Current projects underway in the theoretical chemical and quantum physics group.
Adiabatic processes in solid-state and photonic systems
The adiabatic theorem is one of the fundamental consequences of quantum theory. If the system is modified slowly enough, the current state of the system can smoothly move from one eigenstate to another. This simple observation has interesting consequences when used in controllable nanoscale devices. Electrons moving from one place to another without spending any time in between, photons being steered between waveguides, arbitrary superpositions and entanglement being created as long as everything is done “slowly enough”. This project investigates the use of adiabatic protocols for applications in quantum computing, quantum information theory, measurement and sensing.
Atom-photon interactions and the boundary between quantum optics and condensed-matter
Quantum optics and condensed-matter are traditionally two separate branches of physics. The former being concerned with the interactions between atoms and photons, the latter with the effects of multi-particle systems in which local and non-local effects are observed. Recently, these two fields have started to overlap, resulting in such concepts as circuit-QED and quantum optics experiments in solid-state systems. We are interested in how quantum optics concepts can be implemented in solid-state systems as well as how condensed-matter effects can be observed in interacting atom-photon systems.
Electronic properties of phosporous-doped silicon nanostructures
To make components for a quantum computer, we must use bottom-up approaches to manufacturing. And, while we are not at a stage where large-scale commercial manufacturing is feasible, this technology has developed to the point where it is possible to have control over the manufacturing process with atomic precision. For example, new structures such as quantum dots, layers, and wires have all been made experimentally by doping silicon with phosphorus atoms at extremely high concentrations.
To understand how we can use these devices, we first need to understand their electronic structure. One of the strengths of our group is in performing highly accurate density-functional theory (DFT) calculations to find the ground-state electronic properties of nano-scale structures. The results of these DFT calculations are then used to build effective models for these devices using effective-mass theory, the nonequilibrium Green's functions formalism, and tight-binding theory. This many-pronged approach allows us to model systems that are of direct relavance to current experiments.
Nitrogen-Vacancy defects in Diamond - physics, quantum sensing
There are many different types of impurities in diamond, which affect its optical and structural properties. For each defect species there can be numerous configurations, which results in different absorption and emission spectrum. The most abundant impurity in diamond is nitrogen, with an important nitrogen defect being the nitrogen-vacancy (NV) centre due to its defect levels in the diamond band gap, which results in a strong zero phonon line transition.
The NV centre in diamond, which has multiple charged states, is made up of a substitutional nitrogen atom that is adjacent to a vacant carbon lattice site. The two common charged states of the defect are the neutral centre (NV0) and a centre with an extra electron (NV-). The negatively charged centre has received a lot of attention in recent years due to its possible uses as a single photon source at room temperature and as a qubit in quantum information processing.
More recently, the NV centre has been explored as a quantum limited sensor. Due to the fact that the energy levels of the defect depend on many different material and electromagnetic parameters, an NV centre can be used as a nanoscale magnetometer, electrometer, thermometer and strain sensor.
Open quantum systems and decoherence theory
What defines the boundary between the quantum and classical worlds is one of the most fundamental questions of modern physics. Quantum theory has proven to be spectacularly successful at small length scales, short times and very cold temperatures. Yet, in our everyday world the equations of classical physics have a fundamentally different form and interpretation. How do we smoothly swap between these descriptions? - this is one of the central points of decoherence theory. How do we perturb an otherwise phase coherent theory (quantum mechanics) in order to include the effects of the wider environmental, ie. the effects of dephasing and dissipation. The TCQP group studies this problem in a range of different physical systems and with several different mathematical techniques.
Superconducting circuit theory for information processing and metrology
Electrical circuits operating at sub-Kelvin temperatures display a range of effects, which can only be described by the laws of quantum mechanics. As these circuits can be fabricated "at will", they provide unique opportunities to study quantum effects where a circuit can be designed specifically to study a particular effect. Quantum circuits already find application in the detection of microscopic magnetic and electric fields, ultra-sensitive amplifiers and ultra-fast electronics. This project investigates the behaviour of quantum circuits for both applications in modern technology and to study fundamental physical principles.
Spin physics, spin-chains, quantum magnonics and entanglement theory
The study of spins and their interactions (be they electron-, nuclear- or pseudo-spins) is one of the central components of quantum mechanics. In recent years, the new fields of quantum computing and quantum information processing have link the physics of spins to the fields of information theory, computing theory and cryptography. In this project, we consider the behaviour of interacting few spin systems to study entanglement, transport and measurement. This has applications to quantum computing, quantum sensing as well as the fundamental theory of quantum mechanics.
The physics of Josephson junctions at the microscopic level
Although superconducting devices are based on the dissipation-less properties of metals below the superconducting transition, they suffer from a fundamental drawback. The quantum effects on which these devices rely stem from Josephson junctions, which are ultra-small, ultra-thin insulating barriers. At present the best method for fabricating these junctions is to take advantage of the native amorphous oxide, which forms on many superconducting metals when exposed to oxygen. This oxide layer in turn contains defects, due to its amorphous nature, which provides a new and dominant dissipation channel. Much is known about the effects of these defects, very little is known about their precise microscopic origins. This project aims to uncover the microscopic details of these defects using both theoretical and experimental approaches.