This dissertation reports on advancements in the construction of a quantum sensor for gravimetry based on matter interferometry. Ultra-cold strontium atoms (in the order of 1 μK) within a ring cavity in the bad cavity regime constitute the experimental setup and two distinct approaches were adopted for the comprehension of the systems operation. The first involved simulations of Ramsey-Bordé pulse sequence, specifically the π/2-π-π/2-spin echo sequence using the 689 nm transition of strontium. The sequence is commonly employed in matter interferometers and the simulations provided insights for future measurements. The expected precision of our system is within Δg/g < 10-8, showcasing its potential accuracy. The second approach focused on monitoring Bloch oscillations resulting from the interaction between atoms and cavity light, which induces a frequency proportional to the external force, in our case, gravity. Efforts were made during the experiment towards confirming the systems regime, leading to a significant observation of nonlinear Normal-mode splitting due to high saturation in the strong coupling regime, exhibiting a bi-stable behaviour.
This dissertation reports on advancements in the construction of a quantum sensor for gravimetry based on matter interferometry. Ultra-cold strontium atoms (in the order of 1 μK) within a ring cavity in the bad cavity regime constitute the experimental setup and two distinct approaches were adopted for the comprehension of the systems operation. The first involved simulations of Ramsey-Bordé pulse sequence, specifically the π/2-π-π/2-spin echo sequence using the 689 nm transition of strontium. The sequence is commonly employed in matter interferometers and the simulations provided insights for future measurements. The expected precision of our system is within Δg/g < 10-8, showcasing its potential accuracy. The second approach focused on monitoring Bloch oscillations resulting from the interaction between atoms and cavity light, which induces a frequency proportional to the external force, in our case, gravity. Efforts were made during the experiment towards confirming the systems regime, leading to a significant observation of nonlinear Normal-mode splitting due to high saturation in the strong coupling regime, exhibiting a bi-stable behaviour.
We report on our progress in the construction of a continuous matter-wave interferometer for inertial sensing via the non-destructive observation of Bloch oscillations. At the present stage of the experiment, around 105 strontium-88 atoms are cooled down to below 1μK and transferred to the vertical arm of the optical mode of a ring cavity. Pumped by lasers red-tuned with respect to the 7.6kHz broad intercombination transition of strontium, the two counterpropagating modes of the ring cavity form a one-dimensional optical lattice in which the atoms, accelerated by gravity, will perform Bloch oscillations. The atomic motion can be monitored in real-time via its impact on the counterpropagating light fields. We present the actual state of the experiment and characterize the laser spectrometer developed to drive the atom-cavity interaction.
Atom interferometry is a maturing quantum technology which provides one of the most precise approaches to explore fundamental physics, e.g. to search for dark matter, and one day to observe gravitational waves in the frequency range from 10 mHz - 10 Hz. Standard atom interferometers use Bragg or Raman two-photon transitions for the coherent manipulation of matter waves. In a Michelson configuration for atom-aided gravitational waves detection, two orthogonal arms are built to reject laser noise, in the same way as optical interferometers. Atom interferometers utilizing an optical clock transition for coherent manipulation are less affected by the technical noise of the laser, and as a result that single baseline detectors become feasible.At LP2N, we are building a vertical gravity-gradiometer based on atom interferometry using strontium atoms probed on such a clock transition. The final experimental setup will implement a 6-meter tall atomic fountain to realize gradiometry with two or more separated atomic clouds.In my dissertation, I present the realization of the experimental setup in all its sub-systems, and the first experiments with atoms at millikelvin temperature obtained with the first cooling stage with blue light. The laser system for the second red cooling phase and for the coherent manipulation is ready; meanwhile, we realized a dual-frequency Zeeman slower that improves the atomic flux by a factor of six. We also present a study of an original scheme to achieve pulsed and fast light-atom interaction in a narrow-linewidth cavity by exploiting the Stark shift as a high bandwidth switch; this technique could open to the use of narrow-linewidth cavities – and notably their high power buildup - in several contexts, like atom interferometry and quantum information.
Inertial sensors based on matter-wave interferometry are currently approaching the precision and accuracy of state-of-the-art classical sensors. While these devices are often realised with ultracold but not Bose-condensed atoms as matter waves, employing Bose-Einstein Condensates (BEC) promises to overcome certain limitations, especially those related to the ensemble's expansion. The point-source like character of BECs also enables utilising spatial interference patterns to measure e.g. rotation rates in single-shot experiments. Matter wave-based inertial sensors are considered for experiments ranging from Gravitational Wave detection to tests of the Universality of Free Fall (UFF) to gain insight into the joint between Quantum Mechanics and General Relativity. In the scope of this thesis, matter-wave interferometry with BECs was demonstrated for the first time in a microgravity environment with the QUANTUS-1 apparatus. The same instrument was then employed as a quantum tiltmeter utilising a novel beam-splitting mechanism known as Bragg Double Diffraction. To this end, the QUANTUS-1 apparatus designed as a BEC instrument to be operated in the drop tower at ZARM at University of Bremen was equipped with optics and laser systems required for performing matter-wave interferometry based on Bragg Diffraction. The apparatus employs an atom chip to create BECs of around 10000 Rubidium 87 atoms within 15 s. With a Mach-Zehnder like interferometer scheme, spatial interference fringes were observed after a free evolution time in the interferometer of up to 677 ms. To achieve these long time scales, a method known as Delta-Kick Collimation (DKC) was adapted to slow the expansion of the BEC to a kinetic energy equivalent below 1 nK, and the atoms were transferred to a non-magnetic Zeeman state via an adiabatic rapid passage (ARP). A similar interferometer scheme with a newly developed beam-splitter mechanism known as Bragg Double Diffraction was used to measure the tilt of the instrument on ground with a precision of up to 4.4 AA rad. This thesis presents an overview of the apparatus including ground-based characterisations of all required experimental steps. Results from over 400 free fall experiments are evaluated for expansion studies of the BEC and matter-wave interferometry in microgravity. The time-evolution of first and second order Bragg Double Diffraction beam splitters is studied, and an interferometer sensitive to the tilt of the instrument is implemented. Based on this work, a gravimeter with a new launch mechanism comprising Bragg beam splitters and Bloch oscillations to enable atomic fountains in atom-chip based devices was developed. The microgravity experiments were adapted for the MAIUS-1 sounding-rocket instrument to create the first man-made BEC in outer space and study the feasibility of operating matter-wave interferometers on space-borne platforms. The results of this thesis lay the groundwork for future space-borne missions using matter-wave interferometry for precision measurements of inertial forces.
This thesis describes the first steps to fulfill the main objective of our project: the development of a gravimetry system based on the continuous monitoring of Bloch oscillations performed by ultra-cold strontium atoms.The realization of this project comprises three main stages: the first stage consists in obtaining a magneto-optical trap in the broad transition of 461 nm of strontium, the second stage is the transfer to a magneto-optical trap in the narrow intercombination line of 689 nm, and finally the third stage is the monitoring of Bloch oscillations of the cloud of atoms in a ring cavity.This work details the first stage of the project, the creation of an experimental apparatus which produces ultra-cold strontium atoms in the transition of 461 nm, as well as the characterization of the cloud of atoms, temperature, size, number of atoms, etc.For this project, a new experimental apparatus has been designed and realized.It involves an ultra-high vacuum system, magnetic field sources, laser systems and optical instruments which are required for cooling, trapping and repumping strontium atoms.A novel feature is the implementation of a transversely loaded two-dimensional magneto-optical trap, which serves as a source of a cold strontium atomic beam.A loading rate of the atomic beam into a nearby three-dimensional magneto-optical trap exceeding 10 5 s -1 for the 88 Sr isotope at a moderate dispenser temperature of 600 • C are reported.The temperature of the cloud is about 4 mK and its size 0.3 mm, these characteristics suggest that the state of the atomic trap is in optimal conditions to make the transfer to the optical magneto trap in the intercombination line and thus continue with the second stage of the project.
This dissertation describes the development of a suite of interlinked techniques for precision metrology with matter wave lattices realized in a ceiling-to-floor non-magnetic atom trapping apparatus. This work involves the development of a single-state atom interferometer that relies on the observation of density gratings formed in the atomic ground state of a laser-cooled sample. These gratings, the result of Kapitza-Dirac diffraction of momentum states following a standing wave optical excitation pulse, typically dephase according to their velocity distribution, and motional properties of the sample, including the local value of gravitational acceleration g, are imprinted on the grating contrast and phase. These features can be extracted by coherently backscattering a travelling wave readout field applied within the thermal coherence time of the sample and measuring the grating free-induction decay. Alternatively, a grating echo, resulting from the application of two standing wave pulses separated by t=T_{21}, can be detected using the same backscattering method in the vicinity of the echo time t=2T_{21} on time scales defined by the transit of cold atoms out of the standing wave beam volume. This dissertation presents five main experimental results that impact a new generation of cold atom quantum sensors. Firstly, we investigate a technique for measuring the magnetic moments and sublevel distributions of laser-cooled gases. Secondly, we study optical channell-ing which can be used to increase the scattering efficiency of optical lattices and the sensitivity of grating-echo interferometers. Thirdly, we describe Bragg scattering of a readout pulse from optical lattices and develop a technique to optimize this process in dilute gas lattices, leading to increased signal strengths in echo interferometers. Fourthly, we demonstrate a novel "Ramsey" velocimeter which can measure sample center-of-mass velocity with a precision of 600 µm/s. Finally, we present a measurement of g which represents the first realization of a frequency domain echo-type gravimeter, with a sensitivity to g of 2 parts per million. We outline the basis for improving the precision of these sensors to realize devices capable of measuring g with a sensitivity of 10 parts per billion and probing velocities more precisely than all other atomic velocimeters.
Atom interferometers measure the relative phase shifts between coherent matter-wave paths that arise from interactions with external fields or inertial forces. Due to their exceptional phase sensitivity, atom interferometers became an essential tool for precision measurements and fundamental physics experiments, finding applications in geodesy, gravimetry, and inertial navigation. However, their measurement precision is limited by quantum projection noise, which arises from the Heisenberg uncertainty principle, preventing the measurement of atomic states with absolute precision. The generation of entanglement between the atoms offers a path to surpass this so-called standard quantum limit, thereby enhancing the interferometer’s phase sensitivity beyond classical measurement bounds. This thesis reports on the development of an atom interferometer experiment designed to realize cavity-mediated, squeezed Mach-Zehnder-type interferometry with ultra-cold 87Rb atoms. The experiment combines cavity-aided spin-squeezing with cavity-mediated Mach-Zehnder interferometry to demonstrate entanglement-enhanced phase sensitivity. The experiment is centered on a triangular optical cavity that mediates all relevant atom-light interactions. The cavity provides optical trapping, spin-squeezing, and Raman beam-splitter operations, enabling to perform interferometry on a continuously trapped atomic ensemble. The thesis elaborates on the fundamental theoretical framework, the cavity design, and the full optical setup, including the detailed configuration of the developed laser stabilization methods. Experimentally, continuous loading methods were explored, resulting in an accumulation of up to 4 × 106 atoms in the dipole trap within a cycle time of 500 ms. The AC Stark shift compensation method developed for continuous loading was further applied for in-trap cooling to 10 µK, and optical pumping for efficient atomic state preparation. Coherent state control was verified via observation of microwave-driven Rabi oscillations, and used to characterize atom-cavity coupling. These presented results establish the experimental groundwork for the future development of cavity-mediated, entanglement-enhanced Mach-Zehnder-type atom interferometry.