Quantum technology gives the opportunity to open novel scientific and technological possibilities beyond the current physical and conceptual limitations. For example, an entirely new generation of electronic devices, which will allow technology to advance in the post-CMOS era, can be created. These devices will be based on quantum properties of electrons, such as tunnelling through barriers and spin, which will aim to progress in a range of applications from communications, quantum computing and quantum standard for electrical current to a wide spectrum of spintronics and molecular electronics. However, achieving this is challenging and requires developing novel theoretical methods and fabrication processes. This project aims to combine experiments and simulations to develop a suitable theory and methodology for simulating emerging quantum electronic devices. The main object of research in this proposal will be a single electron transistor (SET). In SETs it is possible to control, with very high precision, the electron flow through the device as individual charges. However, there are still numerous scientific and technical challenges to be overcome in order to create reliable and highly accurate SETs. This proposal aims to address some of these challenges and to answer a simple yet fundamental question: how do electrons flow through aggregates of atoms (quantum dots) in the context of a single electron transistor? The 'rules' for quantum transport in molecules and crystals with perfect symmetry are relatively well established and provide direction to the ongoing experimental effort. In contrast, a similar set of underpinning principles for quantum dots related to transport is clearly absent. A guiding principle in my work, which I follow here, is that theory and calculations should be used in synergy with experiments, addressing fundamental issues and providing insight that leads to improvement of the fabrication processes. This project brings together one UK company, the National Physical Laboratory and two research groups in the University of Glasgow to deliver progress in the field of improving the design parameters and performance of SETs.

Quantum mechanics promises to be the driving force underneath the next technological revolution, and quantum cryptography, providing a commercial solution for the ultimate security, may well be considered the harbinger of this change. With this proposal, we aim at showing how quantum mechanics can allow to overcome the limits faced in the detection of long wavelength radiation, specifically at terahertz frequencies. Terahertz is a portion of the electromagnetic spectrum that is both extremely hard to access to, and incredibly important from a technological standpoint. It is indeed a key player in security (explosives, drugs, and hazard-free concealed weapons detection), telecommunications (increased data-rate of short distance wireless communication), monitoring and quality control (spectroscopy). Despite its high potential, the lack of efficient sources and detectors prevents a widespread commercial application of terahertz time-domain spectroscopy and imaging. Yet, a number of high-tech companies are investing into terahertz technology and recent market studies hint for a 40%/year increase in the turnover associated to this technology. It is therefore vital to identify now strategies for overcoming the limitations of the current terahertz detectors. This proposal aims at developing such a strategy exploiting the unique properties of quantum, entangled states of light. Entangled photons, separated in space but sharing a common wavefunction, can be generated by commercial nonlinear crystals and boost unusual properties not accessible by classical means. Two of these properties, namely the ability to acquire twice the phase of a classical state upon propagation and the reduced amplitude noise below the classical shot-noise limit, offer a mean for increasing the sensitivity of terahertz time-domain detectors, that operate indeed as a differential phase sensor. Combining such an improvement with recent concepts of super-resolved imaging will also result in an increased resolution of long wavelength mapping. Combining for the first time concepts of quantum optics, recognised as a main pillar for our future technology, and of terahertz photonics, boosting a number of underdeveloped application potential, this proposal is in line with the research strategy set by the UK research councils, and promise to deliver impact on a number of different disciplines, such as biology and material science, as well as on the quality control and security inspection activities.

Spectral analysis provides a vital technique to fingerprint the vast array of chemicals, materials and biological matter we encounter on a daily basis. It is central to detecting the presence of noxious gasses or explosives, of contaminants in food, and vitally the correct chemical structure of medicines. This fellowship will deliver a new technology outperforming the state of the art infrared detection, based on recent developments in quantum mechanics. Infrared spectroscopy is far from being a well-established technology, mostly due to the limited sensitivity standard detectors have in the infrared part of the spectrum. A limited sensitivity, in turn, corresponds to a limit in the minimum detectable amount of the chemical compound under scrutiny, hindering the deployment of infrared spectroscopy. This fellowship will address such problem combining two recently developed techniques: time-domain spectroscopy and quantum metrology. Time-domain spectroscopy is an approach developed in the last two decades and relies on measuring a signal that arises from the nonlinear interaction between ultrashort pulses and the infrared field under investigation. In contrast to standard infrared spectroscopy, the measured quantity is not at infrared wavelengths but in the visible region, where detectors have better performances. The detection is therefore not bound to the limited sensitivity of infrared sensors. This technique too is affected by a limit in the sensitivity, which arises from the quantised nature of the radiation in the ultrashort probing pulse and is known as the standard quantum limit. In-tempo will transform infrared spectroscopy, harnessing quantum metrology to overcome the standard quantum limit faced by time-domain spectrometers. Quantum optical metrology studies ways to improve the sensitivity of measurements using quantum states of light, instead of conventional fields. Squeezed and NOON states are the main players in this discipline. Squeezed states have a lower quantum noise on one of their properties, such as the amplitude, in exchange for a higher noise in a conjugate characteristic, such as the phase. NOON states are non-classical wave packets acquiring twice the phase of their classical counterparts when used in interferometers. Twin beams are electromagnetic fields featuring intensity correlations at the quantum level, i.e. more equal than any replica obtained by classical means. This fellowship will use squeezed, NOON and twin beam states instead of classic ultrashort pulses in a time-domain spectroscopy approach. This way it will overcome the standard quantum limit in infrared spectroscopy. The new family of infrared-time domain spectrometers generated by this fellowship will be benchmarked against state-of-the-art traditional spectrometers. Potential market impact and routes to commercialisation will be investigated with the support of the engaged industrial partners.

In the last decade, proof of concepts has been given and small-scale demonstrators have been built to show that the quantum devices allow obtaining unprecedented performances in practical applications. For example, dramatic enhancements can be obtained in the speed and computational power of next-generation computers (Quantum computing) using superconducting qubits. Also, disruptive performance improvements can be achieved in advanced imaging, remote sensing, long distance/secure communication (quantum cryptography) or diagnostic techniques using superconducting nanowire single-photon detectors - SNSPDs. The transition from demonstrators to practical scaled-up devices with a large number of elements is still at an early stage and a significant technological leap is required for a real breakthrough in those fields. The identified challenge in scaling-up the number of elements in quantum circuits, that is virtually identical for superconducting qubits and SNSPDs operating in Radio Frequency regime - RF-SNSPDs -, is represented by efficient multiplexing of these elements since they typically operate at cryogenic temperatures and need multiple connections for control and read-out at microwave frequencies. This makes the electronics complex, costly and difficult to scale beyond 10 to 100 of elements in the commercially available cryostats hampering their use in real-world applications. Single Flux Quantum (SFQ) electronics can operate at cryogenic temperature with unrivalled high frequency and ultra-low power consumption relying on the peculiar current to voltage relation of their basic element: the Josephson Junctions (JJ). Under proper condition, JJs generates ~2 ps width voltage pulses at repetition frequency above 500 GHz, with unprecedented time accuracy, stability and low power consumption. SFQ electronics is intrinsically scalable and we propose to use generated SFQ pulses as a source for precise and low noise frequency signals for multiplexed control and read-out of on-chip integrated qubits and RF-SNSPDs arrays. This transformative approach will allow to finally fill the gap in the existing quantum technology for a step-change at the same time in quantum science and advanced sensing applications. At this aim, we will bring together top UK expertise in nanofabrication and superconducting quantum technology, backed by a strong commitment from the UK world-leading company in SFQ electronics and quantum technologies SeeQC UK. We build on previous work carried out through Innovate UK, Marie Curie, Royal Society and European Research Council funding and make complimentary use of expertise and nanofabrication facilities to significant progress in the development of quantum technology in a 3-years targeted programme. Thanks to the strategic collaboration with National UK Quantum Technology Hubs, we will carry out joint experiments in quantum computing/simulation (Hub in Quantum computing and simulation - HQCS) and in advanced imaging (QuantIC) applications to show the game-changing nature of developed technology. Also, we will leverage support to engage closely with end-users and stakeholder maximizing the impact of the research project. Potential markets for developed technology will be exploited through the collaboration with QT hubs industry partners' network and with the strategic Industrial partners of this proposal like Kelvin Nanotechnology (KNT), Oxford Quantum Circuits (OQC) and SeeQC UK. This project is designed to generate high-quality research outputs and to deploy advanced technology in the field of quantum science. The work strongly resonates with the central themes of Horizon 2020 programmes and with the UK strategic research priorities set by Research Councils. The long-term goal is to establish a world-class experimental research programme which will have a powerful cross-disciplinary impact strengthening the UK's leading position in new science and technology to generate societal and economic benefits.

Semiconductors are commonly used in imaging sensors and solar cells, as they can directly convert light into an electrical current. The highest band of electron energies that are fully occupied is known as the valence band while the lowest unfilled energy band is the conduction band. The energy difference between the conduction and valence bands is known as the bandgap. When electrons from the valence band are excited into the conduction band by absorbing light with energy equals to or greater than the bandgap, the change of charges induces an electrical current. Consequently the bandgap is the most important parameter in the design of semiconductor photodetectors. While visible wavelength photodetectors are widely available, detectors for infrared wavelengths are significantly less mature and more costly. Progress in infrared detectors has been hindered by the limited choice of bandgaps currently available. In this work we will introduce a novel approach, by incorporating Bismuth (Bi) atoms into existing semiconductors such as InAs and InGaAs, to achieve a wide range of bandgap energies to detect infrared signals across a correspondingly wide wavelength range. Achieving this will lead to a new range of infrared detectors that can have transformative impact on applications including night vision imaging, medical diagnostic sensors, environmental monitors and for accurate temperature measurements in manufacturing processes. We will also exploit Bi-alloys to engineer a noiseless charge amplification process in photodiodes known as avalanche photodiodes (APDs). When an electron leaves the valence band a vacant state (a hole) is created. Therefore an electron and a hole are created as a pair of charges in semiconductors. Properties of the conduction and valence bands will determine how electrons and holes gain energy from an applied electric field. In materials such as InAs, electrons gain energy at a much faster rate and travel at higher velocity too, when a voltage is applied. Therefore InAs is an excellent material for high speed electronic devices and also for providing internal signal amplification in APDs. When designed appropriately, the energetic electrons in InAs APD ensure that the amplification process, known as impact ionisation, is coherent so that negligible amplification noise is generated. In this work we will incorporate Bi into InAs to alter the valence band such that only electrons will gain significant energy from the electric field. This ability to suppress energetic holes will allow us to design very high gain APD across a wide range of electric field while concomitantly suppressing the noise associated with impact ionisation. By carefully controlling the fraction of Ga and Bi atoms, we will also develop a range of InGaAsBi APDs suitable for detecting a wide range of infrared wavelengths. The proposed research to introduce a new class of Bi-containing infrared detectors and APDs, will be carried out by a carefully assembled team of world leading researchers from Universities of Sheffield and Surrey, in collaboration with the Tyndall National Institute, as well as partners from LAND Instruments, Laser Components and the UK Quantum Technology Hubs in Enhanced Quantum Imaging. Our work will start with a focus on formulating growth conditions (such as temperature and atomic fluxes) to obtain high quality InGaAsBi crystals. Following an intensive crystal growth programme, we will develop procedures to fabricate the grown InGaAsBi semiconductors into devices for a wide range of measurements to extract key material parameters. A model that accurately describes the bandstructure of InGaAsBi will be developed so that we can use them to design high performance infrared detectors and APDs. These newly engineered devices will be evaluated with our industrial partners for applications ranging from temperature measurements in manufacturing to novel imaging techniques using quantum properties of light.