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.
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.
Atom interferometer measurements are affected by the amount of quantum noise from the laser used to detect atoms. To improve the quantum limited sensitivity of interferometers, there needs to be a way to reduce the amount of quantum noise. Light is composed of two quadratures, where the product of the noise in each quadrature cannot be below a minimum threshold set by the Heisenberg Uncertainty Principle. However, the noise in one quadrature can be reduced at the expense of the other. This form of light is referred to as “squeezed light.” Squeezed light can be produced using four-wave mixing (FWM). Electromagnetically induced transparency (EIT) is an interference phenomenon that occurs when a three-level atom is driven by a coherent field that makes a non-linear medium transparent to the probing field and holds a great deal of similarities to FWM. This thesis developed a theoretical framework that describes the measurement of atomic states and associated noise when quantum light is used to drive the atom and lays the groundwork to produce squeezed light through developing an experiment to produce EIT, as well as discussing the similarities between EIT and FWM to create and further study FWM for improved atom interferometry.
A potential application of squeezed-state light is to improve the sensitivity of high-precision interferometers whose performance is generally limited by photon shot noise or photon-counting statistics. Such interferometers include passive laser gyroscopes and the very sensitive interferometers being developed to detect cosmic gravitational waves. One usually operates a high-precision interferometer in a differential mode, so that noise from the (laser) light source is in principle irrelevant. Operation in a differential mode means that there is a second, normally unused input port. Vacuum noise entering this second port is responsible for the irreducible photon shot noise at the interferometer’s output. Injecting squeezed-state light into the normally unused input port can improve the sensitivity. I discuss the use of squeezed-state light in ring interferometers, delayline Michelson interferometers, and Fabry-Perot interferometers. Included in the discussion are real degrading effects, such as losses, inefficient photodetectors, and laser noise in the presence of nonideal fringe contrast. I explore the compatibility of the squeezed-state technique with other exotic techniques for improving sensitivity, such as Drever's idea for power recycling. Sensitivity estimates for various configurations and techniques are presented.
Exploiting the quantum nature of atoms through the use of matter-wave interferometry has lead to the development of devices sensitive to, among other things, the local gravitational acceleration. Measurements of the gravitational acceleration have applications ranging from detection of subterranean density di erences to observation of general relativistic e ects. Light-pulse atom interferometers provide an absolute measurement of the gravitational acceleration with a sensitivity competitive and even surpassing the performance of classical sensors. The advantages of atom interferometers as inertial measurement devices have lead to e orts in increasing sensitivity and decreasing the physical dimensions of the measurement head, allowing them to be more transportable for eld use applications. With increasing sensitivity, it becomes more critical to address the noise source limitations a ecting the measurement, speci cally the in uence of ground motion on measurements. Vibrations coupling into the inertial reference add a time varying phase shift uncommon to the paths of the interferometer. This limitation is especially pertinent to transportable atom gravimeters measuring in the eld where inertial noise is typically far higher than in relatively quiet laboratory environments. Within this work, implementation and demonstration of inertial noise post-correction in an atom interferometer is shown within low inertial noise environments and simulated strong motion environments. For a high pulse separation time atom interferometer (T = 78 ms) post-correction yielded an increase in the short term stability from 4.4 x 10 ̄6 m/s2/√Hz to 9.2 x 10 ̄7 m/s2/√Hz. This method was reproduced with a di erent motion sensor to perform post-correction in a high motion environment, generated by introducing additional movement onto the inertial reference. By performing post-correction in the high motion environment, we were able to show an increase of short term stability of γ = 73.8. Current limitations to the post-correction resulted from self noise resolution limitations and spectral resolutions limitations. Beyond corrections with commercially available sensors, this work demonstrates the rst post-correction with a next generation compact optomechanical sensor. This optomechanical sensor is formed from monolithic fused silica capable of sensitively measuring accelerations of a harmonic oscillator test mass, which can be read-out optically. This novel motion sensor has the advantage to any previously shown sensor used for post-correction in the capacity that it is vacuum compatible, insensitive to magnetic elds and has the potential to be implemented directly into the inertial reference. In this work, inertial noise post-correction in a simulated high motion environment is shown, correcting from a short term stability of 8 x 10 ̄3 m/s2/√Hz to 5 x 10 ̄4 m/s2/√Hz. Post-correction was limited by the parasitic cavities within the ber required for optical read-out of the harmonic oscillator displacement and intensity noise uctuations of the source laser. The results shown within this work are congruent with previous works on atom chip gravimeters, both of which help move towards portable hand-held gravimeter measurement heads capable of sensitive inertial measurement.
Quantum Sensors, like atom interferometers (AI), can be employed for high-precision measurements of inertial forces, including their application as gravimeters, gradiometers, accelerometers, and gyroscopes. Their measurement principle relies on ultracold atoms that are prepared in quantum-mechanical superposition states in external degrees of freedom. These states can be prepared by a momentum transfer of a Raman laser. Then the superposition state senses the effect of an inertial force, which induce a corresponding relative phase. The phase is read out by a final coupling which converts the interferometric phase into a atom number difference between the two states. The difference provides an estimate of the interferometric phase and the corresponding quantity of interest. The quantum mechanical noise of the atomic ensemble cause a fundamental uncertainty of this estimation, which I analyze for generic AIs. For small atomic densities, the quantum phase noise of the ensemble limits the interferometric sensitivity. For large densities, quantum number fluctuations generate density fluctuations, which generates phase noise. I show that these two competing effects result in an optimal atom number with a maximal interferometer resolution. Squeezed atomic samples allow for a reduction of the quantum noise of one quantity at the expense of an increased noise along of a conjugate quantity. Phase and number are such quantities which obey to a variant of Heisenberg’s uncertainty principle. Neither phase nor number squeezing can improve the maximal interferometer resolution. As one main result of this thesis, I show how an optimal squeezing in between number and phase squeezing, allows for a fundamental improvement. I evaluate possible experimental paths to implement the proposed protocol. Concepts for a squeezing-enhanced operation of external-degree AIs have not yet been demonstrated. I propose and implement an atomic gravimeter, which is designed to accept spin-squeezed atomic states as input states. The interferometer is designed such that the interferometer couplings are performed in spin space, while the phase accumulation is performed in momentum states. For this interferometer, the squeezed input can be directly obtained from spin dynamics in spinor Bose-Einstein condensates. The main noise contributions in the experiment are analyzed, which results in a factor of 84 above the relevant quantum limit, preventing a squeezing enhancement so far. I outline a suppression of the main noise source, uncontrolled AC Stark shift on the squeezed mode and propose future important applications, including test of spontaneous collapse theories and an improvement of large-scale, high-precision gradiometers.
Quantum sensing using atom interferometry possesses immense potential for application in gravimetry, inertial navigation and for gravitational wave detection, apart from carrying out tests of fundamental physics. Atom interferometry can be used to perform absolute inertial measurements with unprecedented long term stability, unlike classical sensors which are prone to drift and incur calibration errors. However, atom interferometers seldom achieve their fundamental sensitivity due to the coupling of ambient vibrations. It is the dominant noise source that not only impairs performance but also affects its operability. Ideal compensation of the effects due to vibration noise using a versatile accelerometer, while concurrently enabling miniaturized atom interferometer setups has not been pioneered. Such a compact hybrid quantum sensor will enable reliable operation with an enhanced performance in field applications, paving the way for robust atom interferometry beyond laboratory conditions. A novel optomechanical resonator has been designed, developed and successfully hybridized with an atom interferometer to suppress the effects of vibration noise, and the results are presented in this thesis. The optomechanical resonator’s inertial test mass is simultaneously used to create a continuous intracavity optical lattice to sense its displacement, and a pulsed optical lattice to manipulate the atoms. For the first time, a broadband accelerometer is seamlessly hybridized with an atom interferometer via mechanical resonator mediated coupling of the optical lattices to measure acceleration. This enables elimination of vibration induced measurement ambiguity in our atom interferometer with a well defined mechanical transfer function. Vibration noise correlation is demonstrated up to our prototype atom interferometer’s most sensitive operating mode of T = 6 ms using double Bragg diffraction, ultimately limited by its technical noise. The optomechanical resonator measures acceleration with a sensitivity down to 1.1×10−6 ms−2/ √Hz corresponding to a displacement sensitivity of 8.6 × 10−15 m/ √Hz. Substantial 1/f noise suppression due to the developed optical readout scheme enables a measurement bandwidth spanning from sub-hertz frequencies up to several kilohertz. Multiple versions of the optomechanical resonator with different characteristics were developed and hybridized with our atom interferometer. This paves the way for developing tailored optomechanical resonators for specific inertial sensing applications.
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.