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arXiv:1404.7117 · New Journal of Physics

Fluorescence detection at the atom shot noise limit for atom interferometry

E Rocco, R N Palmer, T. Valenzuela, Vincent Boyer, A. Freise, Kai Bongs

Published Sep 25, 2014• 36 citations
arXivHTMLPDF

Abstract

Atom interferometers (AIs) are promising tools for precision measurement with applications ranging from geophysical exploration and tests of the equivalence principle of general relativity to the detection of gravitational waves. Their optimal sensitivity is ultimately limited by their detection noise. We review resonant and near-resonant methods to detect the atom number of the interferometer outputs, and we theoretically analyze the relative influence of various scheme dependent noise sources and the technical challenges affecting the detection. We show that for the typical conditions under which an AI operates, simultaneous fluorescence detection with a charge-coupled device sensor is the optimal imaging scheme. We extract the laser beam parameters such as detuning, intensity, and duration required for reaching the atom shot noise limit.

DOI
10.1088/1367-2630/16/9/093046

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Atom interferometers (AIs) are promising tools for precision measurement with applications ranging from geophysical exploration and tests of the equivalence principle of general relativity to the detection of gravitational waves. Their optimal sensitivity is ultimately limited by their detection noise. We review resonant and near-resonant methods to detect the atom number of the interferometer outputs, and we theoretically analyze the relative influence of various scheme dependent noise sources and the technical challenges affecting the detection. We show that for the typical conditions under which an AI operates, simultaneous fluorescence detection with a charge-coupled device sensor is the optimal imaging scheme. We extract the laser beam parameters such as detuning, intensity, and duration required for reaching the atom shot noise limit.
Resonant frequency modulation spectroscopy has been previously used as a highly sensitive method for measuring the output of cold atom interferometers. Using a detailed model that accounts for optical saturation, laser intensities and atomic densities that vary spatially, and radiation pressure on the atoms, we theoretically investigate under what parameter regimes the optimum signal-to-noise ratio is found under experimentally realistic conditions. We compare this technique to the standard method of fluorescence imaging and find that it outperforms fluorescence imaging for compact interferometers using condensed atomic sources or where the photon collection efficiency is limited. However, we find that fluorescence imaging is likely to be the preferred method when using squeezed atomic sources due to limited atom number.
Atom interferometers possess great potential as sensitive and stable acceleration and rotation sensors. Currently, the DoD is interested in leveraging atom interferometry research to improve inertial sensors and develop portable, compact devices based on atom interferometers with strategic-grade performance. Typical atom interferometers measure the acceleration or rotation signal with an on-resonance detection laser field, but these laser fields have inherent noise that degrades the signal-to-noise ratio. The laser noise in one quadrature of the field can be reduced below the so-called shot noise limit at the expense of the other quadrature. Fields with this property are called “squeezed fields.” Although lowering the noise in one quadrature increases noise in the other quadrature (due to Heisenberg’s uncertainty principle), the noise in the other quadrature can be made irrelevant by designing a detection scheme that does not use that quadrature. By reducing light-source noise, atom interferometer measurement precision can be increased. This thesis aims to establish the groundwork for creating squeezed light to be used for the detection of atoms in the ground or excited state of the NPS Atom Interferometry Laboratory atom interferometer.
Abstract Recent advances in quantum sensing—optical clocks at 5.5 × 10 − 19 systematic uncertainty, frequency-dependent squeezing below the standard quantum limit, quantum magnetometers approaching fundamental sensitivity limits—raise a natural question: can these technologies detect gravitational waves directly, or enhance existing detectors beyond current capabilities? We show that the answer is primarily determined by the coupling mechanism between the gravitational wave and the sensor. Starting from the tidal Hamiltonian in Fermi normal coordinates, we identify three physically distinct mechanisms by which a gravitational wave couples directly to a quantum system, and derive their transducer gains within linearized general relativity and non-relativistic quantum mechanics. Internal atomic coupling (tidal distortion of electronic wavefunctions) yields a transducer gain G A = 2.4 × 10 − 20 , with vanishing first-order energy shifts for all J = 0 clock states—a ∼ 10 35 deficit relative to laser interferometry that exceeds any projected quantum enhancement. Center-of-mass coupling (Doppler shifts from geodesic motion) reaches strain sensitivities of ∼ 10 − 18 , still 10 4 above LISA requirements. Light propagation coupling (phase accumulation over macroscopic baselines) provides the enormous transducer gain that makes laser interferometry—and atom interferometry—viable. For detectors exploiting this third mechanism, we quantify how much improvement quantum sensors can provide through the detector’s noise architecture: LISA’s noise budget is predominantly classical, limiting combined quantum enhancement to E ≈ 1.04 , while ground-based detectors in the shot-noise-dominated regime achieve E = 1.6 – 2.1 . Atom interferometers exploit the same light-propagation mechanism to target the 0.01–10 Hz band between the LISA and LIGO ranges.
Interferometric gravitational wave detectors (such as advanced LIGO) employ high-power solid-state lasers to maximize their detection sensitivity and hence their reach into the universe. These sophisticated light sources are ultra-stabilized with regard to output power, emission frequency and beam geometry; this is crucial to obtain low detector noise. However, even when all laser noise is reduced as far as technically possible, unavoidable quantum noise of the laser still remains. This is a consequence of the Heisenberg Uncertainty Principle, the basis of quantum mechanics: in this case, it is fundamentally impossible to simultaneously reduce both the phase noise and the amplitude noise of a laser to arbitrarily low levels. This fact manifests in the detector noise budget as two distinct noise sources—photon shot noise and quantum radiation pressure noise—which together form a lower boundary for current-day gravitational wave detector sensitivities, the standard quantum limit of interferometry. To overcome this limit, various techniques are being proposed, among them different uses of non-classical light and alternative interferometer topologies. This article explains how quantum noise enters and manifests in an interferometric gravitational wave detector, and gives an overview of some of the schemes proposed to overcome this seemingly fundamental limitation, all aimed at the goal of higher gravitational wave event detection rates. This article is part of a discussion meeting issue ‘The promises of gravitational-wave astronomy’.
Atom interferometers (AIFs) are highly precise inertial sensors and are considered promising instruments for the detection of gravitational waves (GWs) and certain dark matter (DM) candidates in the mid-frequency band. While GW detection with AIFs was initially proposed for space-based experiments with baselines spanning thousands of kilometers, recent developments suggest that terrestrial setups with baselines of at least 100 meters may also be capable of detecting GWs. In this work, we analytically derive the GW phase response formula of a ground detector, find an additional term compared to the existing literature and check it our findings numerically. Based on this treatment, we analyze the optimal geometric parameters for earth-bound AIF experiments. Subsequently, we numerically simulate GW detection schemes for these optimized interferometers using an open-source Python algorithm. Numerical simulations of these schemes have are not available to the community, yet crucial for the accurate modeling of noise and non-trivial gravitational backgrounds.
The periodicity inherent to any interferometric signal entails a fundamental trade-off between sensitivity and dynamic range of interferometry-based sensors. Here, we develop a methodology for substantially extending the dynamic range of such sensors without compromising their sensitivity, stability, and bandwidth. The scheme is based on simultaneous operation of two nearly identical interferometers, providing a moiré-like period much larger than 2π and benefiting from close-to-maximal sensitivity and from suppression of common-mode noise. The methodology is highly suited to atom interferometers, which offer record sensitivities in measuring gravito-inertial forces but suffer from limited dynamic range. We experimentally demonstrate an atom interferometer with a dynamic-range enhancement of more than an order of magnitude in a single shot and more than three orders of magnitude within a few shots for both static and dynamic signals. This approach can considerably improve the operation of interferometric sensors in challenging, uncertain, or rapidly varying conditions.