続紙 １ ) 京都大学 博 士(理 学) 氏名 柴田 康介 論文題目 High-sensitivity in situ imaging of atoms in an optical lattice with narrow optical transitions (狭線幅光学遷移を用いた光格子中の原子の高感度その場イメージング) (論文内容の要旨)Development of laser cooling techniques in recent decades has enabled us to study neutral atoms in a quite controlled way.Cold neutral atoms have been used as a platform to study intriguing quantum many-body phenomena such as Bose-Einstein condensation and Bardeen-Cooper-Schriefer-Bose-Einstein condensation crossover.The excellent controllability of a cold atom system allows us precise measurements with unprecedented accuracy, which provides wide applications, such as an atomic clock.Another important application of cold atoms is quantum information processing.Internal degrees of freedom of a trapped cold atom can be regarded as a well-defined quantum bit (qubit) and quantum gate operations are realized by moderate internal or external interaction.In such cold atom research, a probing method plays an important role.In fact, one of the advantages of cold atom research is the ability to "see" atoms directly by taking images of the atoms via resonant or near-resonant light to the atoms.Among various developments in probing techniques, high-sensitivity detection is of particular importance, especially in the study of a simple system of a few atoms.While high-sensitivity detection and imaging were demonstrated with alkaline metals, there has been increasing interest on a different kind of atomic species.One attractive atomic species is ytterbium.Ytterbium is an alkaline-earth-like-metal atom and has several unique features, such as the rich variety of isotopes of two fermions and five bosons, and the existence of long-lived metastable electronic states.These features offer various possibilities in quantum many-body physics research and are also advantageous to quantum information processing as well as precision measurement.Therefore, a high-sensitivity imaging of ytterbium atoms will bring about a significant development in cold atom research.Under these backgrounds, this thesis reports on the successful demonstration of highsensitivity in situ imaging of ytterbium atoms in a two-dimensional optical lattice.The applicant demonstrated fluorescence imaging of ytterbium atoms in an optical lattice using optical molasses with a narrow optical transition of the wavelength 556 nm ("green molasses"), providing the Doppler cooling limit temperature low enough compared to the typical optical lattice depths.The applicant successfully observed a spatial modulation of fluorescence intensity with a period of approximately 6 μm as a result of the interference between the lattice period of 266 nm and the period of the standing-wave optical molasses of 278 nm.This pattern indicates that the temperature was sufficiently lower than the lattice potential so that the atoms emitted fluorescent photons around the bottom of the lattice sites
続紙 １ ) 京都大学 博 士(理 学) 氏名 柴田 康介 論文題目 High-sensitivity in situ imaging of atoms in an optical lattice with narrow optical transitions (狭線幅光学遷移を用いた光格子中の原子の高感度その場イメージング) (論文内容の要旨)Development of laser cooling techniques in recent decades has enabled us to study neutral atoms in a quite controlled way.Cold neutral atoms have been used as a platform to study intriguing quantum many-body phenomena such as Bose-Einstein condensation and Bardeen-Cooper-Schriefer-Bose-Einstein condensation crossover.The excellent controllability of a cold atom system allows us precise measurements with unprecedented accuracy, which provides wide applications, such as an atomic clock.Another important application of cold atoms is quantum information processing.Internal degrees of freedom of a trapped cold atom can be regarded as a well-defined quantum bit (qubit) and quantum gate operations are realized by moderate internal or external interaction.In such cold atom research, a probing method plays an important role.In fact, one of the advantages of cold atom research is the ability to "see" atoms directly by taking images of the atoms via resonant or near-resonant light to the atoms.Among various developments in probing techniques, high-sensitivity detection is of particular importance, especially in the study of a simple system of a few atoms.While high-sensitivity detection and imaging were demonstrated with alkaline metals, there has been increasing interest on a different kind of atomic species.One attractive atomic species is ytterbium.Ytterbium is an alkaline-earth-like-metal atom and has several unique features, such as the rich variety of isotopes of two fermions and five bosons, and the existence of long-lived metastable electronic states.These features offer various possibilities in quantum many-body physics research and are also advantageous to quantum information processing as well as precision measurement.Therefore, a high-sensitivity imaging of ytterbium atoms will bring about a significant development in cold atom research.Under these backgrounds, this thesis reports on the successful demonstration of highsensitivity in situ imaging of ytterbium atoms in a two-dimensional optical lattice.The applicant demonstrated fluorescence imaging of ytterbium atoms in an optical lattice using optical molasses with a narrow optical transition of the wavelength 556 nm ("green molasses"), providing the Doppler cooling limit temperature low enough compared to the typical optical lattice depths.The applicant successfully observed a spatial modulation of fluorescence intensity with a period of approximately 6 μm as a result of the interference between the lattice period of 266 nm and the period of the standing-wave optical molasses of 278 nm.This pattern indicates that the temperature was sufficiently lower than the lattice potential so that the atoms emitted fluorescent photons around the bottom of the lattice sites
We developed a dual molasses technique which enabled us to perform high-sensitivity in situ fluorescence imaging of ytterbium (Yb) atoms in a two-dimensional optical lattice prepared in a thin glass cell. This technique successfully combines two different kinds of optical molasses for Yb atoms, that is, the one using the 1S0–1P1 transition which provides high-resolution in the in situ fluorescence imaging and the other using the 1S0–3P1 transition for cooling the atoms in the optical lattice. We performed in situ imaging of 174Yb atoms and could observe a Moiré pattern with a period of about 6 µm produced by the molasses beam with 556 nm and the optical lattice with 532 nm, which implies that the temperature was kept below the lattice depth during the fluorescence imaging. The number of photons per atom is estimated to be enough for single atom detection with our imaging system. This result is quite promising for the realization of an Yb quantum gas microscope.
We demonstrate site-resolved imaging of individual bosonic 174Yb atoms in aHubbard-regime twodimensional optical lattice with a short lattice constant of 266 nm. To suppress the heating by probe light with the 1S0- 1P1 transition of the wavelength λ = 399 nmfor high-resolution imaging and preserve atoms at the same lattice sites during the fluorescence imaging, we simultaneously cool atoms by additionally applying narrow-line optical molasses with the 1S0 3P1 transition of the wavelength λ = 556 nm.We achieve a lowtemperature of T = 7.4(13) μK, corresponding to amean oscillationquantum number along the horizontal axes of 0.22(4) during the imaging process.Wedetect, on average, 200 fluorescence photons from a single atom within a 400 ms exposure time, and estimate a detection fidelity of 87(2)%. The realization of a quantum gas microscope with enough fidelity for Yb atoms in a Hubbard-regime optical lattice opens up the possibilities for studying various kinds of quantum many-body systems such as Bose and Fermi gases, and their mixtures, and also long-rangeinteracting systems such as Rydberg states.
In this thesis I report a microscope system for fluorescence imaging of ultra-cold ytterbium atoms trapped in a two-dimensional optical lattice with single-site resolution.Having a rich variety of isotopes, ytterbium atoms trapped in two-dimensional optical lattices are a promising tool to study many-body quantum phenomena resulting from strongly interacting systems.In particular, quantum simulators using fermionic atoms are useful for studying the Fermi-Hubbard model, which is expected to be the key to elucidate the mechanism of high temperature superconductors.In order to observe atoms trapped in an optical lattice, a large number of photons is required to obtain a well resolved image.In contrast with to the conventional method of lasercooling the atoms while observing their fluorescence, in this experiment a deep potential was created using a combination of a shallow ground-state and a deep excited-state potentials, which confines the heated atoms during the imaging process.The resulting quantum gas microscope was able to resolve individual lattice sites in an optical lattice with a 544 nm spacing.And I'm not happy with all the analyses that go with just the classical theory, because nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical, and by golly it's a wonderful problem, because it doesn't look so easy.
Engineering controllable, strongly interacting many-body quantum systems is at the frontier of quantum simulation and quantum information processing. Arrays of laser-cooled neutral atoms in optical tweezers have emerged as a promising platform because of their flexibility and the potential for strong interactions via Rydberg states. Existing neutral atom array experiments utilize alkali atoms, but alkaline-earth atoms offer many advantages in terms of coherence and control, and also open the door to new applications in precision measurement and time keeping. In this Letter, we present a technique to trap individual alkaline-earth-like ytterbium (Yb) atoms in optical tweezer arrays. The narrow ^{1}S_{0}-^{3}P_{1} intercombination line is used for both cooling and imaging in a magic-wavelength optical tweezer at 532 nm. The low Doppler temperature allows for imaging near the saturation intensity, resulting in a very high atom detection fidelity. We demonstrate the imaging fidelity concretely by observing rare (<1 in 10^{4} images) spontaneous quantum jumps into and out of a metastable state. We also demonstrate stochastic loading of atoms into a two-dimensional, 144-site tweezer array. This platform will enable advances in quantum information processing, quantum simulation, and precision measurement. The demonstrated narrow-line Doppler imaging may also be applied in tweezer arrays or quantum gas microscopes using other atoms with similar transitions, such as erbium and dysprosium.
Ultra-cold atoms have become indispensable in quantum sensing, quantum computing, and quantum communication applications, as well as in fundamental sciences. In quantum sensing, they enable precision measurements with outstanding sensitivities that can be of great benefit in navigation, geodesy, and fundamental physics applications. The sensitivity of the atom interferometers used herefore typically scales with the square of the interrogation time. Therefore, in atom interferometric measurements, one seeks ensembles that feature a particularly low internal kinetic energy and can thus be detected after a long time-of-flight. The lowest values are achieved using Bose-Einstein condensates in conjunction with delta-kick collimation, a technique for the collimation of the atomic wave function. Bose-Einstein condensation can be achieved using a multi-stage laser cooling scheme, followed by evaporative cooling, which finally brings the temperature down to the critical temperature, typically in the nanokelvin regime. In addition to the low thermal spread, microgravity enables particularly long observation times, as it avoids limitations of the interrogation time due to vacuum chamber constraints, which are present when falling freely under the influence of gravity. Additionally, microgravity enables measurements that are not possible under normal gravity conditions, such as buoyancy-free miscibility studies, truly uniform potentials, and advanced cold atom quantum memories, which are projected to feature unprecedented time-bandwidth products. For the preparation of a Bose-Einstein condensate, two techniques are widely used in science, namely magnetic traps and optical dipole traps. For microgravity missions, magnetic traps on atom chips have been used in nearly all setups so far due to their particularly low SWAP budget and high cold atom flux. In contrast, ground-based experiments often prefer optical traps due to their higher versatility, full optical access, and the absence of surfaces in proximity to the atoms, which is fundamental for precision measurements. Additionally, optical traps enable the manipulation of intra- and inter-species scattering lengths using Feshbach resonances. Compared to atom chip traps, they also feature an intrinsically better symmetry of the trapping potential, in theory leading to fewer aberrations in matter-wave optics. The latter could enable the generation of ensembles in the yet unreached femtokelvin regime using delta-kick collimation with fewer perturbations. This thesis is about the realization of a 1064nm optical dipole trap set up for the microgravity environment of the Bremen Drop Tower. It aims to highlight the advantages of optical traps in conjunction with microgravity and prepare optical trapping technology for use in future microgravity and space missions. The thesis describes the experimental setup, the direct loading process, the characterization of the optical potential, and the optimization and investigation of the evaporative cooling in the optical dipole trap. For the evaporation process, the influence of gravity is comprehensively investigated through analytical calculations and microgravity tests. Bose-Einstein condensation is demonstrated, proving the achievement of condensation through absorption imaging of the spatial distribution, tests on the free evolution of the ensemble, and investigation of the phase-space densities achieved. Up to (74 ± 4)% condensation of an ensemble featuring (30 000 ± 13 000) atoms at a kinetic temperature of (37 ± 3) nK is demonstrated. The evaporation shows high efficiency down to a 3 s evaporation time, while a critical phase-space density can be achieved within (1.29 ± 0.07) s using an even faster evaporation ramp. This opens the door to fundamental physics experiments on Bose-Einstein condensates in microgravity, as well as applications pursuing atomic ensembles with exceptionally low expansion velocities.
Imagine a new generation of computers that dramatically surpasses the most powerful supercomputers, being able to tackle the most challenging of computational problems. These problems arise, for example, in quantum physics where the computational power required to model a many-particle system increases exponentially with the number of its constituents, and as a consequence the calculations become prohibitively time-consuming. Nobel laureate Richard Feynman proposed this new type of computer in the early 1980s: ‘Let the computer itself be built of quantum mechanical elements which obey quantum mechanical laws.’ Such a quantum simulator would allow researchers to tackle challenging problems in condensed-matter physics. In particular many-body fermionic quantum systems are very difficult to compute due to the antisymmetric nature of the wave function, and the resulting sign problem for quantum Monte Carlo methods. This is why a quantum simulator will be an excellent test bed to investigate phenomena and properties of strongly correlated fermionic quantum systems. A powerful experimental platform to realize a quantum simulator is laser-cooled atoms [1] held in an artificial crystal of light, generated by superimposing laser beams. The atoms are kept in this optical lattice similar to marbles in the hollows of an egg carton, and they can mimic the behaviour of electrons in a real crystal. These systems interface atomic physics with condensed matter physics in a new way, as the tools of atomic physics allow for exquisite control of many experimental parameters, which are often inaccessible in solid state experiments. In-situ imaging techniques [2] using ‘quantum-gas microscopes’ constitute a new approach to the study of many-body quantum systems in an optical lattice with unprecedented resolution at the single-particle and single-site level [3,4]. This new method is complementary to earlier optical lattice experiments measuring distributions in momentum space after a time-of-flight expansion. A quantum-gas microscope uses an optical imaging system (Fig. 1a) for collecting the fluorescence light of ultracold atoms in an optical lattice with high spatial resolution comparable to the lattice spacing of ≈ 500 nm. (a) Atoms in optical lattice atoms are imaged with a high-resolution microscope objective [4]. (b) Single-site addressing is achieved by focussing a laser beam through the microscope onto selected atoms, bringing them into resonance with microwave radiation which produces spin flips [12]. (c) and (d) Single-atom-resolved fluorescence images of bosonic Mottinsulators [4,9]. (e) Deterministically prepared atom distribution using single-site addressing [12]. (f) Direct observation of single-atom quantum walks [15]. Fluorescence imaging of atoms in a quantum-gas microscope has made it possible to directly observe bosonic Mott insulators with single-atom resolution [4–6], giving access to in-situ measurements of temperature and entropy distributions. Subsequently, ground-breaking experiments in many-body quantum dynamics have been performed, such as the first direct observation of correlation spreading after quenches of the system to lower lattice depths [7]. Other experiments include the quantum simulation of an antiferromagnetic spin-1/2 chain [8], the realization of algorithmic cooling [9], the direct measurement of correlations across the superfluid-to-Mott-insulator transition [10], or the observation of the ‘Higgs’ amplitude mode [11]. The high-resolution imaging system is also capable of optically addressing individual atoms with sub-diffraction-limited resolution. For this purpose, a laser beam is focussed though the microscope objective onto selected lattice sites, bringing the atoms into resonance with microwave radiation which produces spin flips [12,13]. The preparation of arbitrary atom distributions in the lattice not only opens perspectives for neutral-atom quantum information processing, but also allows the deterministic creation of specific out-of-equilibrium situations, and their ensuing system dynamics can be studied with single-atom resolution. Examples include the creation of spin impurities and the study of their dynamics in a many-body system [14], the observation of quantum walks [15] and magnon-bound states [16], and the measurement of entanglement entropy [17]. The technique used for single-site-resolved detection of individual atoms in the optical lattice is fluorescence imaging. It requires laser cooling of the atoms to sub-Doppler temperatures while detecting the fluorescence photons emitted during this process. A challenge is to achieve a low particle loss rate and to prevent the atoms from hopping between lattice sites. In the case of the first quantum-gas microscopes, this has been implemented with bosonic rubidium-87 atoms, and also recently for ytterbium-174. However, it has proven more challenging for fermionic alkaline species such as potassium-40 and lithium-6, as their low mass and small excited state hyperfine splitting-prevented standard sub-Doppler cooling from working. As a result, it took several years to extend the quantum-gas microscope technique to fermionic species, and in 2015 several groups have demonstrated single-atom-resolved imaging of fermions [18]. In these setups, the direct observation of band insulators [19], fermionic Mott insulators [20,21] and even antiferromagnetic correlations [22] has been reported most recently. These developments provide fascinating opportunities to study many-body fermionic quantum systems with single-particle access. The fermionic nature of the electron is vital to understanding a range of phenomena within solid-state physics, such as electron pairing in superconductivity, and quantum magnetism. In this context, the quantum simulator will allow, e.g. for direct measurement of ordered quantum phases and out-of-equilibrium dynamics, with access to quantities such as spin--spin correlation functions and the growth of many-body entanglement. The high-resolution imaging systems could also be used to project tailored time-varying light fields onto the atoms, allowing the removal of high-entropy regions by locally modifying the confining potential to attain very low-entropy states. Attaining these low-entropy many-particle states is one of the key challenges in our field. In summary, quantum-gas microscopes offer unprecedented access to many-body quantum systems, and they will allow us to gain an even deeper insight into the intriguing world of quantum physics that governs the properties of strongly correlated materials. Using a well-controlled quantum system to simulate other quantum systems could help resolve some of the most challenging problems scientists face in condensed matter physics and materials science, but it will also provide new insights in quantum chemistry, high-energy physics and biology. SK acknowledges funding from the EU (ERC FERMILATT, SIQS, QuProCs) and SUPA.