This thesis presents the investigations of Rubidium atoms in magneto-optical traps and triply charged Thorium ions in electrodynamic traps for future advances in long-distance quantum telecommunication, next generation clocks, and fundamental tests of current physical theories. Experimental realizations of two core building blocks of a quantum repeater are described: a multiplexed quantum memory and a telecom interface for long-lived quantum memories. A color change of single-photon level light fields by several hundred nanometers in an optically thick cold gas is demonstrated, while preserving quantum entanglement with a remotely stored matter excitation. These are essential elements for long-distance quantum telecommunication, fundamental tests of quantum mechanics, and applications in secure communication and computation. The first trapping and laser cooling of Thorium-229 ions are described. Thorium-229 nuclear electric quadrupole moment is revealed by hyperfine spectroscopy of triply charged Thorium-229 ions. A system to search for the isomer nuclear transition in Thorium-229 is developed and tested with the excitation of a forbidden electronic transition at 717 nm. Direct excitation of the nuclear transition with laser light would allow for an extremely accurate clock and a sensitive test bed for variations of fundamental physical constants, including the fine structure constant.
This thesis presents the investigations of Rubidium atoms in magneto-optical traps and triply charged Thorium ions in electrodynamic traps for future advances in long-distance quantum telecommunication, next generation clocks, and fundamental tests of current physical theories. Experimental realizations of two core building blocks of a quantum repeater are described: a multiplexed quantum memory and a telecom interface for long-lived quantum memories. A color change of single-photon level light fields by several hundred nanometers in an optically thick cold gas is demonstrated, while preserving quantum entanglement with a remotely stored matter excitation. These are essential elements for long-distance quantum telecommunication, fundamental tests of quantum mechanics, and applications in secure communication and computation. The first trapping and laser cooling of Thorium-229 ions are described. Thorium-229 nuclear electric quadrupole moment is revealed by hyperfine spectroscopy of triply charged Thorium-229 ions. A system to search for the isomer nuclear transition in Thorium-229 is developed and tested with the excitation of a forbidden electronic transition at 717 nm. Direct excitation of the nuclear transition with laser light would allow for an extremely accurate clock and a sensitive test bed for variations of fundamental physical constants, including the fine structure constant.
Trapped cold ions are tools which we used to approach two very disparate areas of physics, strong coupling between Ba+ ions and optical resonators, and investigations of a low-energy nuclear isomer of 229-Th. The first part of this thesis describes our progress towards the integration of a miniature Paul (rf) ion trap with a high finesse (F=30000) optical cavity. Ba+ ions were trapped and cooled for long periods and a new scheme for isotope selective photoionization was developed. The second part of this thesis describes our progress towards controlled excitation of the low energy nuclear isomer of 229-Th, which may provide a bridge between the techniques of cold atomic and nuclear physics. As a step towards this goal, 232-Th3+ ions were confined in rf ions traps and cooled via collisions with a buffer gas of helium. A sophisticated scanning program was developed for controlling ion trap loading, tuning lasers, and running a CCD camera to look for fluorescence. The low-lying electronic transitions of Th3+ at 984 nm, 690 nm and 1087 nm were observed via laser fluorescence.
The laser-accessible nuclear transition in the thorium-229 isotope has been identified as a promising candidate for the realization of an optical nuclear clock. Such a nuclear clock might rival or outperform current optical clocks based on electron-shell transitions in atoms or ions, is expected to be more robust against external perturbations, and provides enhanced sensitivity in clock-based tests of fundamental principles of physics. Here, we implement a thorium-229 nuclear clock by stabilizing a continuous-wave laser to the 148 nm nuclear transition with rapid feedback based on continuous absorption spectroscopy. The thorium-229 nuclei are embedded into a millimeter-sized, room temperature calcium fluoride crystal. A subharmonic of the 148 nm radiation is continuously compared to a Yb+ single-ion clock. The nuclear clock shows a simple shot-noise limited scaling of the fractional frequency instability of 3⋅10−12τ/s where τ is the averaging time, approaching 10−15 instabilities over 1 day of continuous operation. Improvements of the instability by several orders of magnitude can be projected for future solid-state nuclear clocks. We use the nuclear clock to constrain models of ultralight dark matter by searching for periodic fluctuations and slow drifts in the nuclear transition energy, on time scales between 20 s and 1 day. Drawing benefit from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force and quarks.
Atoms cooled and trapped using laser light have become the backbone of modern quantum physics research. Various new applications based on laser-cooled atoms are in active development. The work described in this dissertation is dedicated to this endeavor. I present experimental and numerical investigations on several fundamental properties of the laser-cooled atoms in different quantum states and how they may translate into applications within several emerging atom-based technologies. First, I describe the precision measurements of the dynamic polarizability and photo-ionization cross-section at 1064-nm light of the 5D3/2 state in rubidium, which is of interest for the portable atomic clocks. Two different spectroscopic methods are implemented to extract the polarizability from the experimental data. The results are statistically equivalent: -524(17) [-499(59)] atomic units from the first [second] method. The photo-ionization cross section of the 5D3/2 state at 1064~nm is measured to be 44(1) Mb. The aforementioned photo-ionization cross section is suitable for efficient ion generation and the subsequent application in cold-atom-based ion sources. This is demonstrated in the second study presented here. The ion source employs a novel method to monitor the electric microfields of the ions using the Stark effect of the highly-excited (Rydberg) atoms seeded within the origin of the ion source. Rydberg atoms are extremely sensitive to electric fields and serve as an embedded real-time sensor and tracker of the ion-source performance because of their quasicontinuous creation and detection. The principles of such ion-microfield sensing are discussed and analyzed in detail and can be utilized in any cold-atom-based focused-ion-beam source. The hyperfine structure of the 4D3/2 state in rubidium is investigated next. The measurement utilizes two-photon optical absorption spectroscopy and a detailed analysis of light shifts. All four hyperfine components are resolved in the experimental data. The magnetic-dipole and electric-quadrupole hyperfine constants are obtained to be 7.419(45) MHz and 4.19(19) MHz, respectively. Future applications of the 4D3/2 state in optical atomic clocks and Rydberg-atom physics are discussed. Next, I investigate a Rydberg-atom-ion molecule bound by long-range multipolar forces. The molecule is the first example of a bound state between a Rydberg atom and an ion. Strong interactions between these two particles lead to large molecular binding energy (reaching a few GHz) and large binding length (exceeding 1 mum). The Born-Oppenheimer approximation does not hold for the Rydberg-atom-ion molecule because the motion of the Rydberg-atom nucleus is on the same timescale as the dynamics of the ion. The Born-Huang theory is utilized to investigate the resultant nonadiabatic dynamics of the molecule and to make the first fully quantum predictions of its lifetime as a function of the vibrational state and quantum principal number of the Rydberg atom. Lastly, I present a novel technique for atom interferometry that employs uninterrupted three-dimensional confinement and manipulation during the interferometric protocol. The atoms in the existing apparatuses have to follow several-meter-long trajectories along which various detrimental effects such as wave-packet dispersion erase useful phase information. ``Tractor atom interferometry" presented here promises to suppress the harmful effects and improve the interferometers’ sensitivity; provide flexible control over the atomic trajectories; and significantly reduce the experimental footprint. The suitability of this approach for measuring background acceleration (such as due to the gravitational field) is demonstrated. The sensitivity estimations, experimental considerations, and initial laboratory work are discussed.
The realization of a large-scale quantum network of interconnected quantum nodes is one of the most promising directions in quantum technology. Such a network distributes entanglement between remote matter qubits via single photons in optical fibers, enabling quantum repeaters and long-distance secure communication and supporting emerging applications such as distributed quantum computing and network-enhanced metrology. To realize this vision, proposed architectures require quantum nodes that provide an efficient quantum light-matter interface for entanglement generation, serve as a quantum memory allowing long-lived storage of quantum states, and have the possibility to connect to telecommunication fiber-based photonic channels. This thesis presents the development and experimental realization of a quantum network node based on a single Rb-87 atom. A single atom is confined in a tightly focused optical dipole trap, and entanglement is generated between the polarization of an emitted photon and the Zeeman state of the atom. To enable long-lived quantum storage, we coherently map the atomic qubit to a basis less sensitive to magnetic fields using Zeeman-state-selective Raman transfer, reducing magnetic field sensitivity by a factor of 500. Such hyperfine qubit control is implemented via a two-photon Raman process driven by a laser field that is amplitude modulated at the qubit splitting using a fiber-based Mach-Zehnder interferometer. With this approach, we achieve a 1/e storage lifetime of 10 ms, extending coherence by a factor of 30 compared with the initial qubit encoding. Residual dephasing arises primarily from position-dependent effective magnetic fields caused by the tightly focused optical dipole trap beam together with atomic motion, and the differential AC-Stark shifts come from the trap detuning. While the first can be fully canceled via a standing-wave dipole trap, which needs to change the trap setup. We mitigate the second effects with a spin-echo pulse, extending coherence by a factor of two. Nonetheless, the significantly longer coherence time already allows high-fidelity entanglement distribution over fiber links with a length of a few hundred kilometers. To reduce fiber attenuation for long-distance distribution, we convert the emitted 780 nm single photons to the low-loss telecom S band using difference-frequency generation in a periodically poled lithium niobate waveguide, pumped at 1607 nm to produce 1517 nm photons. By combining long-lived quantum memory with a telecom interface, we demonstrated long-distance atom-photon entanglement distribution through 50 km and 101 km of spooled fiber with fidelities of 85% and 71%, respectively, establishing a key component for scalable quantum communication. These results establish neutral atom nodes as promising building blocks for metropolitan-scale quantum networking and outline a scalable path toward repeater protocols based on multiplexing memories and Rydberg-mediated interactions.
Quantum mechanics is leading to many modern technologies, e.g. based on laser physics. Emerging fields are for example nonlinear optics, hybrid systems, or quantum communication. Despite these applications, there are still limitations of our understanding of quantum mechanics. Even though it is broadly accepted that decoherence limits the size of entangled states, we currently do not know how to overcome these limitations. How large can commonly entangled states be? Can we observe macroscopic quantum phenomena with coarse-grained detectors or even with our bare eyes? This thesis contributes towards answering these fundamental questions and to technological advances. The experiments are based on a light-matter platform of quantum optical states and atomic ensembles. The ensembles consist of rare-earth ion-doped crystals. Additionally, the atomic frequency comb protocol turns the ensemble into a quantum memory. In the scope of quantum communication we work on a quantum repeater. First, we investigate fundamental quantum mechanical states and their validity at macroscopic scales. We show that the detection of a collectively emitted excitation can be used to measure the size of the entangled ensemble. The entangled ensemble shared this excitation in a coherent superposition. This superposition of an atomic excitation in an ensemble is called W-state. We certify a new lower bound to the possible size of an entangled W-state between at least 16 million atoms. The proof of entanglement of this size relies on two measures: one determines the number of atoms involved in the storage process. The other one determines the state "quantumness" by examining the re-emitted light. Another experiment studies a macroscopically distinguishable superposition state. The superposition survives the storage in an atomic ensemble. Second, we perform two more experiments that contribute to a quantum repeater. One experiment probes the quantum states simultaneously stored in a multi-mode quantum memory. In this work we also develop the concept of indirect entanglement witness. For the other experiment, a frequency down-conversion platform is set up. The conversion overcomes the wavelength compatibility issue between some quantum memories and the telecommunication C-band. The narrow spectral filtering of the conversion performs better than expected. In summary, our work analyzes large entangled states and building blocks for a quantum repeater. The presented quantum memories are capable of temporal multi-mode storage. Its temporally precise re-emission is due to the re-phasing of the atoms of the W-state. We also show a good way to set up frequency down-conversion for the sake of quantum repeaters. This work emphasizes the atomic frequency comb protocol as a viable and reliable platform for quantum repeaters.
The work presented in this thesis investigates theoretically and experimentally quantum memories with atomic ensembles, which are suited for long-distance quantum communication. For this purpose laser-cooled, ultracold atomic clouds of Rubidium 87 are used as memory for nonclassical single-photon states. Base on the atom-light interface of the protocol from Duan, Lukin, Cirac, Zoller (DLCZ) for long distance quantum communication the single photons states are stored and retrieved. By the help of non-classical correlations and the quantum memory a deterministic single photon source is demonstrated. The Hong-Ou-Mandel dip between two single photons created in this way has been measured. Moreover it is reported on the realization of a long-lived quantum memory by using magnetic-field insensitive atomic states in an optical trap and that by extending the spinwave's wavelength the dephasing of the memory due to thermal motion could be suppressed. Measured storage times of over a millisecond represent a significant progress towards the realization of long distance quantum communication.