Phase-matched few-cycle high-harmonic generation: ionisation gating and half-cycle cutoffs

For the direct exploration of electron dynamics in molecules, e.g. during a chemical reaction, a short pulsed radiation source is required, delivering flashes of duration less than a femtosecond. Due to their wavelengths conventional laser pulses cannot be shortened enough to reach such pulse durations. High-harmonic generation (HHG) is currently the key to the subfemtosecond regime with wavelengths in the extreme-ultraviolet and soft-X-ray range. HHG is a very inefficient process and, therefore, the radiation produced by every atom involved has to be phase-matched to obtain a macroscopic signal. The intrinsic characteristics of phase matching provide the possibility to produce even single attosecond pulses. A simulation will show, how phase matching acts as temporal gate and allows HHG only at the leading-edge of the driving laser pulse. The behaviour of the leading-edge gating will be analysed for different experimental conditions, such as peak intensity of the laser pulse, density of the gaseous generation medium and the distance between focus and generation region. Half-cycle cutoff (HCO) analysis allows an experimental access to observing the leading-edge gate, that will be compared to the simulation. The HCO-analysis can also be used to estimate the duration of the driving laser pulse. In addition the position- and pressure dependence of the HHG process will be analysed, too

Measuring test mass acceleration noise in space-based gravitational wave astronomy

The basic constituent of interferometric gravitational wave detectors -- the test mass to test mass interferometric link -- behaves as a differential dynamometer measuring effective differential forces, comprising an integrated measure of gravity curvature, inertial effects, as well as non-gravitational spurious forces. This last contribution is going to be characterised by the LISA Pathfinder mission, a technology precursor of future space-borne detectors like eLISA. Changing the perspective from displacement to acceleration can benefit the data analysis of LISA Pathfinder and future detectors. The response in differential acceleration to gravitational waves is derived for a space-based detector's interferometric link. The acceleration formalism can also be integrated into time delay interferometry by building up the unequal-arm Michelson differential acceleration combination. The differential acceleration is nominally insensitive to the system free evolution dominating the slow displacement dynamics of low-frequency detectors. Working with acceleration also provides an effective way to subtract measured signals acting as systematics, including the actuation forces. Because of the strong similarity with the equations of motion, the optimal subtraction of systematic signals, known within some amplitude and time shift, with the focus on measuring the noise provides an effective way to solve the problem and marginalise over nuisance parameters. The $\mathcal{F}$-statistic, in widespread use throughout the gravitation waves community, is included in the method and suitably generalised to marginalise over linear parameters and noise at the same time. The method is applied to LPF simulator data and, thanks to its generality, can also be applied to the data reduction and analysis of future gravitational wave detectors.Comment: 10 pages, 3 figures, 1 tabl

The ReaxFF reactive force-field : development, applications and future directions

The reactive force-field (ReaxFF) interatomic potential is a powerful computational tool for exploring, developing and optimizing material properties. Methods based on the principles of quantum mechanics (QM), while offering valuable theoretical guidance at the electronic level, are often too computationally intense for simulations that consider the full dynamic evolution of a system. Alternatively, empirical interatomic potentials that are based on classical principles require significantly fewer computational resources, which enables simulations to better describe dynamic processes over longer timeframes and on larger scales. Such methods, however, typically require a predefined connectivity between atoms, precluding simulations that involve reactive events. The ReaxFF method was developed to help bridge this gap. Approaching the gap from the classical side, ReaxFF casts the empirical interatomic potential within a bond-order formalism, thus implicitly describing chemical bonding without expensive QM calculations. This article provides an overview of the development, application, and future directions of the ReaxFF method

Quantum key distribution beyond the repeaterless secret key capacity

Quantum communications promise to provide information theoretic security in the exchange of information. However, unlike their classical counterpart, they utilise dim optical pulses whose amplification is prohibited. Consequently, their transmission rate and range is confined below a theoretical limit known as repeaterless secret key capacity. Overcoming this limit with today’s technology was believed to be impossible until the recent proposal of Twin-field (TF) quantum key distribution (QKD), a scheme that uses phase-coherent optical signals and an auxiliary measuring station to distribute quantum information. Here, TF-QKD and its main variations are initially explored and compared in simulations, to assess their performance in different attributes. Such schemes are also practically implemented for the first time in two experiments. The first is a proof-of-principle implementation over significant channel losses, in excess of 90 dB. In the second, the setup is developed further and the protocol is implemented over real fibre channels exceeding 600 km in length, representing the first fibre-based secure quantum communication beyond the barriers of 600 km and 100 dB. In both cases, in the high loss/distance regime, the resulting secure key rates exceed the repeaterless secret key capacity, a result never achieved before.EPSRC, Toshiba Research Europ

Provably Secure and Practical Quantum Key Distribution over 307 km of Optical Fibre

Proposed in 1984, quantum key distribution (QKD) allows two users to exchange provably secure keys via a potentially insecure quantum channel. Since then, QKD has attracted much attention and significant progress has been made in both theory and practice. On the application front, however, the operating distance of practical fibre-based QKD systems is limited to about 150 km, which is mainly due to the high background noise produced by commonly used semiconductor single-photon detectors (SPDs) and the stringent demand on the minimum classical- post-processing (CPP) block size. Here, we present a compact and autonomous QKD system that is capable of distributing provably-secure cryptographic key over 307 km of ultra-low-loss optical fibre (51.9 dB loss). The system is based on a recently developed standard semiconductor (inGaAs) SPDs with record low background noise and a novel efficient finite-key security analysis for QKD. This demonstrates the feasibility of practical long-distance QKD based on standard fibre optic telecom components.Comment: 6+7 pages, 3 figure

Development of a femtosecond field synthesizer for ultrafast science

In this thesis, I present several new technologies aimed at improving ultrafast spectroscopy at attosecond timescales. The focus of the work was on femtosecond field synthesis, but work on attosecond streaking spectroscopy is also presented. We used CEP few-cycle near-infrared laser pulses to generate XUV isolated attosecond pulses for attosecond streaking spectroscopy on solid surfaces. We characterised the few-cycle pulses using a SEA-F-SPIDER and constructed a combined Second Harmonic/Transient Grating FROG to permit the characterisation of pulses from the UV to the short wavelength infrared. The isolated attosecond pulses were applied, with the few-cycle pulses, to the measurement of the photoemission time delay from gold and silver surfaces. We studied the effect of the few-cycle pulse's Gouy phase on streaking measurements and found that, for our system, the effect was larger than previous reports suggested. We then developed and implemented a bi-material target for surface streaking. This system minimised the systematic error resulting from Gouy phase shifts. We then used the system to measure the photoemission delay between the two materials, finding that gold valence photoelectrons were delayed by 171 ± 49 as relative to silver valence photoelectrons. We constructed a three-colour field synthesizer. The purpose of this system was to generate sub-cycle shaped pulses that could optimise attosecond pulse generation. The output of a CEP stable NIR amplifier was split with one channel being the CEP-stable few-cycle NIR pulse. The other two channels were a 40 fs duration, short wavelength infrared pulse centred at 1300 nm, generated using a commercial OPA system, and a 46 fs duration UV pulse centred at 405 nm generated by second-harmonic generation. The relative stability of each channel and was measured to be 0.179 rad for the second harmonic relative to the few-cycle pulse. For the infrared pulse relative to the few-cycle pulse, the stability was 13.2 rad, which was larger due to the phase instability in commercial OPA. The spatial quality of the beams was measured and found to be suitable for the driving HHG. Two of the three pulses in the synthesizer were used to generate attosecond pulses at 90 eV photon energy with a two-fold increase in flux compared to the single colour case. Synthesized-field waveform-dependent shifts in the cutoff were observed in the HHG spectrum. The resulting attosecond pulses were characterised using gas-phase attosecond streaking, showing that their duration was minimally affected by the presence of the weaker field due to spectral filtering by a multilayer mirror. From the streaking trace, we were able to accurately retrieve the spectrum of the multi-cycle pulse.Open Acces

LISA technology and instrumentation

This article reviews the present status of the technology and instrumentation for the joint ESA/NASA gravitational wave detector LISA. It briefly describes the measurement principle and the mission architecture including the resulting sensitivity before focussing on a description of the main payload items, such as the interferomtric measurement system, comprising the optical system with the optical bench and the telescope, the laser system, and the phase measurement system; and the disturbance reduction system with the inertial sensor, the charge control system, and the micropropulsion system. The article touches upon the requirements for the different subsystems that need to be fulfilled to obtain the overall sensitivity.Comment: 37 pages, 18 figures, submitted to CQ