Professor Kevin Heng (Center for Space and Habitability, University of Bern, Bern, Switzerland)

TBA

Using atom-like spins in semiconductors toward scalable quantum computing

Dr Frederico Martins (Hitachi Cambridge Laboratory)

Abstract: In a world where the amount of data to process is steadily increasing, the quantum nature of matter offers new possibilities to develop concepts, which may overcome nowadays technologies. Implications are expected in research areas that can range from quantum computation, cryptography, and quantum simulation.
To be useful, a qubit (the elementary quantum unit of information) needs to be both isolated from its environment and precisely controllable, which places strict requirements on its physical realization. In particular, spins in solids are one of the most promising realizations due to their potential for scalability and miniaturization. Furthermore, in these systems, quantum control has been established and electron spin coherence times now exceed several seconds. Even so, a critical challenge in these systems consists of developing a robust two-qubit gate that can be scaled up to a larger network.
In this seminar, I will overview some of the challenges of this field and introduce a new mechanism for “long-range” interaction. Making use of independent readout of two electron spins, we demonstrate coherent exchange interaction mediated by a multielectron quantum dot. This result provides a possible route to the realisation of multi-qubit quantum circuits based on single spins.

*CANCELLED*

Abstract: We know that activity from the Sun can affect the Earth's ionosphere with the resulting disruption of electrical grids and therefore human activity. I will place the Sun in the context of other stars and outline how Solar flares are generated. One key question is how often Solar 'Super' flares occur. With telescopes such as Kepler and TESS, we are now able to study many stars which are similar to the Sun and
determine how often Super-flares occur on Solar-type stars. Finally I will briefly discuss how stellar activity can affect both the detection of exo-solar planets and their atmospheres.

Contemporary cosmological hydrodynamical simulations are reaching unprecedented levels of sophistication and complexity. Numerical programs like IllustrisTNG (www.tng-project.org) are allowing us to model reasonably realistic populations of galaxies across an ever-wider range of masses, environments, evolutionary stages and cosmic epochs. There we resolve and model the structural details of thousands of galaxies together with the large-scale cosmic web by solving the equations of gravity and magnetohydrodynamics and by including prescriptions for star formation, stellar evolution, metal enrichment, cooling and heating of the gas, galactic outflows and feedback from the supermassive black holes, all within the boundary conditions of our standard cosmological paradigm. In this talk, I will give an overview of our efforts to generate and effectively exploit these types of models and I will showcase some of the astrophysical insights that they are allowing us to uncover and quantify. Finally, I will show how the combination of Deep Learning methods and large-volume galaxy simulations allows us to infer unobservable quantities, such as information about the past assembly and merger history of galaxies, from their observable properties.”

IceCube: Cosmic Neutrinos and Multimessenger Astronomy

Professor Francis Halzen (Wisconsin IceCube Particle Astrophysics Center and the Department of Physics, University of Wisconsin–Madison)

Below the geographic South Pole, the IceCube project has transformed one cubic kilometer of natural Antarctic ice into a neutrino detector. IceCube detects more than 100,000 neutrinos per year in the GeV to 10 PeV energy range. From those, we have isolated a flux of high-energy neutrinos of cosmic origin, with an energy flux that is comparable to that of high-energy photons. We have also identified the first source: on September 22, 2017, following an alert initiated by a 290-TeV neutrino, observations by other astronomical telescopes pinpointed a flaring active galaxy, powered by a supermassive black hole. We will review recent progress in measuring the cosmic neutrino spectrum and in identifying its origin.

Release of magnetic energy due to reconnection is believed to drive such transient phenomena as solar flares, eruptions, and jets. This energy release should be associated with a decrease of the coronal magnetic field. Quantitative measurements of the evolving magnetic field strength in the corona are required to find out where exactly and with what rate this decrease takes place. The only available methodology capable of providing such measurements employs microwave imaging spectroscopy of gyrosynchrotron emission from nonthermal electrons accelerated in flares. Here, we report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field at the cusp region; well below the nominal reconnection X point. The field decays at a rate of ~5 Gauss per second for 2 minutes. This fast rate of decay implies a highly enhanced, turbulent magnetic diffusivity and sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. Moreover, spatially resolved maps of the nonthermal and thermal electron densities derived from the same microwave spectroscopy data set allow us to detect the very acceleration site located within the cusp region. The nonthermal number density is extremely high, while the thermal one is undetectably low in this region indicative of a bulk acceleration process exactly where the magnetic field displays the fast decay. The decrease in stored magnetic energy is sufficient to power the solar flare, including the associated eruption, particle acceleration, and plasma heating. We discuss implications of these findings for understanding particle acceleration in solar flares and in a broader space plasma context.

The kelvin redefined and its implications

Professor Graham Machin (National Physical Laboratory)

In May 2019 the International System of Units (the SI) underwent what was its biggest change since its introduction when the definition of four of the seven SI base units were changed to be based on defined values of fundamental physical constants. Since the change, the kelvin is now defined in terms of the Boltzmann constant, the ampere on the electron charge, the kilogram on the Planck constant and the mole on the Avogadro constant.

The redefinition of the kelvin has opened several new possibilities for traceable thermometry direct to the kelvin definition. These could include using primary thermometry to calibrate sensors at National Measurement Institutes (NMIs) and, in the medium term, in calibration laboratories dispensing with traceability to the defined scales (ITS-90, PLTS-2000) and so disseminating thermodynamic temperature. In the longer term these changes could lead to the rise in potential, paradigm changing approaches to temperature sensing such as traceability at the point of measurements both through self-validating thermometers and more radically by the deployment of practical primary thermometry based on fundamental physics and where temperature sensor itself will, unlike today, no longer need calibrating to provide traceability.

In this talk an introduction to how the kelvin was redefined and to the mise en pratique for the definition of the kelvin (MeP-K) will be given. How traceable temperatures are attained will be discussed, both presently, through the defined scales and how, in the medium and long term, this is likely to change; with thermodynamic temperature approaches becoming increasingly prevalent. The talk will end by introducing novel approaches to temperature traceability including provision of NMI like uncertainty thermodynamic temperatures in calibration laboratories and the rise of in-situ/in-process traceability and the implications, particularly in the context of digitalisation and the need for "point-of-truth" in for example autonomous sensor networks.

Abstract: The unitary evolution of a quantum system preserves its coherence, but interactions between the system and its environment result in decoherence, a process in which the quantum information stored in the system becomes degraded. A spin-polarized positively charged muon implanted in a fluoride crystal realizes such a coherent quantum system, and the entanglement of muon and nearest-neighbor fluorine nuclear spins gives rise to an oscillatory time dependence of the muon polarization that can be detected and measured. The decohering effect of more distant nuclear spins can be modelled quantitatively, allowing a very detailed description of the decoherence processes coupling the muon-fluorine “system” with its “environment,” and allowing us to track the system entropy as the quantum information degrades. I discuss the implications of these results for more general experiments in which spin-polarised particles, such as the muon, can act as probes of local magnetic behaviour.

It is well known that the atmosphere exhibits turbulence with a -5/3 energy spectrum on small scales (on the order of 100 km and smaller). It is also well known (especially to airline passengers) that transient localised enhancements to this background turbulence are hazardous to aircraft. Understanding this aviation-affecting subset of turbulence is an important and challenging application of geophysical fluid dynamics. This talk will describe a new theory and mechanism for the generation of aviation turbulence. It will also test the skill of the mechanism by comparing observations of turbulence with predictions made from the theory. These tests are successful, to the extent that the algorithm is now being used operationally to predict turbulence for the aviation sector every day. Finally, I will explain why climate change is strengthening aviation turbulence, potentially leading to bumpier flights in future.

Professor Bruce Drinkwater (University of Bristol) 

Abstract: Acoustic (or ultrasonic) beams are used widely in engineering, science and medicine. Beams are integral to medical and non-destructive testing imaging systems where the focal spot size determines resolution. However, these beams can become distorted, e.g., when propagating through polycrystalline metals or across the skull into the brain. Beams are also key to techniques that use the acoustic radiation force to transfer momentum to manipulate objects, i.e., acoustical tweezers or tractor beams. Here the beams must be carefully shaped to create the correct local force field. This talk first describes the properties of acoustic beams, how they are distorted and how this can be corrected. The various experimental techniques for the creation of beams are then explored. Acoustic holograms and metasurfaces lead to particularly exquisite control of beam shape yet are currently static. Arrays of individual emitters can provide dynamic beam control but are currently lower in resolution. In this talk these approaches are explained and example devices introduced. Throughout the talk the various applications of acoustic beams are highlighted including both imaging and biomedical manipulation. The latter has enabled the creation of artificial tissues such as muscle.

RESCHEDULED on January 17

Abstract: Invisibility cloaks are fantastic devices in popular culture from Harry Potter to Star Trek. But even in the real world so-called metamaterials (synthetic composite materials with emergent new properties) can act as (partial) cloaks both against light (vision) and sound (acoustics). We recently discovered that the 65 million year old arms race with their echolocating bat predators has equipped silkmoths (Saturniidae) with remarkable acoustic metamaterials on their wings and bodies. These ultrathin sound absorbers offer protection because the strength of the echo bouncing off the moth's body determines the distance over which bats can detect it. In the talk we will use innovative acoustic tomographies to visualise how fur on bodies and scales on wings of moths provide acoustic cloaking. Turning the moth wing into bio-inspired thinner and better sound absorbers ('sonic wallpaper') can help us in the struggle to maintain healthy living and working environments in our ever noisier world.

The Moiré pattern of the magic-angle twisted bilayers graphene and twisted bilayer MoS₂ leads to localization of the low energy electrons in the AA-stacking regions, reflected by very flat bands. This strong reduction of the kinetic energy enhances the importance of interactions and thus renders the bilayer systems much more susceptible to correlation effects, as show experimentally by the discovery of correlated insulators and superconductivity. Despite numerous theoretical and experimental studies, the understanding of this new electronic localization is still incomplete. The rotation angle is of course a key parameter, but it has also been shown that slight expansion or contraction of one layer relative to the other (“heterostrain”) can strongly modify the flat bands.

We will present theoretical study of the electronic structure and quantum transport properties of electronic flat bands, considering as well as possible the structural parameters that condition them (rotation angle, bias voltage, heterostrain, local defects or atomic relaxation). We also investigate the magnetic instabilities in twisted bilayer graphene using a real-space Hartree-Fock mean-field theories. Starting from a tight-binding description of the non-interacting bilayer systems we add a local Coulomb interaction U in order to model the Coulomb repulsion between electron. At half filling, localized magnetic states emerge for U

values well below those required to render an insulated layer antiferromagnetic.

On Thursday 25th July (10:00-11:15), Professor Stéphane Udry from the University of Geneva will be delivering a talk at the University of Warwick titled 'The New Worlds of the Cosmos: A Fantastic Human Quest of our Time'. This event is open to all, and the content of the talk will be aimed at all knowledge levels.

For those who register to attend, we will send an update nearer the time to inform you where the talk will be taking place on campus.

If you would like to attend the talk, please complete the form. Light refreshments will be provided, please let us know if you have any dietary requirements.

Thursday 25th July, 10:00-11:15, Room TBC

Speakers TBA