Micro-combs: a novel generation of optical sources

The quest towards the integration of ultra-fast, high-precision optical clocks is reflected in the large number of high-impact papers on the topic published in the last few years. This interest has been catalysed by the impact that high-precision optical frequency combs (OFCs) have had on metrology and spectroscopy in the last decade [1–5]. OFCs are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally-spaced spectral lines that represent precise marks in frequency. Their importance was recognised worldwide with the 2005 Nobel Prize being awarded to T.W. Hänsch and J. Hall for their breakthrough in OFC science [5]. They demonstrated that a coherent OFC source with a large spectrum – covering at least one octave – can be stabilised with a self-referenced approach, where the frequency and the phase do not vary and are completely determined by the source physical parameters. These fully stabilised OFCs solved the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Very recent advancements in the fabrication technology of optical micro-cavities [6] are contributing to the development of OFC sources. These efforts may open up the way to realise ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates, in the form of compact and integrated devices. Indeed, the fabrication of high-quality factor (high-Q) micro-resonators, capable of dramatically amplifying the optical field, can be considered a photonics breakthrough that has boosted not only the scientific investigation of OFC sources [7–13] but also of optical sensors and compact light modulators [6,14]

Analysis of laser radiation using the Nonlinear Fourier transform

Modern high-power lasers exhibit a rich diversity of nonlinear dynamics, often featuring nontrivial co-existence of linear dispersive waves and coherent structures. While the classical Fourier method adequately describes extended dispersive waves, the analysis of time-localised and/or non-stationary signals call for more nuanced approaches. Yet, mathematical methods that can be used for simultaneous characterisation of localized and extended fields are not yet well developed. Here, we demonstrate how the Nonlinear Fourier transform (NFT) based on the Zakharov-Shabat spectral problem can be applied as a signal processing tool for representation and analysis of coherent structures embedded into dispersive radiation. We use full-field, real-time experimental measurements of mode-locked pulses to compute the nonlinear pulse spectra. For the classification of lasing regimes, we present the concept of eigenvalue probability distributions. We present two field normalisation approaches, and show the NFT can yield an effective model of the laser radiation under appropriate signal normalisation conditions

Strong field sub-femtosecond electronic processes

This thesis is comprised of theoretical investigations on several different strong field and/or sub-femtosecond processes resulting from the interaction between atoms and short laser pulses. Said theory is based on the numeric solution of the time-dependent Schro\"odinger equation (TDSE) by high performance computing methods. Specifically, Chapter 2 examines the Attoclock, a strong field problem designed to clock the escape of an electron as it tunnel ionises. In which, we present both the result of a collaboration yielding the first agreement between ab initio theory and experiment [Sainadh et al., Nature 568, 75 (2019)], and a straightforward model based on classical scattering for an idealised version of the problem [Bray et al., Phys. Rev. Lett. 121, 123201 (2018)]. Chapter 3 considers reconstruction of attosecond beating by interference of two-colour transitions (RABBITT) in which an attosecond pulse train 'pump' and infrared pulse 'probe' simultaneously impinge on a target with a precisely controlled delay between them. The oscillating phase of the ionisation probability as a function of this delay yields the angular anisotropy parameter and Wigner time delay for its corresponding energy. We calculate and present these quantities for the valence p-shell of various noble gas atoms [Bray et al., Phys. Rev. A 97, 063404 (2018)] and additionally examine the effect of an encapsulating C60 fullerene cage on the 4d shell of Xe [Bray et al., Phys. Rev. A 98, 043427 (2018)]. In Chapter 4 we look at the effect of electron correlation on high harmonic generation (HHG), the process by which attosecond pulses are produced, from one and two colour fields. We perform single active electron calculations for the 5p shell of Xe and model the correlation as an enhancement factor taken as the ratio between photoionisation cross-sections computed with and without said correlations. Doing so we report solid agreement with experimentally observed spectra for both field setups [Bray et al., Phys. Rev. A 100, 013404 (2019)]. Finally, Chapter 5 investigates the non-dipole problem of the state resolved strong field acceleration of neutral species. This requires the solution of the coupled two-body TDSE of the centre of mass and reduced mass electron, each with three degrees of freedom, in a non-spatially uniform field. Accordingly it necessitates its own dedicated solution method. Developing and applying said method to atomic hydrogen we compute an acceleration for each state consistent with experimental observation [Bray et al., Phys. Rev. Lett., Submitted]. Additionally our method allows us, via comparison with classical expressions, to derive the time at which each excited state was produced and, by similar means, an effective polarisibility for the ground state. Most interestingly this latter value is of opposite sign to the typical +9/2, providing an unambiguous signature of having entered the Kramers-Henneberger regime

Laser Technologies for Applications in Quantum Information Science

Scientific progress in experimental physics is inevitably dependent on continuing advances in the underlying technologies. Laser technologies enable controlled coherent and dissipative atom-light interactions and micro-optical technologies allow for the implementation of versatile optical systems not accessible with standard optics. This thesis reports on important advances in both technologies with targeted applications ranging from Rydberg-state mediated quantum simulation and computation with individual atoms in arrays of optical tweezers to high-resolution spectroscopy of highly-charged ions. A wide range of advances in laser technologies are reported: The long-term stability and maintainability of external-cavity diode laser systems is improved significantly by introducing a mechanically adjustable lens mount. Tapered-amplifier modules based on a similar lens mount are developed. The diode laser systems are complemented by digital controllers for laser frequency and intensity stabilisation. The controllers offer a bandwidth of up to 1.25 MHz and a noise performance set by the commercial STEMlab platform. In addition, shot-noise limited photodetectors optimised for intensity stabilisation and Pound-Drever-Hall frequency stabilisation as well as a fiber based detector for beat notes in the MHz-regime are developed. The capabilities of the presented techniques are demonstrated by analysing the performance of a laser system used for laser cooling of Rb85 at a wavelength of 780 nm. A reference laser system is stabilised to a spectroscopic reference provided by modulation transfer spectroscopy. This spectroscopy scheme is analysed finding optimal operation at high modulation indices. A suitable signal is generated with a compact and cost-efficient module. A scheme for laser offset-frequency stabilisation based on an optical phase-locked loop is realised. All frequency locks derived from the reference laser system offer a Lorentzian linewidth of 60 kHz (FWHM) in combination with a long-term stability of 130 kHz peak-to-peak within 10 days. Intensity stabilisation based on acousto-optic modulators in combination with the digital controller allows for real-time intensity control on microsecond time scales complemented by a sample and hold feature with a response time of 150 ns. High demands on the spectral properties of the laser systems are put forward for the coherent excitation of quantum states. In this thesis, the performance of active frequency stabilisation is enhanced by introducing a novel current modulation technique for diode lasers. A flat response from DC to 100 MHz and a phase lag below 90° up to 25 MHz are achieved extending the bandwidth available for laserfrequency stabilisation. Applying this technique in combination with a fast proportional-derivative controller, two laser fields with a relative phase noise of 42 mrad for driving rubidium ground state transitions are realised. A laser system for coherent Rydberg excitation via a two-photon scheme provides light at 780 nm and at 480 nm via frequency-doubling from 960 nm. An output power of 0.6 W at 480 nm from a single-mode optical fiber is obtained . The frequencies of both laser systems are stabilised to a high-finesse reference cavity resulting in a linewidth of 1.02 kHz (FWHM) at 960 nm. Numerical simulations quantify the effect of the finite linewidth on the coherence of Rydberg Rabi-oscillations. A laser system similar to the 480 nm Rydberg system is developed for spectroscopy on highly charged bismuth. Advanced optical technologies are also at the heart of the micro-optical generation of tweezer arrays that offer unprecedented scalability of the system size. By using an optimised lens system in combination with an automatic evaluation routine, a tweezer array with several thousand sites and trap waists below 1 μm is demonstrated. A similar performance is achieved with a microlens array produced in an additive manufacturing process. The microlens design is optimised for the manufacturing process. Furthermore, scattering rates in dipole traps due to suppressed resonant light are analysed proving the feasibility of dipole trap generation using tapered amplifier systems

Allomorphy as a mechanism of post-translational control of enzyme activity

Enzyme regulation is vital for metabolic adaptability in living systems. Fine control of enzyme activity is often delivered through post-translational mechanisms, such as allostery or allokairy. β-phosphoglucomutase (βPGM) from Lactococcus lactis is a phosphoryl transfer enzyme required for complete catabolism of trehalose and maltose, through the isomerisation of β-glucose 1-phosphate to glucose 6-phosphate via β-glucose 1,6-bisphosphate. Surprisingly for a gatekeeper of glycolysis, no fine control mechanism of βPGM has yet been reported. Herein, we describe allomorphy, a post-translational control mechanism of enzyme activity. In βPGM, isomerisation of the K145-P146 peptide bond results in the population of two conformers that have different activities owing to repositioning of the K145 sidechain. In vivo phosphorylating agents, such as fructose 1,6-bisphosphate, generate phosphorylated forms of both conformers, leading to a lag phase in activity until the more active phosphorylated conformer dominates. In contrast, the reaction intermediate β-glucose 1,6-bisphosphate, whose concentration depends on the β-glucose 1-phosphate concentration, couples the conformational switch and the phosphorylation step, resulting in the rapid generation of the more active phosphorylated conformer. In enabling different behaviours for different allomorphic activators, allomorphy allows an organism to maximise its responsiveness to environmental changes while minimising the diversion of valuable metabolites

Self-Induced Transparency Solitons in Nanophotonic Waveguides

This thesis explores the existence and properties of self-induced transparency (SIT) solitons in nanophotonic waveguides. SIT solitons are shape-preserving solutions of the semi-classical Maxwell-Bloch equations, a system of nonlinearly coupled differential equations. In a first investigation, collisions of counterpropagating simultons (SIT solitons in absorbing three-level systems) are studied numerically in the plane-wave approximation and a polarisation- and group-velocity dependent soliton birth is uncovered. Apart from their fundamental interest, such light-light interaction effects may be of use for optical computing applications if they can be transferred to tightly confined light pulses. Confining light is usually achieved by using dielectric waveguides that exhibit group velocity dispersion leading to chirped pulses, which experience absorption when entering an absorbing medium. If the chirp is strong enough and the pulse intense enough, they can even completely invert an absorber. When investigating chirped pulse propagation through a dense ensemble of two-level system it is found that the chirped pulses dynamically reshape into unchirped pulses experiencing transparency. Furthermore, the conditions on the waveguide geometry to enable SIT are analysed, identifying a nanophotonic slot waveguide with a low-index gap, exhibiting high electric field enhancement and a homogeneous field profile, as the ideal candidate system for guided SIT solitons. This analysis is supported by two-dimensional numerical calculations that show the solitary character is maintained during propagation if the absorber density is high enough to ensure a slow-down of the pulse and to thus counteract the waveguide dispersion. Finally, the soliton birth due to simulton collisions and optical memory schemes proposed for plane-wave SIT are investigated in the two-dimensional slot waveguide and found to also be possible in this geometry

Materials Analysis Using a THz Imaging System Based on Atomic Vapour

This thesis studies the response of the interaction between Rydberg atomic vapour and a THz frequency field. When Caesium atoms at room temperature are excited to a Rydberg state using three infrared lasers and a 0.55 THz field resonant with the 14P3/2 → 13D5/2 transition is applied, the atoms respond by emitting a green optical fluorescence corresponding to the 13D5/2 → 6P3/2 decay. This response is exploited to investigate the absorption coefficient for different polymer materials that transmit well in the THz frequency range using the Beer–Lambert law. We calibrate the system to obtain a measure of THz intensity. As the THz imaging system is highly sensitive to environmental changes, and to show that our results are consistent, we provide a comparison of results between our atomic detection method and a commercial thermal power meter. Additionally, we measure the absorption coefficient of the same materials at a frequency of 1.1 THz, and the results are compared with those measured at 0.55 THz. The THz imaging system is also used to perform some experiments in order to demonstrate its effectiveness in real-world applications. The system provides an interesting image contrast in the case of a sample containing two different polymer materials measured at two THz frequencies. The result is a proof-of-concept that multispectral THz imaging can provide additional information and is motivation to improve our THz imaging system by introducing a dual-species THz imager. We also investigate the polarisation spectroscopy of an excited-state transition of rubidium vapour at room temperature as a step towards a rubidium THz imaging system. The narrow dispersive signal produced by this spectroscopy technique is ideal for laser frequency stabilisation of excited-state transitions

Characterisation and control of trapped-ion qubit

Trapped ions are one of the promising platforms that realise a quantum bit, or qubit, in quantum computation. The fundamental quantum operations, such as single- and two-qubit gates, have been demonstrated. However, the fidelity of a quantum gate is easily compromised by inadequate qubit initialisation or incorrect settings of experimental parameters. This thesis aims to address those issues, allowing for the complete coherent control of the trapped ion qubit. This thesis describes the construction and testing of a new ion trap apparatus for optimal control of a trapped ion qubit, which ideally makes the quantum gates more robust against experimental parameters. This thesis extends the two-level Ramsey interferometry to higher order in a trapped ion. Creation and certification of the motional superposition require excellent control of the trapped ion qubit. We prepare a motional superposition state with undesired AC Stark shift due to the off-resonant carrier compensated by our compensation scheme. We successfully certify the superposition consists of three motional Fock states, $(\ket{0}+\ket{1}+\ket{2})/\sqrt{3}$, using a robust certifier derived from statistical moments of the interference pattern. The thesis presents the results of a Bayesian estimator that estimates the Rabi frequency and detuning frequency by processing the measurement records via Bayes' theorem. We compare the estimate from the Bayesian estimator and the standard fitting method, in which Rabi frequency and detuning are estimated by fitting the Rabi oscillation and the frequency spectrum of the ion, and found the Bayesian estimator can estimate those unknown parameters as accurately as the standard fitting method, but only requires less than one-hundredth of measurements necessary for the fitting method. We also experimentally demonstrate measurement-based cooling, which is an alternative way to cool the ion to its ground state. Contrary to resolved sideband cooling, which is routinely used for the ground state cooling in our experiment, this cooling method probabilistically prepares the ion in its motional ground state. We perform a state-dependent mapping operation that maps the ion's atomic state to either $\ket{g}$ or $\ket{e}$ conditioned on its motional states: the ion's atomic state is mapped to $\ket{e}$ if its motional state is $\ket{0}$; otherwise the ion's atomic state is mapped to $\ket{g}$. The following projective measurement of the atomic state of the ion enables the discrimination between the motional ground state and the motional excited states. Therefore we can prepare the ion in the motional ground state by selecting the instances where the ion is measured to be in $\ket{e}$, which heralds the motional ground state. In the near future, the group will begin a project to implement and evaluate recent proposals for making quantum gates robust against a mis-set of frequency using a polychromatic light field.Open Acces