University of St Andrews

Optomechanical devices monitor the displacement of a small mechanical oscillator using the light of an optical cavity with exquisite sensitivity, allowing for instance the recent detection of gravitational waves by the LIGO experiment (2017 Nobel prize). The optomechanical interaction can also lead to cooling and has allowed cooling to (or near) the quantum ground state of a range  of mechanical devices, with the assistance of cryogenic cooling. Over the past decade or so, cooling the centre of mass motion of nanoparticle (on the 100-500 nm scale, about the size of a virus) levitated by light in a cavity has been a challenge for several experimental groups [1]. Levitation by light at high vacuum would provide a high degree of decoupling from environmental heating and decoherence. Nevertheless, the problems are formidable: the oscillator must be cooled from room temperature (about  n~10^7 quanta above the ground state) down to typical occupancies n<1.  The key bottleneck was trapping the particle stably at high vacuum; particle loss issues held up the field for years, but in 2015-2017 it became possible to trap and cool down to $n~100$ including at UCL [2].  This year, a technical breakthrough [4] means that the milestone of cooling of a levitated nanoparticle to the quantum ground state (n<1) is in view (and has already been tentatively reported at summer conferences by groups in Austria and Switzerland). In the talk I will review progress in levitated optomechanics, in particular cavity optomechanics, and including our work at UCL. A key driver of the field is for applications in quantum sensing, thus cheaper, so -called “table-top” versions of LIGO. Which may be used for quantum accelerometers or for exploring fundamental physics. I will review the Standard Quantum Limit (SQL) of optomechanical sensing which represents a trade-off between incoming imprecision noise (quantum shot noise of light) and measurement back-action noise. An unusual  and interesting feature of the new levitated set-ups is that they are intrinsically 3D, while in contrast, standard optomechanics cools only a single degree of freedom so I will also look at the SQL in 3D.

Gravitational waves are ripples in spacetime, produced by the most

violent events in the cosmos: exploding stars, colliding black holes, perhaps even the Big Bang itself.  Since September 2015, when the twin LIGO (Laser Interferometer Gravitational-Wave Observatory) instruments made the first-ever direct detection of gravitational waves from the merger of two massive black holes more than a billion light years distant, the LIGO Scientific Collaboration and  the Virgo Collaboration have reported the confirmed detection of ten further compact binary mergers - including the merger of two neutron stars detected in August 2017 that was also observed across the entire electromagnetic spectrum.  Moreover since April 2019 the LIGO and Virgo detectors have been engaged in their third observing run, with enhanced sensitivity, and during the first four months of “O3” they reported a further 22 candidate detections. The gravitational-wave window on the cosmos is now well and truly open!

In this lecture I will review the latest LIGO and Virgo observations of the mergers of binary black holes and neutron stars – and how these observations are shaping and improving our understanding of strong gravity, cosmology and fundamental physics.  I will then outline the gravitational-wave “roadmap” for the coming decades, including the planned LIGO and Virgo observing runs and upgrades, the addition of KAGRA and LIGO India to the ground-based network, the “ third generation” ground-based interferometers proposed for the 2030s and the enormous potential of spaceborne detectors that will probe the lower frequency gravitational-wave spectrum.  I will describe how these observations should provide a much deeper and more complete view of the Gravitational Universe over the next two decades, offering new and complementary insights into a wide range of astrophysical questions – from the origin of black holes and the evolution of galaxies to the nature of dark matter and dark energy and perhaps even the fundamental properties of spacetime itself.

The discovery of novel properties, effects or phenomena in modern materials science is often driven by the quest for the coexistence and/or coupling of several functional properties into a single system. Within this framework, I will focus on the microscopic mechanisms leading to the interplay between spin and dipolar degrees of freedom, along with their theoretical modelling. I will first address the coupling of long-range magnetic and electric dipolar orders in multiferroics, in particular in electronic ferroelectrics, where the spin/charge/orbital order induces an electric polarization. Second, I will discuss (non-magnetic) ferroelectric semiconductors, where the spin-orbit interaction leads to a tight link between Rashba spin-splitting in the electronic structure, spin-texture and electric polarization. The common denominator is to achieve the electric-field control of magnetism and, therefore, the long-sought integration of spintronics with ferroelectricity.