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arXiv:2003.03491 · Physical Review Letters

Quantum-Enhanced Velocimetry with Doppler-Broadened Atomic Vapor

Zilong Chen, Hong Ming Lim, Chang Cang Huang, Rainer Dumke, Shau-Yu Lan

Published Mar 6, 2020• 14 citations
arXivHTMLPDF

Abstract

Traditionally, measuring the center-of-mass (c.m.) velocity of an atomic ensemble relies on measuring the Doppler shift of the absorption spectrum of single atoms in the ensemble. Mapping out the velocity distribution of the ensemble is indispensable when determining the c.m. velocity using this technique. As a result, highly sensitive measurements require preparation of an ensemble with a narrow Doppler width. Here, we use a dispersive measurement of light passing through a moving room temperature atomic vapor cell to determine the velocity of the cell in a single shot with a short-term sensitivity of 5.5 μm s^{-1} Hz^{-1/2}. The dispersion of the medium is enhanced by creating quantum interference through an auxiliary transition for the probe light under electromagnetically induced transparency condition. In contrast to measurement of single atoms, this method is based on the collective motion of atoms and can sense the c.m. velocity of an ensemble without knowing its velocity distribution. Our results improve the previous measurements by 3 orders of magnitude and can be used to design a compact motional sensor based on thermal atoms.

DOI
10.1103/physrevlett.124.093202

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Traditionally, measuring the center-of-mass (c.m.) velocity of an atomic ensemble relies on measuring the Doppler shift of the absorption spectrum of single atoms in the ensemble. Mapping out the velocity distribution of the ensemble is indispensable when determining the c.m. velocity using this technique. As a result, highly sensitive measurements require preparation of an ensemble with a narrow Doppler width. Here, we use a dispersive measurement of light passing through a moving room temperature atomic vapor cell to determine the velocity of the cell in a single shot with a short-term sensitivity of 5.5 μm s^{-1} Hz^{-1/2}. The dispersion of the medium is enhanced by creating quantum interference through an auxiliary transition for the probe light under electromagnetically induced transparency condition. In contrast to measurement of single atoms, this method is based on the collective motion of atoms and can sense the c.m. velocity of an ensemble without knowing its velocity distribution. Our results improve the previous measurements by 3 orders of magnitude and can be used to design a compact motional sensor based on thermal atoms.
Measuring the motion of quantum particles has been playing a significant role in performing high precision inertial sensing and studying fundamental physics.While most of the motion sensing schemes with cold atoms are based on single-particles.In this thesis, a new measuring method of using a collective state of atoms for motion quantum sensing is introduced.Two experiments were demonstrated to investigate its feasibility.One is related to the light-dragging effect in an electromagnetically induced transparent (EIT) cold 85 Rb atomic ensemble.The dragging coefficient F d was enhanced to 1.67 × 10 5 , which was three orders of magnitude better than the previous experiments.With a large enhancement of the dragging effect, we realised an atom-based velocimeter that has a sensitivity of 1 mm/s, which was two orders of magnitude higher than the velocity width of the atomic medium used before.Such a demonstration could pave the way for motion sensing using the collective state of atoms in a room temperature vapour cell or solid-state material.Another experiment is related to the motion sensing in a driven periodic potential.The motion of the atomic ensemble undergoing Bloch oscillation was measured using the light dragging method.In order to have efficient Bloch oscillation of atoms, the first Raman sideband cooling of 85 Rb to pre-cool atomic ensemble close to the recoil temperature (357 nK) was achieved by us.The phase shift measurements showed the linear-like relation to the accelerating time with the data precision 0.00036 rad (0.005 ns, 0.7 mm/s), instead of the stepwise oscillation period τ B .To observe the stepwise motion, it is required to reduce the lattice field intensity and implement the velocity selection technique to select atoms with a narrow velocity width.
As one of the most influential experiments on the development of modern macroscopic theory from Newtonian mechanics to Einstein's special theory of relativity, the phenomenon of light dragging in a moving medium has been discussed and observed extensively in different types of systems. To have a significant dragging effect, the long duration of light travelling in the medium is preferred. Here we demonstrate a light-dragging experiment in an electromagnetically induced transparent cold atomic ensemble and enhance the dragging effect by at least three orders of magnitude compared with the previous experiments. With a large enhancement of the dragging effect, we realize an atom-based velocimeter that has a sensitivity two orders of magnitude higher than the velocity width of the atomic medium used. Such a demonstration could pave the way for motional sensing using the collective state of atoms in a room temperature vapour cell or solid state material.
Accurate velocimetry measurements with fine spatial resolution are important for weather forecasting and detection of clean air turbulence and wind shear. This work experimentally demonstrates velocimetry measurements using an actively controlled atomic vapor filter, which has applications as an edge-filter-based direct detection Doppler lidar system. Here, the Doppler shift is determined by measuring the change in absorption of the signal along the edge of a transmission line in an atomic vapor medium. A counter-propagating pump beam resonant with a second ground state absorption line results in deep holes in the transmission spectrum near resonant velocity classes of the vapor. The steeper transmission slope gives an increased sensitivity to the Doppler shift of the probe beam. The location of the slope may be shifted by modifying the pump wavelength and power, essentially creating an actively controllable edge filter. The filter is used to estimate the Doppler shift of light scattered from a rotating disk with tangential speeds between 10 and 45 m/s. Improvements up to 70% in the velocity sensitivity are demonstrated when propagating the pump beam through the cell. Further improvements are anticipated with optimized cell and pump beam parameters. The filter is suggested for future lidar measurements of aerosol velocities and can be applied to other wavelengths resonant with energy level transitions in atomic vapors in the UV, visible, and IR regimes.
Abstract Atom-based sensing with optical readout is fundamentally limited by photon shot noise. Squeezed light can effectively suppress this noise of optical readout, thereby enabling measurement sensitivities beyond the standard quantum limit, but their application in light–atom hybrid systems is often hindered by absorption losses of atomic medium. Here, we demonstrate the first realization of electromagnetically induced transparency spectrum in a Rydberg-atom system using a squeezed probe field that surpasses the photon shot noise limit. Our theoretical model identifies atomic transit and absorption-induced losses as the primary factors limiting squeezing preservation. Experimentally, we engineer the atomic ensemble as a medium with tunable transmittance by employing Doppler-matched velocity-selective excitation to suppress absorption, achieving a squeezing transmission of 90.4 %\%% % for -1.7 dB squeezed light through a cesium vapor cell. This work is a critical step towards quantum-enhanced Rydberg atom sensors.
The transient dynamics of electromagnetically induced transparency (EIT) are fundamental to understanding coherent light–atom interactions and the advancement of quantum technologies such as optical switching and quantum memory. However, in room-temperature atomic vapors, Doppler broadening significantly alters these dynamics, yet a comprehensive understanding of its impact on the transient EIT response remains lacking. In this study, we combine analytical and numerical methods to investigate the absorption dynamics of a weak probe field in a three-level Λ-type system driven by a strong coupling field, based on the optical Bloch equations and Laplace transform techniques. Our results show that the transient response is highly sensitive to both the atomic spontaneous emission rate and the Rabi frequency of the coupling field. Increasing the coupling field intensity not only accelerates the approach to steady state but also induces oscillatory dynamics and negative absorption. Under Doppler broadening, the time required to reach steady state increases by approximately three orders of magnitude compared to the Doppler-free case—an effect that is surprisingly insensitive to temperature variations across the 100–400 K range. Moreover, restoring a short steady-state time under broadened conditions necessitates increasing the coupling laser intensity by two orders of magnitude. These findings provide key insights into the influence of Doppler broadening on coherent transient processes and offer practical guidelines for the design of room-temperature atomic devices, including quantum memories and optical modulators.
We studied experimentally the influence of thermal atomic motion on light propagation in a vacuum atomic vapour cell above room temperature. We found that atomic motion introduces sizable changes in the spectral properties of the medium, such as dispersion and absorption, if the conditions for coherent population trapping are fulfilled. In particular, studying the group delay of light in the atomic vapour, we confirm the theoretical predictions of Kocharovskaya et al. (2001, Phys. Rev. Lett., 86, 628) and demonstrate that a coherent atomic medium has light dragging abilities large enough to make feasible the realization of frozen light based on atomic motion. We also demonstrate that dragging can be observed in measurements of the electromagnetically induced transparency resonance width, as well as in group delay measurements.