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arXiv:2208.00736 · Optics Letters

Zeeman optical pumping of 87 Rb atoms in a hollow-core photonic crystal fiber

Tomasz Krehlik, Artur Stabrawa, R. Gartman, K. T. Kaczmarek, Robert Löw, Adam M. Wojciechowski

Published Oct 13, 2022• 6 citations
arXivHTMLPDF

Abstract

Preparation of an atomic ensemble in a particular Zeeman state is a critical step of many protocols for implementing quantum sensors and quantum memories. These devices can also benefit from optical fiber integration. In this work we describe experimental results supported by a theoretical model of single-beam optical pumping of 87 Rb atoms within a hollow-core photonic crystal fiber. The observed 50% population increase in the pumped F = 2, m F = 2 Zeeman substate along with the depopulation of remaining Zeeman substates enabled us to achieve a threefold improvement in the relative population of the m F = 2 substate within the F = 2 manifold, with 60% of the F = 2 population residing in the m F = 2 dark sublevel. Based on theoretical model, we propose methods to further improve the pumping efficiency in alkali-filled hollow-core fibers.

DOI
10.1364/ol.471091

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•Jan 1, 2021•0 citations
Preparation of an atomic ensemble in a particular Zeeman state is a critical step of many protocols for implementing quantum sensors and quantum memories. These devices can also benefit from optical fiber integration. In this work we describe experimental results supported by a theoretical model of single-beam optical pumping of 87 Rb atoms within a hollow-core photonic crystal fiber. The observed 50% population increase in the pumped F = 2, m F = 2 Zeeman substate along with the depopulation of remaining Zeeman substates enabled us to achieve a threefold improvement in the relative population of the m F = 2 substate within the F = 2 manifold, with 60% of the F = 2 population residing in the m F = 2 dark sublevel. Based on theoretical model, we propose methods to further improve the pumping efficiency in alkali-filled hollow-core fibers.
We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of  30~30 30,000 rubidium atoms, which creates a medium with an optical depth of  180~180 180 inside the fiber. Compared to our earlier study [1] this represents a sixfold increase in the maximum achieved optical depth in this system.
We present an experimental realization of a noise-free and high-bandwidth quantum memory scheme using a rubidium vapor that is confined within the hollow core of a photonic crystal fiber. We achieve the same internal efficiencies as similar free-space experiments (30%) for 4.5-ns-long optical pulses but with a 100-fold reduction in the control-field power required. Modeling indicates that this efficiency could be improved to 88% with higher control powers and the implementation of techniques such as light-induced atomic desorption to increase the optical depth. The compactness, robustness, and low drive power of this approach lends itself to direct integration into large-scale fiber-based quantum processors.
We theoretically investigate the process of coupling cold atoms into the core of a hollow-core photonic-crystal optical fiber using a blue-detuned Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam to couple the atoms, the blue-detuned hollow beam can confine cold atoms to the darkest regions of the beam, thereby minimizing shifts in the internal states and making the guide highly robust to heating effects. This single optical beam is used as both a funnel and a guide to maximize the number of atoms into the fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical trap (MOT) above a vertically oriented optical fiber. We observe a gravito-optical trapping effect for atoms with high orbital momentum around the trap axis, which prevents atoms from coupling to the fiber: these atoms lack the kinetic energy to escape the potential and are thus trapped in the laser funnel indefinitely. We find that by reducing the dipolar force to the point at which the trapping effect just vanishes, it is possible to optimize the coupling of atoms into the fiber. Our simulations predict that by using a low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a 20-$\ensuremath{\mu}$m-radius core hollow fiber, it is possible to couple 11% of the atoms from a MOT 9 mm away from the fiber. When the MOT is positioned farther away, coupling efficiencies over 50% can be achieved with larger core fibers.
The generation of large multiphoton quantum states—for applications in computing, metrology and simulation—requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light–matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90 ± 1% of atoms are prepared in the ground state. We measure high optical depths (3 × 10 4 ) and narrow homogeneous linewidths (6 ± 2 MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Λ-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects.
Cold atoms trapped and guided in hollow-core photonic crystal fibers provide a scalable diffraction-free setting for atom–light interactions for quantum technologies. However, due to the mismatch of the depth and spatial extension of the trapping potential from free space to the fiber, the number of cold atoms in the fiber is mainly determined by the loading process from free space to waveguide confinement. Here, we provide a numerical study of the loading dynamics of cold atoms into a hollow-core photonic crystal fiber. We use the Monte Carlo method to simulate the trajectories of an ensemble of cold atoms from free space trapping potential to optical potential inside a hollow-core fiber and calculate the temperature, loading efficiency, and geometry of the ensemble. We also study the noise sources that cause heating and a loss of atoms during the process. Our result could be used to design and optimize the loading process of cold atoms into a hollow-core fiber for cold atom experiments.
Non-classical states of light, such as squeezed states, enable a wide variety of scientific and engineering applications in optical communication, precision measurements and quantum computing. Therefore, engineering non-classical states of light in different systems for different purposes is interesting and appealing. Non-classical states of light, particularly squeezed states, can be generated from light in a coherent state or vacuum state by using certain optical nonlinear interactions. To efficiently introduce such optical nonlinear interactions, a high nonlinear coefficient of the medium and high intensity of light in the medium are the key factors, where the hollow core photonic crystal fibers filled with atomic vapor shows their great benefits. In this thesis, we report on our approach to apply hollow core photonic crystal fibers (HCPCFs) with the goal to generate squeezed states using strong optical nonlinearities of atomic vapor. In a HCPCF, light with high intensity has a well confined mode over long distances along the fiber. To apply the HCPCF in squeezed state generation, we experimentally investigated unexpected noise properties of the HCPCF and determined the parameter range to reduce excess noise from the fiber. Furthermore, we have filled the hollow core of a piece of HCPCF with rubidium vapor, which provides a high nonlinearity when the confined light is on resonance with the rubidium dipole transition. Fundamentally, our planned approach relies on self induced transparency (SIT), which offers high nonlinearity and absorption-free propagation at the same time. We have theoretically investigated SIT solitons in rubidium vapor to analyze the required parameters to achieve SIT solitons in our rubidium filled HCPCF. The experimental and theoretical approach presented in this thesis suggests an efficient method for generating squeezed light.