Quantum information networks will facilitate the use of a broad range of quantum technologies including quantum computers, quantum sensors, and quantum communication systems, among others. An important component of these networks are devices that enable light-atom interactions, as they interface photons from communication channels with the matter-based nodes, allowing for processing of photonic quantum information. These devices will need to be scalable and easily integrated to expand the size of networks. One platform of interest is vapour-filled hollow-core fibres (HCF). By overlapping light and atomic vapour within the same volume, strong interactions can occur, including important nonlinear effects such as cross-phase modulation and wave mixing processes. HCFs can be directly integrated into fibre systems, making them a convenient technology for the expansion of quantum networks. This thesis reports on three projects themed around scalable technologies for quantum networks. The first project involved the injection-locking of a diode laser. Nanosecond long pulses had their 20 GHz frequency chirps reduced to near the Fourier-transform limit, enabling them to interact efficiently with rubidium vapour. This project demonstrated that the mature, cost effective, and scalable technology of diode lasers can be compatible with essential atomic ensemble quantum networking devices such as quantum memories and quantum frequency converters. In the second project, a high-bandwidth quantum memory protocol called off-resonant cascaded absorption (ORCA) was investigated. The ORCA memory can be used to synchronise high speed photonic operations that occur in optical quantum information processing. This project used a Kagom´e HCF to enhance the light-atom interactions, resulting in two orders of magnitude reduction in optical power requirements. The third project again used the HCF platform, this time to facilitate a quantum frequency conversion scheme that converts rubidium-compatible, near-IR wavelengths to the telecommunications C-band, which is ideal for long distance fibre transmission. This project demonstrated the FWM process has reduced optical pump powers and higher efficiency in the HCF when compared to a rubidium-filled cell. The research described in this thesis presents a strong argument for the use of HCFs as a scalable technology that could be implemented in quantum networks.
Quantum information networks will facilitate the use of a broad range of quantum technologies including quantum computers, quantum sensors, and quantum communication systems, among others. An important component of these networks are devices that enable light-atom interactions, as they interface photons from communication channels with the matter-based nodes, allowing for processing of photonic quantum information. These devices will need to be scalable and easily integrated to expand the size of networks. One platform of interest is vapour-filled hollow-core fibres (HCF). By overlapping light and atomic vapour within the same volume, strong interactions can occur, including important nonlinear effects such as cross-phase modulation and wave mixing processes. HCFs can be directly integrated into fibre systems, making them a convenient technology for the expansion of quantum networks. This thesis reports on three projects themed around scalable technologies for quantum networks. The first project involved the injection-locking of a diode laser. Nanosecond long pulses had their 20 GHz frequency chirps reduced to near the Fourier-transform limit, enabling them to interact efficiently with rubidium vapour. This project demonstrated that the mature, cost effective, and scalable technology of diode lasers can be compatible with essential atomic ensemble quantum networking devices such as quantum memories and quantum frequency converters. In the second project, a high-bandwidth quantum memory protocol called off-resonant cascaded absorption (ORCA) was investigated. The ORCA memory can be used to synchronise high speed photonic operations that occur in optical quantum information processing. This project used a Kagom´e HCF to enhance the light-atom interactions, resulting in two orders of magnitude reduction in optical power requirements. The third project again used the HCF platform, this time to facilitate a quantum frequency conversion scheme that converts rubidium-compatible, near-IR wavelengths to the telecommunications C-band, which is ideal for long distance fibre transmission. This project demonstrated the FWM process has reduced optical pump powers and higher efficiency in the HCF when compared to a rubidium-filled cell. The research described in this thesis presents a strong argument for the use of HCFs as a scalable technology that could be implemented in quantum networks.
Light is an ideal information carrier for quantum networks: its quantum properties are not degraded by noise in ambient conditions and it has a large information capacity owing to its high bandwidth. However, quantum technologies based on photonic networks have been hampered by the exponentially poor scaling of the underlying probabilistic quantum operations. Quantum optical memories, devices that store, manipulate, and release on- demand quantum light, have been identified as an indispensable network component, because they facilitate scalability. Noise-free operation of the memory is critical, since even small additional noise can render the memory classical by destroying the quantum character of the light. Here I introduce a new broadband quantum memory protocol - the off-resonant cas- caded absorption (ORCA) memory - that is inherently noise-free and operates in ambient conditions. I model ORCA theoretically, after which I characterise the classical perfor- mance of a proof-of-concept implementation in warm caesium vapour, using weak near- infrared coherent states. In order to verify quantum operation of the ORCA memory, I interface it with a broadband heralded single-photon source. I measure and compare the quantum statistics of the stored and retrieved light, observing for the first time their full preservation in a room-temperature atomic quantum memory. Finally, I make headway into integrating the ORCA vapour memory by investigating an alkali-filled hollow-core fibre platform. I use light-induced atomic desorption (LIAD) to demonstrate record- breaking alkali vapour densities in fibre, a prerequisite for efficient memory operation. Because of its technical simplicity and integrability, its high bandwidth and its low noise, ORCA provides a viable route towards next generation, photonic quantum network technologies.
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.
[Truncated] This thesis concerns a new photonic technology based on hollow-core photonic-crystal fibre (HC-PCF) that has been loaded with an atomic vapour. It was demonstrated that this technology may have important applications in the fields of metrology and quantum optics. Furthermore, the small size and robustness of HC-PCF allows these devices to potentially be miniaturised for possible commercial and industrial applications. By filling the HC-PCF with an atomic vapour, excellent light-atom coupling can be attained as both the atoms and light are confined within the same volume. The small transverse dimensions of the fibre’s optical mode leads to high intensities at low input powers, enabling efficient driving of non-linear transitions which would otherwise be difficult with free-space optical systems. Additionally, long interaction lengths, obtained through an arbitrary length of HC-PCF, lead to large optical depths on these typically weak non-linear transitions. The non-linear transition employed here is the 5S1/2 → 5D5/2 rubidium (Rb) two-photon transition. This system is of great interest due to its non-linearity, long excited state lifetime, efficient excitation and large optical depth attained within the HC-PCF. A number of methods for loading a HC-PCF with Rb are discussed, along with proposed ideas for making this process more efficient. Spectroscopic techniques were used to investigate the effect of the fibre’s confined geometry upon the atomic spectra. Extensive theoretical modelling was employed to confirm the observed spectra and give insight to the mechanisms behind the observations. Using this experimental and theoretical knowledge, two applications were targeted: optical atomic frequency standards; and cross phase modulation for quantum logic gates.
We present a low-loss, compact, hollow core optical fibre (HCF) cell integrated with single mode fibre (SMF). The cell is designed to be filled with atomic vapour and used as a component in photonic quantum technologies, with applications in quantum memory and optical switching. We achieve a total insertion loss of 0.6(2) dB at 780 nm wavelength via graded index fibre to ensure efficient mode matching coupled with anti-reflection coatings to minimise loss at the SMF-HCF interfaces. We also present numerical modelling of these interfaces, which can be undertaken efficiently without the need for finite element simulation. We encapsulate the HCF core by coupling to the SMF inside a support capillary, enhancing durability and facilitating seamless integration into existing fibre platforms.
Having a strong interaction between hot alkali vapour and light is important for Rydberg physics and nonlinear optics. In optical fibres, the tight confinement by the core offers high laser intensities for even moderate powers. With the invention of hollow-core photonic crystal fibres (HC-PCFs), the interaction of alkali atoms with high intensities over long distances could be studied. In the past, setups combining alkali atoms and a HC-PCF have suffered from long waiting times until atoms are evenly distributed along the fibre. The large size of the used vacuum chambers entailed technical challenges when trying to heat the system uniformly. In this thesis, an all-glass vapour cell incorporating a hollow-core fibre is presented. Its compact size allows fast and precise control of the temperature. Measurements have shown that this cell drastically reduces the time required for equilibration to only hours. Furthermore, the cell’s all-glass design permits unobstructed optical access from all sides. The optical access can be used to record the fluorescence emitted by atoms along the inside of the fibre core. In a first experiment, the frequency-dependent fluorescence along the fibre position was monitored while alkali vapour was diffusing out of the fibre. The atomic density was successfully retrieved using a numerical model based on the optical Bloch equations. Promising technical improvements of the cell design using fibre tapers are discussed, which could lead to an all-fibre vapour cell with fibre-integrated light delivery. The high laser intensity inside a HC-PCF can easily exceed the linear spectroscopy regime. Staying below the saturation intensity inside a waveguide might not always be easy—or even desired. However, established numerical software packages, such as ElecSus, are limited to the weak-probe regime. For that reason, an open-source software based on the Lindblad master equation was developed in this work. The software allows predicting the absolute value of the electric susceptibility for many experimental parameters of hot alkali vapours, including the isotope ratio, temperature or magnetic field strength. Most distinctly, it includes the laser intensity and effects such as optical pumping and transit-time broadening. To verify the model accuracy, spectra of 87Rb-filled cells were measured in a magnetic field of 0.6 T. Entering the hyperfine Paschen-Back regime, individual atomic resonances can be addressed, allowing further benchmarking of the model based on open and closed transitions. Comparing the experimental data with simulated spectra, very good agreement for three different beam diameters and for laser intensities over a range of more than five orders of magnitude was found. Finally, Rydberg states of rubidium—with their multitude of possible transitions—are discussed as a possible source of light in the terahertz (THz) region. In contrast to some other existing sources, THz from hot atomic vapour is expected to be very narrowband. In one report, the successful generation of THz in a four-level-system in hot rubidium vapour was observed. Here, a very similar setup was employed, but unfortunately no THz emission was detected. It is unclear if the reason is of fundamental or experimental nature. One potential route to enhance the THz signal strength at the detector is to channel the signal as much as possible by guiding it inside a waveguide. Appropriate waveguides were investigated, which in contrast to optical frequencies use polymers as a material. Using polymers, waveguides using the 3D printing technique were fabricated and guidance of THz light was observed.
Coherent manipulation and interaction between atoms and light are the core ingredients in modern quantum science and technology. Various applications such as quantum computation, quantum memories and quantum sensors which are promising in future technologies, are currently under exploration. However, due to the diffraction nature of light, miniaturizing quantum systems involving atom-light interactions has been highly limited. To avoid diffraction, I load cold atoms into a hollow-core fiber (HCF) and demonstrate four major experiments inside the fiber: inertia-sensitive Mach-Zehnder atom interferometer, long-lived quantum spin coherence, light storage and creation of an array of Schr\"odinger cat states. In this thesis, I show the theoretical framework of each experiment and demonstrate experimentally a cold atoms quantum laboratory realized a HCF. Cold 85Rb atoms of temperature ∼\SI{6}{\micro\kelvin} are loaded into a Kagome type photonic crystal hollow-core fiber (HCF) through an optical trapping beam coupled into the HCF. I first demonstrate an inertia-sensitive Mach-Zehnder atom interferometer using two counter-propagating Raman beams. However, the short quantum spin coherence time of ∼\SI{100}{\micro\second} due to the differential AC stark shift from the trapping beam is a major obstacle to reaching high sensitivity of the optically trapped atom interferometer inside the HCF. By introducing a vector light shift from the dipole trap to cancel the differential scalar light shift, the quantum spin coherence time is increased to hundreds of milliseconds. The long quantum spin coherence time opens up possibilities for various quantum experiments in HCF. Here, I demonstrate light storage using atoms as the storage medium in the HCF. A maximum storage efficiency of \SI{3}{\percent} and storage time of over \SI{20}{\milli\second} is shown in this thesis. With the implementation of dynamical decoupling, the storage time is improved to ∼\SI{100}{\milli\second}. Moreover, I create a 1D array of Schr\"odinger cats states in facilitated by an optical lattice in the HCF and study its potential for applications in quantum sensing and quantum simulation in HCF. Both odd and even cat states are validated and their coherence properties are studied. This thesis realized the state-of-the-art quantum experiments that are miniaturized into a HCF. The results signify a promise in future quantum sciences and technologies with compact fiber-based devices.