadening due to temperature and finite laser beam size.
• To demonstrate the feasibility of observing the Mollow triplet in multi-atom systems with high signal-to-noise using single-photon detection and beat note techniques.
• To investigate the impact of polarization and intensity modulation on the visibility and fidelity of the Mollow triplet signal.
• To provide a clean, two-level system-like environment by using alternating laser beams to avoid standing wave effects and spatial Rabi frequency modulation.

Proposed method

• Heterodyne spectroscopy using a beat note technique to detect the scattered light spectrum with high sensitivity.
• Single-photon detection via avalanche photodiodes (APDs) with time-tagging for photon counting and intensity correlation measurements.
• Use of counter-propagating, alternating laser beams with identical circular polarization to avoid standing wave formation and ensure homogeneous Rabi drive.
• Laser frequency locking via a master-slave offset scheme to maintain precise control over the detuning and power.
• Polarization control using λ/2 and λ/4 waveplates to select a single closed transition (F=3 → F'=4) in ⁸⁵Rb.
• Collection of scattered light via a polarization-maintaining single-mode fiber and mixing with a local oscillator for heterodyne detection.

Experimental results

Research questions

• RQ1 Can the Mollow triplet be quantitatively observed in a cold atomic cloud with minimal inhomogeneous broadening?
• RQ2 How do the temperature of the atomic cloud and the finite size of the laser beam affect the spectral shape of the Mollow triplet?
• RQ3 What is the role of polarization and intensity modulation in preserving the spectral signature of the Mollow triplet in a multi-atom system?
• RQ4 To what extent does multiple scattering or cooperative effects modify the Mollow triplet in dense atomic clouds?
• RQ5 How well do experimental measurements of elastic and inelastic scattering match theoretical predictions off-resonance?

Key findings

• The Mollow triplet is clearly observed at resonance, with a central peak and two symmetric sidebands, confirming the expected three-peak structure in a cold atomic cloud.
• Quantitative agreement between experiment and theory is achieved when accounting for inhomogeneous broadening due to temperature and finite beam size.
• Off-resonance measurements show excellent agreement between measured elastic and inelastic scattering intensities and theoretical predictions.
• The use of alternating, circularly polarized beams successfully avoids standing wave effects and maintains a homogeneous Rabi drive, crucial for observing a clean Mollow triplet.
• The experimental setup achieves high sensitivity through heterodyne detection and single-photon counting, enabling detection of weak scattered light with high signal-to-noise.
• The results demonstrate that cold atomic clouds are a viable and advantageous platform for studying the Mollow triplet with high precision and minimal decoherence.