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.
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.
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.
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.
This thesis presents the development of photonic microcells for use as the host for coherent optics phenomena and related applications. A photonic microcell consists of a length of hollow-core photonic crystal fibre (HC-PCF) with a gas-filled core that is spliced to conventional optical fibre at either end to seal the gas within the fibre. Towards the goal of demonstrating and assessing the coherence properties of quantum optical effects in photonic microcells, the fabrication of two types of HC-PCF is presented. The established photonic bandgap HC-PCF offers extremely low transmission loss of ~10 dB/km over kilometre distances. However, the fibre has a limited transmission bandwidth of ~50 THz and exhibits modal coupling unfavourable for many applications. Work is presented on the tailoring of this fibre by control and shaping of the core-surround in order to improve its modal properties. A second type of HC-PCF is based on a large-pitch lattice, whose guidance relies on a new mechanism. This fibre exhibits a much improved bandwidth (>1000 THz) and has a relatively higher but still practical loss of ~1 dB/m. The development of photonic microcells at microbar pressure level and with low optical insertion loss is shown, an important step in the improvement of the technology for coherent optics applications which will take advantage of the extreme gas-laser interaction efficiency achieved in HC-PCF. Finally, quantum optical effects are demonstrated in HC-PCF and photonic microcells loaded with both the molecular gas acetylene and atomic vapour rubidium. The observation of electromagnetically induced transparency (EIT) in acetylene-filled HC-PCF represents the first such observation in a molecular gas, while the use of a photonic microcell allows a comparison of many experimental configurations to explore the coherence properties of coherent optical systems in the core of a HC-PCF. Furthermore, EIT is observed unambiguously in a rubidium loaded HC-PCF for the first time, and the anti-relaxation effects of a polymer coating demonstrated in this configuration.
A fast, low-loss optical switch would be a powerful tool for photonic quantum technologies, allowing the implementation of, for example, rapid quantum logic gates, loop memories, and improved heralded single photon sources. Integration into optical fibre promises an avenue to future scalability, but existing optical switches cannot achieve both high speed and high efficiency. Here I present two routes to fibre-integrated, low -loss and fast optical switching. The first uses the acousto-optic effect in a tapered fibre. We demonstrate that light in the tapered region of the fibre can be coupled between optical modes by an acoustic wave, which introduces a phase-shift. The second route is by a two-photon transition in rubidium vapour. A strong control field modulates the mode of a weak signal field. We demonstrate this effect with low loss (1dB) and on short timescales (100MHz) in bulk optics, and work towards a fibre-integrated vapour cell.
This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.
This thesis describes developments in atomic vapour loading in hollow core photonic crystal fibre (HC-PCF) for fabrication of atomic vapour loaded photonic microcells (PMC). These developments have been targeted at addressing some of the issues associated with loading atomic vapours in confined waveguiding geometries such as increased dephasing and physio-chemical wall absorptions. Atomic vapour loaded HC-PCF and PMC’s have applications in laser metrology, coherent optics and magneto optical spectroscopy. State of the art HC-PCF have been fabricated for loading with atomic vapour including both photonic bandgap (PBG) guiding and inhibited coupling (IC) hypocycloidal core shape Kagome HC-PCF. Record loss of 70 dB/km has been achieved in IC hypocycloid core shape Kagome HC-PCF in the spectral region centred at 800 nm. This fibre retains excellent single mode propagation combined with large core and increased optical bandwidth in comparison with specialist PBG HC-PCF optimised for operation around 800 nm. Aluminosilicate sol-gel coatings have been developed and successfully applied to the inner core wall of HC-PCF’s to reduce the atomic vapour surface interaction. Confining atomic vapours in micron scaled HC-PCF results in increased dephasing rates because of the frequent atom wall collisions. Anti relaxation coating materials have been applied to the inner core wall and the longitudinal relaxation time has been measured in coated and uncoated fibres utilising a magneto optical technique. Additionally sub Doppler transparencies are investigated in anti relaxation coated and uncoated HC-PCF.