## 23 September 2020

## Robert Bennett

**Thinking about nothing: Interpretations of the quantum vacuum**

A fundamental debate throughout the history of science concerns the (im)possibility of truly empty space. Nowhere is this more evident than in quantum mechanics and quantum field theory, where the vacuum state is often envisaged as a bubbling sea of short-lived fluctuations. The (indirect) consequences of this are very well known, having been measured more precisely than any other physical quantity. Some recent experiments aim to directly observe the fluctuating electromagnetic vacuum field, and have reported positive results. In this colloquium I will give an account of the subtleties that must be appreciated when dealing with vacuum fluctuations, and show that after a century of quantum mechanics we are not necessarily any closer to an ontology of empty space.** **

## 21 October 2020

## Joan Vaccaro

**What makes the universe tick: searching for the quantum origins of time**

The violation of the discrete symmetries of charge conjugation (C), parity inversion (P), and time reversal (T) observed in high energy physics are clearly fundamental aspects of nature. A new quantum theory [1,2] proposes the possibility that the violations have large-scale physical effects. The new theory does not presume any conservation laws or equations of motion. In particular, if T violation is turned off, matter is represented in terms of virtual particles that exist momentarily only. However, with T violation turned on, what was the mathematical structure of a virtual particle now traces out an unbounded world line that satisfies conservation laws and an equation of motion. The theory is then analogous to the 5 dimensional “proper time” formalism introduced by Feynman, extended by Nambu in the 1950’s, and developed as parameterized relativistic quantum theories [3]. The important point here is that time evolution and conservation laws are not built into the new theory, but rather they emerge phenomenologically from T violation. In other words, the new theory proposes that T violation is the origin of dynamics and conservations laws. The theory has experimentally testable predictions and offers new insight into the quantum nature of time. The talk will include a brief analysis of the nature of the T violation from known and expected sources such as mesons, neutrinos, and a Higgs-like scalar field, and give a brief description of a proposed experimental test.

[1] J.A. Vaccaro, Quantum asymmetry between time and space, Proc. R. Soc. A 472, 20150670 (2016). https://dx.doi.org/10.1098/rspa.2015.0670

[2] J.A. Vaccaro, The quantum theory of time, the block universe, and human experience, Phil. Trans. R. Soc. Lond. A 376, 20170316 (2018). https://dx.doi.org/10.1098/rsta.2017.0316

[3] J.R. Fanchi, Review of invariant time formulations of relativistic quantum theories, Found. Phys. 23, 487-548 (1993).

## 4 November 2020

## Timothy Moorsom

##### The Molecular Toolbox: Using organics to take control of magnetism

Alchemists once dreamed of transmuting base metals into gold through mystical or magical means. We now understand that the unique properties of different metals arise from their electronic structure, but it is still challenging to take control of these properties at the nano-scale. For the past eight years, I have been investigating how to use organic molecules to do just that. Through the formation of a highly polarised interface state, called a spinterface, it is possible to develop new hybrid meta-materials with unique properties not found in other systems. In this talk, I will outline how spinterfaces can be used to create designer magnets, turning metals like Cu and Mn into room temperature ferromagnets, or using pure carbon to develop the strongest magnets ever seen outside of the rare-earths, with the potential to transform renewable energy production. I will discuss some of the emerging applications of spinterfaces to technology, such as the development of a single-spin capacitor and the potential for molecules to control skyrmion motion. Finally, I will discuss my plans to use molecules to control the behaviour of topological insulators and design a new generation of high-speed, low energy logic devices.

## 18 November 2020

## Roberta Zambrini

##### From classical to quantum synchronization

Spontaneous synchronization is a paradigmatic phenomenon in complex systems also beyond physics.

In classical systems it is usually characterized through trajectories in phase-space making a generalization in quantum systems a challenging issue open to many approaches.

In this talk I will present an introduction to the topic and then present some recent results on transient synchronization in quantum systems and open questions.

## 3 February

## Jacqui Romero (U Queensland)

##### Adventures in Hilbert Space: Quantum Information Using the Shape of Light

Information is physical, it is encoded in physical systems, for example in the electrons that flow through our digital devices. In this way, the rules that information follow is dictated by physics: when the information is encoded in a quantum system, quantum physics rules! The shape of light has emerged in recent years as a promising platform for encoding quantum information, for the multiple levels that it affords and the ease with which shape can be controlled. I will first start by highlighting the differences between classical and quantum information. Then I will proceed to discuss two recent experiments.

The first is on ignorance—one might ask if ignorance of a whole system implies ignorance of its parts. Our classical intuition tells us yes, however quantum theory tells us no: it is possible to encode information in a quantum system so that despite some ignorance of the whole, it is impossible to identify the unknown part. I will give an experimental evidence that supports this counterintuitive fact. The second is on learning, we implemented a self-learning tomographic technique wherein the experiment guides itself to estimate an unknown quantum state. This is especially important for systems that are of high dimensionality, where making tomographically complete measurements become impractical.

## 17 February 2021

## Lijian Zhang (Nanjing University)

Measuring the quantum measurement

Accurate knowledge of a quantum-optical detector is essential for its utilization, be it in foundational investigations or technological applications. Different from their classical counterparts, quantum detectors cannot be simply described by several parameters including detectivity, spectral sensitivity and noise equivalent power. Instead, a quantum detector should be characterized by its full set of measurement operators, the POVM, and benchmarked by its quantum utility. The former is normally achieved by quantum detector tomography with a set of pre-calibrated probe states and specific reconstruction algorithms. I will give a brief introduction on how the detector tomography is implemented experimentally, and talk about several methods to improve its reliability and alleviate its complexity. As an example of the utility of quantum detectors, we apply the resource theory to quantify the capability of a quantum detector to detect coherence within the input state.

MS Stream link:

## 22 February 2021

## Norbert Lütkenhaus (Waterloo)

##### Some weird quantum stuff

##### Colloquium in honour of Professor Steve Barnett's 60th birthday.

## Wednesday, 3 March 2021

## Christopher Berry (Glasgow)

#### The secret lives of black holes

Gravitational-wave astronomy provides a unique insight into the lives of black holes. Since the beginning of the advanced-detector era in September 2015, we have observed gravitational waves from over 40 binary black hole systems. From the measured gravitational-wave signal we can infer the properties of their source systems, and uncover new insights into their formation. There are currently many mysteries around how massive stars evolve and binaries form in order to create the population of binary black holes. I will explain how we can use the growing catalogue of gravitational-wave observations to unravel these mysteries, reviewing our discoveries to date and highlighting our expectations for future breakthroughs.

Zoom link:

https://uofglasgow.zoom.us/j/93593792666?pwd=YnYwMVZqaVhwSWpNN1VPd2tFNHYvQT