A series of talks by visitors to the AMQP Group, Department of Physics, Swansea University.

I review the notions of bare (unprotected) quantum devices (QD), externally protected QDs [1] and internally protected QDs. Quantum topological states are used as a resource to achieve for the first time a fully-fledged quantum device like a quantum computer, including self-correcting initialization, measurements and quantum gates [2]. This construction is based on Topological Color Codes [3,4] which have very nice and powerful transversality properties which allow them to perform quantum tasks like topological quantum distillation, teleportation, dense coding etc. There are recent experimental small realizations of Topological Color Codes using a trapped-ion platform [5]. The underlying physical phenomena in these new quantum devices is the notion of topological order: a new paradigm in strongly correlated systems. We show how quantum topologically protected states can be constructed not only in 2D space but also in higher dimensional systems [6].

REFERENCES:

-[1]-"Error Threshold for Color Codes and Random 3-Body Ising Models" H. G. Katzgraber, H. Bombin, M. A. Martin-Delgado; Phys. Rev. Lett. 103, 090501 (2009)

-[2]-"Self-Correcting Quantum Computers" H. Bombin, R. W. Chhajlany, M. Horodecki, M.A. Martin-Delgado; New Journal of Physics 15 (5), 055023 (2013).

-[3]-"Topological Quantum Distillation" H. Bombin, M.A. Martin-Delgado; Phys.Rev.Lett. 97 (2006) 180501

-[4]-"Topological Computation without Braiding" H. Bombin, M.A. Martin-Delgado; Phys.Rev.Lett.98:160502, (2007)

-[5]- "Quantum computations on a topologically encoded qubit" D. Nigg, M. Muller, E. A. Martinez, P. Schindler, M. Hennrich, T. Monz, M. A. Martin-Delgado, R. Blatt; Science Vol. 345 no. 6194 pp. 302-305 (2014).

-[6]-"Exact Topological Quantum Order in D=3 and Beyond: Branyons and Brane-Net Condensates" H. Bombin, M.A. Martin-Delgado; Phys.Rev.B75:075103, (2007)

Fluorescence microscopy is popular in the life sciences due to its molecular specificity and non-invasiveness. Here, I will outline three methods we are using to study how biophysical mechanisms regulate the activation of T cells, a key regulator of the adaptive immune system.

Firstly, single-molecule super-resolution microscopy combined with novel statistical analysis which we use to describe the nanoscale clustering of cell surface molecules and the nanoscale ultrastructure of the cell skeleton. Second, structured illumination super-resolution microscopy combined with image correlation approaches to quantify molecular flows in cells including those of the cell skeleton and the cell membrane. Finally, environmentally sensitive membrane dyes which report on their local biophysical and biochemical environments to study molecular organisation in the cell membrane. Together, these methods show how macroscale events - such as an immune response - can be regulated by nanoscale spatio-temporal molecular organisation.

I will describe the applicability to problems of slow relaxation and non-equilibrium in the dynamics of quantum many-body systems of concepts more frequently associated with classical soft matter systems such as glasses. I will discuss how quantum slow dynamics arises from local dynamical constraints, how collective relaxation is a phenomenon dominated by spatial fluctuations in the dynamics, and whether quantum non-ergodicity - termed many-body localisation - can arise in systems without disorder. I will also discuss experimentally relevant issues such as the effect of dissipation and how to probe the growth of entanglement and coherence in these systems.

We will discuss our trapping and cooling experiments of optically levitated nanoparticles [1]. We will report on the cooling of all translational motional degrees of freedom of a single trapped silica particle to ~1mK simultaneously at vacuum of 10^{-5} mbar using a parabolic mirror to form the optical trap. We will further report on the squeezing of a thermal motional state of the trapped particle by rapid switch of the trap frequency [2].

We will further discuss ideas to experimentally test quantum mechanics by means of collapse models [3] by both matter-wave interferometry [4] and non-interferometric methods [5]. While first experimental bounds by non-interferometric tests have been achieved during the last year by a number of different experiments [4], we at Southampton work on setting up the Nanoparticle Talbot Interferometer (NaTalI) to test the quantum superposition principle directly for one million atomic mass unit particles.

We will further discuss some ideas to probe the interplay between quantum mechanics and gravitation by (levitated) optomechanics experiments. One idea is to seek experimental evidence about the fundamentally quantum vs classical nature of gravity by using the torsional motion of a non-spherical trapped particle, while another idea is to test the existence of a gravity related shift of energy levels of the mechanical harmonic oscillator, which is predicted by semi-classical gravity (the so-called Schrdinger-Newton equation) [6].

- Vovrosh, J., M. Rashid, D. Hempston, J. Bateman, and H. Ulbricht, Controlling the Motion of a Nanoparticle Trapped in Vacuum, arXiv:1603.02917 (2016).
- Rashid, M., T. Tufarelli, J. Bateman, J. Vovrosh, D. Hempston,M. S. Kim, and H. Ulbricht, Experimental Realisation of a Thermal Squeezed State of Levitated Optomechanics, Phys. Rev. Lett. 117, 273601 (2016)
- Bassi, A., K. Lochan, S. Satin, T.P. Singh, and H. Ulbricht, Models of Wave-function Collapse, Underlying Theories, and Experimental Tests, Rev. Mod. Phys. 85, 471 - 527 (2013)
- Bateman, J., S. Nimmrichter, K. Hornberger, and H. Ulbricht, Near-field interferometry of a free-falling nanoparticle from a point-like source, Nat. Com. 5, 4788 (2014); Wan, C., et al. Free Nano-Object Ramsey Interferometry for Large Quantum Superpositions, Phys. Rev. Lett. 117, 143003 (2016).
- Bahrami, M., M. Paternostro, A. Bassi, and H. Ulbricht, Non-interferometric Test of Collapse Models in Optomechanical Systems, Phys. Rev. Lett. 112, 210404 (2014); Bera, S., B. Motwani, T.P. Singh, and H. Ulbricht, A proposal for the experimental detection of CSL induced random walk, Sci. Rep. 5, 7664 (2015).
- Grossardt, A., J. Bateman, H. Ulbricht, and A. Bassi, Optomechanical test of the Schroedinger-Newton equation, Phys. Rev. D 93, 096003 (2016).

One of the main challenges to the construction of a scalable quantum computer is the extreme sensitivity of quantum information to environmental noise and instrumental imperfections. On the theoretical front, several strategies have been proposed to mitigate errors and perform successful computations in the presence of noise. Most of these quantum error correction (QEC) strategies have been designed for incoherent noise, which typically arises from stochastic interactions with the environment. Recently, it has been pointed out by several authors that coherent errors, which arise from instrumental miscalibrations, can corrupt quantum information to a much higher degree than incoherent errors. It is not clear whether or not traditional QEC methods can be protective against this type of noise. To tackle this problem, we have devised a simulation technique to accurately measure how effective QEC is to mitigate coherent errors. We have found that QEC transforms noise from coherent to effectively semi-coherent [1]. Furthermore, ongoing work in our group suggests that subsequent levels of QEC can transform coherent noise to completely incoherent. This means that, although coherent noise can be extremely deleterious to quantum information, a clever application of QEC can reduce the noise to tolerable levels, which relaxes the stringent experimental demands needed to build a quantum computer.

[1] M. Gutierrez *et al*., Phys. Rev. A *94*, 042338 (2016)

The distinction between fundamental noise (such as that induced by effective collapse or quantum gravity models) and environmental one (due to the coupling to a well identified environment) is a challenging objective, since the two forms of noise have identical dynamical consequences.

Inspired by the notion that environmental noise is in principle observable, whilst fundamental noise would not be, we study the estimation of the diffusion parameter induced by wave function collapse models under continuous monitoring of the environment. We will thus show that monitoring can break the symmetry between the roles of fundamental and environmental noise parameters, and thus improve the upper bounds one can put on the former.

In this pedagogical talk, we will derive our result in detail from first principles, taking into account finite measurement efficiencies and, in order to quantify the advantage granted by monitoring, also analysing the quantum Fisher information associated with a diffusion parameter.

The study of light propagation through thermal atomic vapours subject to external magnetic fields is a flourishing area of research. At Durham we have spent 15 years studying the spectroscopy of alkali-metal vapours, culminating in the publication of our electric susceptibility code ElecSus (Zentile et al. Computer Physics Communications 189 162-174 (2015)). The applications range from devices (a compact optical isolator, narrow-line filters) to fundamental physics (measuring the cooperative Lamb shift, a possible new method to measure Boltzmann's constant).

In the talk I will describe experimental and theoretical work to model the electric susceptibility of a vapour of alkali-metal atoms; and finish with an outlook for cooperative quantum optics in thermal vapours.

- D. J. Whiting et al. Physical Review A 93 043854 (2016)
- M. A. Zentile et al. Optics letters 40 2000-2003 (2015)
- D. J. Whiting et al. Optics letters 40 4289-4292 (2015)
- K. A. Whittaker et al. Physical Review Letters 112 253201 (2014)
- M. A. Zentile et al. J Phys B 47 075005 (2014)
- J. Keaveney et al. Physical Review Letters 109 233001 (2012)
- L. Weller et al. J Phys B 45 215005 (2012)
- J. Keaveney et al. Physical Review Letters 108 173601 (2012)

While ultracold atoms have been very successful in probing closed equilibrium condensed matter phenomena, open non-equilibrium quantum systems have attracted a strong and growing interest recently because of their rich dynamics and nontrivial steady states. Exciton-polaritons (polaritons) have emerged as a prime candidate for the non-equilibrium system of interacting bosons.

Polaritons are mixed light-matter bosons resulting from the strong coupling of photons in a microcavity and excitons in a quantum well which can condense into macroscopically coherent many-body states. As a prime example of the non-equilibrium nature of polariton condensates, we have shown recently that polariton condensates can spontaneously magnetize, and we can control their spin optically and electronically. Interestingly, the direction of the spin of two coupled condensates can be also controllably aligned (or anti-aligned). Hence, a lattice of polariton condensates is expected to model a non-equilibrium interacting spin system with unusual properties. I will address this and show you some very interesting and surprising recent results from our labs.

In this talk, I will review the main properties of topological insulators, i.e. holographic phases of matter with an insulating bulk but conducting boundaries, characterised by a topological invariant.

I will emphasise the connection between condensed matter and lattice Gauge theory concepts, and review possible implementations in the field of atomic atomic, molecular, and optical (AMO) physics.

This connection allows us to introduce lower-dimensional 'toy' models where strong-correlation effects brought by interactions can be understood both analytic and numerically, and where cold-atom experiments can be used to test our predictions.

Cold atom experiments come in many forms, usually in large scale vacuum chambers too bulky for practical devices beyond the laboratory, but many are being translated to more compact forms known as 'atom chips'. These are microfabricated wires patterned onto a substrate through which currents are passed to form various magnetic traps for atoms. Currently these chips, which are no larger than a postage stamp, are still housed in large vacuum chambers surrounded by a vast array of optics, laser, detectors and control electronics. If Quantum Technology is to truly make a significant impact we need to focus on miniaturizing and integrating this infrastructure as well. We aim to do this not with scaled-down copies of conventional components, but with mass-producable planar fabrication techniques, developed for semiconductor devices and MEMS, which we adapt to perform the roles of large scale apparatus.

Protecting resources in the form of quantum correlations, such as entanglement and quantum discord, against environmental noise, is of paramount importance towards the development of quantum technology. In this talk, we discuss how quantum correlations can be "frozen" over time, despite their exposure to environmental noise. We show that bipartite quantum correlations, such as quantum discord and other "discord-like" quantities, can be frozen by properly tuning the initial state subjected to the noise. We discuss a possible situation where bipartite entanglement, although being more fragile than the "discord-like" quantities, can be frozen by a suitable choice of quantum many-body system. We also demonstrate that the freezing of entanglement is a scale-invariant phenomenon in specific phases of some quantum spin models.

Bringing quantum science and technology to the space frontier offers exciting prospects for both fundamental physics and applications such as long-range secure communication and space-borne quantum probes for inertial sensing with enhanced accuracy and sensitivity. But despite promising proposals to exploit the significant advantages of space quantum missions, large-scale quantum test beds in space are yet to be realised due to the high costs and lead times of traditional 'Big Space' satellite development. But the 'small space' revolution, spearheaded by the rise of nanosatellites such as CubeSats, is an opportunity to greatly accelerate the progress of quantum space missions by providing easy and affordable access to space and encouraging agile development. We review space quantum science and technology, CubeSats and their rapidly developing capabilities and how they can be used to advance quantum satellite systems.

Preserving Quantum Information (coherence) is the main technical challenge on the road to Quantum Technology. The purpose of this talk is to introduce the noise models one commonly studies, and the countermeasures against decoherence which have been brought forward in the field of quantum control. We will try to define the jargon "non-Markovian" which is commonly discussed in this context and observe some intriguing mathematical contradictions. These will lead us to suggest a test for the validity of Schrödinger's Equation.