Physics: Current Research Highlights
Physics is the scientific discipline that addresses the most fundamental questions, from the very nature of matter to the origin of the Universe. Moreover Physics is important to the economy of the nation; many past technologies have been and many future technologies will be based on research carried out in physics departments. Bristol Physics has the strength and vision to pursue the newly emerging areas of nanoscience, high temperature superconductors, smart materials, quantum information and revolutionary instrumentation for probing biological systems as well as carrying out world class research in elementary particle physics and cosmology.
1 Astrophysics (Professors M Bremer and S Phillipps)
One of the biggest questions in astrophysics and cosmology is how galaxies formed and evolved to the population of galaxies that we see in the universe today. To understand this it is necessary to obtain a full census of the galaxy population, both locally and in the distant universe - which is seen as it was billions of years ago because of the time the light has taken to reach Earth. Over the past decade, astrophysics research group members have been at the forefront in both endeavours. Through spectroscopic surveys of all the objects visible in the direction of some nearby galaxy clusters, the group discovered a population of tiny galaxies dubbed “ultra-compact dwarfs”, a breakthrough called by Sidney van den Bergh, the foremost world expert on galaxy classification, "the first time since ... 1938 that an entirely new class of galaxy has been discovered!" [Nature 423, 519 (2003)].
At the other end of the scale, using the largest optical telescopes in the world, Bristol researchers were the first to systematically study galaxies at redshifts around 5 - corresponding to a time less than one billion years after the Big Bang when the universe was only a few percent of its current age - and their contribution to the so-called “epoch of reionization”, when energetic photons from the first objects to form ionised the cosmic plasma.
In modern technology, most devices such as computers, televisions and magnetic information storage depend on quantum mechanics for their operation. Research by the Correlated Electron Systems group focuses on a special problem in quantum physics: the collective behaviour of the electrons in solids. Just as the interactions between human beings can produce highly ordered entities such as societies, the interactions between electrons in a solid can produce a huge variety of exotic physical states. These states are not only of fundamental interest they provide a wealth of key technologies.
A particular focus of the group is superconductors: materials that are able to transport electrical current without loss of energy when cooled to low temperatures. Superconductors are used to produce high magnetic fields or transport large amounts of electrical power using a small conduit. Using many experimental probes, researchers are investigating why a particular material becomes superconducting, what determines the temperature at which this occurs and the nature of the quantum states of the electrons and their collective excitations.
Most materials that superconduct at high temperatures consist of copper-oxide (cuprate) planes, and the electron current that flows within the planes can be 10,000 times higher than that between the planes. The electronic state appears to be two-dimensional, but the Bristol group was the first to measure the energy contour of the fastest moving electrons (the Fermi surface) in any high temperature superconductor and show that the state is actually three-dimensional. [Nature 425, 814(2003)]. Very recent experiments using ultra high magnetic fields that last a fraction of a second provide new insight into the fundamental mechanism that leads to “quantum criticality” in cuprate materials [Science 323,603(2009)].
3 Photonic Quantum Information Science (Professor J O’'Brien)
Quantum mechanics provides a completely unexpected description of how the world works at the microscopic level. It paints a picture that is fundamentally probabilistic, where a single object can be in two places at once —“superposition”— and where two objects in remote locations can be instantaneously connected —“entanglement”. These curious properties have been observed experimentally. Today scientists are learning how to harness these properties to realize entirely new “quantum” technologies, including communication systems whose security is based directly on the laws of quantum physics, and quantum computers with unprecedented computational power for particular tasks.
Photons, single particles of light, are ideal quantum systems for the realization of quantum technologies, owing to their high-speed, low-noise properties, and the ease with which they can be manipulated. However, the standard approach of using cm-sized mirrors and beam splitters mounted on large optical tables to realize quantum circuits that control photons has reached its practical limit. In 2006, a new activity was established at Bristol to address this issue via the invention of “integrated quantum photonics”: the use of waveguides on silicon chips to create quantum photonic circuits. This approach has been used to demonstrate the logic operations that lie at the heart of a quantum computer [Science 320, 646 (2008)], to manipulate four entangled photons [Nature Photonics 3, 346 (2009)] and most recently to make a small-scale quantum computer on a chip that implements a key quantum algorithm namely one which finds the prime factors of an input [Science 325, 1221 (2009)].
4 Nanophysics (Professor M Miles)
Atomic force microscopy (AFM) is enormously successful over a wide range of fields because of its ability to produce atomic resolution images in liquid and gaseous environments as well as in vacuum. However, it has the drawback that it typically requires minutes to record a single image, thus preventing the study of the many important processes occurring over shorter time periods. The Nanophysics and Soft Matter group, led by Professor Mervyn Miles, has invented two different new high-speed force microscopes. The first is a high-speed contact mode AFM which relies on super lubrication of the tip and sample to decrease the shear-force damage to soft samples. Rates of 1300 frames per second have been achieved on biological samples, making this new microscope capable of imaging at over 100,000 times the rate of a conventional instrument. Researchers can follow processes occurring on short time-scales, examine large areas at high resolution, and write structures, via electrochemical oxidation, by the high-speed tip over large areas.
A major problem in AFM is the level of tip-sample force interaction and the resulting distortion of delicate (bio) structures. In order to overcome this problem Miles and his team have developed the non-contact transverse dynamic force microscope, ideal for following processes at the single biomolecule scale. The instrument “senses” the sample surface, without coming into contact, through changes in the mechanical properties of water confined in the nanoscale gap between the vertical probe tip and the sample.
An advanced holographic optical tweezers system, the dynamic holographic assembler, was developed in collaboration with Glasgow University (Padgett Group) for assembling three-dimensional micro and nanostructures. The assembler has been used to construct nanotools. The assembler can generate and control up to about 100 traps simultaneously, and a multi touch table interface enables flexible control. Forces on the “fingertips” of the optically-trapped microhand or forces acting on a nanotool can be measured in real time. The measured forces are used in haptic feedback to a cyberglove allowing the user to “feel” the structures in the micro and nano world, permitting, for example, individual living cells to be palpated for disease.