Quantum talks archive

Browse our archive for previous talks from our quantum researchers.

7 December 2021 - Tasha Bierrum (Quantum Systems and Devices group): Towards Multiple Bose-Einstein Condensates on an Atom Chip.

Through radio-frequency dressing, a Bose-Einstein condensate (BEC) can be coherently split in two creating a beam splitter style interferometer. This technique has been applied to study BEC properties1 and Josephson junctions. In this talk I will discuss how this can be extended with multiple radio-frequencies to coherently split a single BEC into three or more BECs, with the aim of investigating relative quantum phase2. [1] Schumm, Matter-wave interferometry in a double well on an atom chip, Nature, p.57-62, 2005. [2] Leggett, Is ”relative quantum phase” transitive?, Found Phys, 25, 1, p.113-122, 1995.

23 November 2021 - Hannah Wood (Materials Physics group): Size-dependent Porosity Defines Conductivity in Liquid-exfoliated Nanosheet Networks.

Transition metal dichalcogenides such as molybdenum disulfide (MoS2) are layered materials of great interest due to their interesting thickness-dependent properties and potential for optoelectronic devices. MoS2 can be prepared in dispersion by liquid phase exfoliation, size selected by liquid cascade centrifugation and deposited by vacuum filtration, spray deposition and printing techniques. Here, the dependence of the electrical conductivity of MoS2 nanosheet networks on the average lateral size is investigated. Size-selected dispersions are characterised by statistical atomic force microscopy and UV-visible extinction spectroscopy using established metrics. We find that network conductivity increases as nanosheet size decreases, suggesting that nanosheet packing dominates over the increasing density of inter-nanosheet junctions. The conductivity in the networks of the smallest nanosheets reaches 10-3 S/m, a thousandfold increase on that of the larger nanosheets reported in the literature. This conductivity enhancement is understood in terms of nanosheet packing and porosity, enabling their development towards applications in printed optoelectronic devices. 

9 November 2021 - Graham Stutter (Ion Trap Cavity-QED and Molecular Physics group): Laser Cooling of Antihydrogen.

Performing precision laser spectroscopy on the 1S–2S transition in antihydrogen is a long-standing goal of the antimatter community and a common aim of many of the experiments based in the Antiproton Decelerator (AD) facility at CERN. Comparing this frequency to the equivalent transition in hydrogen — which has been measured to a precision of a few parts in 1015 — provides a direct test of CPT theorem, which dictates that the spectra of hydrogen and antihydrogen must be identical. In 2017, the ALPHA collaboration performed detailed spectroscopy of the 1S–2S transition in antihydrogen for the first time and found agreement with the expected hydrogen transition to a relative precision of 2x10-12. These measurements were limited by transit time broadening, a result of the velocity of antihydrogen in our magnetic minimum trap. To reduce this velocity, we have pursued Doppler cooling on the Lyman-α transition at 121nm. In this talk I will present results from these efforts and their effect on our observed 1S–2S lineshape.

26 October 2021 - Vittorio Cecconi (Emergent Photonics Laboratory): Full-wave Control of Ultrafast THz Pulses in Complex Media.

We present a theoretical investigation of broadband, spatiotemporal control of terahertz light in random media based on the nonlinear conversion of spatially modulated ultrashort pulses and time-domain field detection. 

12 October 2021 - Christophe Valahu (Ion Quantum Technology group): Robust Entanglement of Trapped Ions with an Indirect Spin-Spin Interaction.

Trapped ions placed in a static magnetic field gradient are subject to a spin-dependent force. Interestingly, this force allows the spins of two ions to couple to one another. This can be used as the basis for an entangling two-qubit gate, however the spins are very susceptible to magnetic field noise which limits the gate's fidelity. We show how to combine the spin-spin coupling interaction with dressed states, a decoherence free subspace that mitigates the effects of magnetic field noise. The resulting entangling gate is very robust to motional decoherence, which strongly alleviates experimental complexities.

28 September 2021 - Aikaterini Gialopsou (Quantum Systems and Devices group): Magnetocephalography using Optically Pumped Magnetometers (OPMs).

Magnetoencephalography (MEG) is a widely used neuroimaging technique with numerous clinical applications. Technological developments with Optically Pumped Magnetometers (OPMs) has enabled new non-invasive brain function mapping capabilities with OPM-MEG, offering improved sensor placement flexibility, and closer positioning to the scalp, compared to superconducting quantum interference devices (SQUIDs). OPMs also offer an improved spatial resolution with increased source localisation. Here we provide a detailed explanation of the OPM MEG application and the significant advantages over the SQUID MEG in neuroscience.

14 September 2021 - Chris Brown (Materials Physics group): Tuneable Synthetic Reduced Graphene Oxide Scaffolds Elicit High Levels of Three-dimensional Glioblastama Interconnectivity in Vitro.

Three-dimensional tissue scaffolds have utilised nanomaterials to great effect over the last decade. In particular, scaffold design has evolved to consider mechanical structure, morphology, chemistry, electrical properties, and of course biocompatibility – all vital to the performance of the scaffold and how successful they are in developing cell cultures. We have developed an entirely synthetic and tuneable three-dimensional scaffold of reduced graphene oxide (rGO) that shows good biocompatibility, and favourable mechanical properties as well as reasonable electrical conductivity. Importantly, the synthesis is scaleable and suitable for producing scaffolds of any desired geometry and size, and we observe a high level of biocompatibility and cell proliferation for multiple cell lines. In particular, one of the most devastating forms of malignant brain cancer, glioblastoma (GBM), grows especially well on our rGO scaffold in vitro, and without the addition of response-specific growth factors. We have observed that our scaffold elicits spontaneous formation of a high degree of intercellular connections across the GBM culture. This phenomenon is not well documented in vitro and nothing similar has been observed in synthetic scaffolds without the use of response-specific growth factors – which risk obscuring any potential phenotypic behaviour of the cells. The use of scaffolds like ours, which are not subject to the limitations of existing two-dimensional substrate technologies, provide an excellent system for further investigation into the mechanisms behind the rapid proliferation and success of cancers like GBM. These synthetic scaffolds can advance our understanding of these malignancies in the pursuit of improved theranostics against them, whilst also reducing the current reliance on animal testing.

20 July 2021 - Laura Blackburn (Ion Trap Cavity-QED and Molecular Physics group): Towards High Resolution Spectroscopy of Molecular Nitrogen Ions.

High resolution spectroscopy of molecules is a prime candidate to measure potential temporal changes in the proton-to-electron mass ratio, μ. By measuring a vibrational transition within a molecule with unparalleled precision and comparing it to an optical atomic transition, potential changes in μ can be detected. In our experiment we use N2+, which has systematic shifts even better than the currently best optical atomic clocks. To perform precision spectroscopy, a single 14N2+ ion will be co-trapped, in a linear Paul trap, with a 40Ca+ ion which will act as a frequency reference and be used for the cooling and state detection of the nitrogen ion. A vibrational Raman transition in the nitrogen ion will be compared to a quadrupole transition in the calcium ion. Prerequisite to this is the preparation of 14N2+ in a specific rovibronic state. Recently, a 2+1’ resonance-enhanced multiphoton ionisation (REMPI) scheme was developed, using the a1Σg+(ν=6) ← X1Σg+(ν=0) band in 14N2 for the resonant excitation. This scheme demonstrated a fidelity of >99% for loading into the rovibronic ground state. Simulations indicate that the high amplitude and inhomogeneous electric fields of the ion trap will broaden the ionisation threshold and prevent state-selective loading in many cases. Rapidly switching the trap off during loading can reduce the broadening and may mitigate the broadening.

6 July 2021 - Robyn Tucker (Emergent Photonics Laboratory): Video Rate Terahertz Near-Field Hot Carrier Microscopy.

Semiconductor composite devices, structured materials, and metasurfaces require intricate knowledge of ultrafast carrier dynamics to characterise their physical mechanisms. Because Terahertz (THz) radiation interacts classically with free carriers, material physics widely utilises THz time-domain spectroscopy coupled with a photo-exciting pump to cause photon absorption of carriers so that relaxation, recombination, and other complex processes can be observed with optical probing. Unfortunately the coarse THz diffraction limit restricts the resolution of the retrieved spatial morphology distribution in non-uniform media; even well-established point-scan methodologies are extremely slow and fail to resolve non-local interactions between regions of a 2D sample. This talk proposes and presents the implementation and results of a novel nonlinear, fully parallel and wide-area near field methodology, known as Optical-Pump Terahertz Near Field Microscopy (OP-TNFM). This methodology enables the mapping of hot carrier distributions on material surfaces and their characteristic dynamics with resolutions exceeding the THz diffraction limit. OP-TNFM allows for the wide-area assessment of heterogeneous systems with arbitrary excitation distributions while simultaneously providing the ultrafast time-domain evolution of THz field.

22 June 2021 - Alex Owens (Ion Quantum Technology group): Around the Bend: Linking 1D Planar Ion Traps in a 2D Array.

Linear RF ion traps have been employed to great effect intermediate scale quantum computing experiments, as they provide excellent isolation of small quantum systems from the decohering effects of the outside world. To tackle large scale computing problems with ions will require distribution of information over an array of traps, though the isolation ion traps provide makes coupling them together somewhat tricky. We approach this coupling issue by physically shuttling ions between linear traps through ‘X-junctions’ where the RF electrodes of 4 linear traps meet.  I will be talking about design considerations for traps with junctions, what makes a good or bad ion trajectory and approaches to modelling the problem along with some discussion of experimental results/bumps in the road! 

8 June 2021 - Rob Shah (Quantum Systems and Devices group): Coherence Properties of Bose Gases with Tuneable Dimensionalities.

Bose-Einstein condensation (BEC) is a quantum many-body phenomenon that is most typically realised by cooling a gas of alkali atoms down to ultracold temperatures, usually a few hundred nano-Kelvin. A stand-out feature of a BEC is it’s fully coherent nature, and they are often labelled as the matter-wave equivalent to a laser beam. This presentation describes how we produce BECs in the laboratory, and how experimentally we can manipulate the gas’ confining potential to control the dimensionality. Interestingly, as the dimensionality is reduced there is also a loss in phase coherence which we observe during imaging as the formation of an interference pattern within the atomic density, these gases are known as quasi-condensates. The coherence properties of BECs and quasi-condensates are then discussed with support from our experimental investigation.

25 May 2021 - Keiran Clifford (Materials Physics group): Size-Dependent Conductivity in Graphene Networks Enables Thermoelectric Applications.

Graphene is the most well-known and comprehensively studied two-dimensional layered material, with a remarkable and unique combination of electrical, mechanical, and thermal properties. Liquid-phase exfoliation (LPE) techniques facilitate the production of pristine graphene on a large scale at relatively low-cost. Size selection of these LPE dispersions, followed by a thermal annealing step, results in the highest reported value of electrical conductivity (1.2 x 10S m-1) of any solution-processed graphene to date, enabling applications in printed electronics and energy storage devices. Recent work on this material’s thermoelectric performance indicates that it shows promise as a low-cost, green material for thermal energy harvesting applications.

11 May 2021 - Corentin Pignot (Ion Trap Cavity-QED and Molecular Physics group): Towards Ion-photon Entanglement in the Strong Coupling Regime Within a Fibre-based Fabry-Perot Cavity.

Ions and photons are some of the most promising qubits to create a quantum network. They give the possibility to create and transfer information at the quantum level. Quantum applications such as quantum key distribution or quantum computing rely on efficient entanglement processes between qubits. In this talk, I will present one scheme to entangle a single calcium 40 ion with a single photon within an optical fibre cavity operating in the strong coupling regime. I will also present a way to measure the fidelity of the process with the use of a second mapping photon. This scheme is being implemented in one of our ion traps and some preliminary results will be shown. 

27 April 2021 - Vivek Kumar (Emergent Photonics Laboratory): Seeing Through Scattering.

Scattering of light is critical to many applications such as deep tissue biological imaging, laser surgery, geophysics and Lidar based applications. However, light scattering prevents focusing beyond a certain depth inside the medium which limits all these applications to shallow depths, so it is highly desired to break this diffusion limit and focus light deep inside the medium. In this talk, I will present spatiotemporal refocusing of THz waves following a direct measurement of propagation properties of the scattering medium.

13 April 2021 - Dr Mariam Akhtar (Ion Quantum Technology group): Towards Improving Quantum Coherence - In-situ RF Microplasmas with Energies suited to In Situ Selective Cleaning of Surface Adsorbates in Ion Microtraps.

The coherent control of trapped ions has many applications in quantum technologies from quantum information processing to quantum metrology and optical atomic clocks. However, motional decoherence due to electric field noise remains a limiting factor. A dominant source of noise is thought to be caused by surface adsorbates. The use of in-situ RF microdischarges has the potential to selectively sputter contamination. This work demonstrates a capacitively-coupled, radio-frequency (RF) microplasma inside the 3D electrode structure of an ion microtrap device. Spectroscopic analysis of the He I 667 nm and Hα 656 nm emission lines yielded the gas temperature and electron density, which enabled calculation of the mean ion bombardment energy. For He sputtering of hydrocarbon adsorbates on Au, we calculate that the high energy tail of the distribution should remove adsorbate monolayers in as little as 1 min of processing. We also calculate that the distribution is insufficiently energetic to have any significant effect on the Au electrode surface within that duration. Our results suggest that the microplasma technique is suited to in situ selective removal of surface adsorbates from ion microtrap electrodes.

30 March 2021 - Thomas Coussens (Quantum Systems and Devices group): Spin-Exchange-Relaxation-Free Optically Pumped Magnetometers and their Application.

Since the 1960s, SQUIDs have been the benchmark for sensitive magnetic field measurement. However, developments in the Spin-Exchange-Relaxation-Free (SERF) optically pumped magnetometers (OPMs) has allowed for similar, if not better sensitivity compared to SQUIDs, as well as a number of additional advantages. One particular benefit of OPMs is the reduced distance from a magnetic source to the sensor, which is of particular interest for magnetoencephalography (MEG), where OPMs are able to offer improved spatial resolution than previously possible. This talk will outline the principles of SERF magnetometry and discusses the potential benefits in both industry and medical settings, including the monitoring of currents within electric vehicle batteries.

16 March 2021 - Dr Manoj Tripathi (Materials Physics group): Regulating Graphene Wrinkles in the Vertically Stacked Hybrid Structure.

The artificial stacking of (2D) heterostructure brings remarkable applications of tuned physical, electrical and optical properties. The van der Waals interaction at heterointerface can generate desirable physics of nanomaterials that leads to the emerging field interface engineering. Some of the crucial contributions of hetero-layer devices are processed by regulating strain and doping at the interface by fabricating structural disorders at the nanoscale. Nevertheless, a critical understanding of nanoscale mechanics and interfacial vdW interaction is required to fabricate the desired architecture. In the present work, I will demonstrate the tuning of interfacial interaction between graphene-MoS2 heterostructure for enhancing the geometry of graphene wrinkles. I will discuss the role of generated strain at the interfacial region regulates shear force which is responsible for the sliding of the top layer on a solid-state lubricant, MoS2. The present work clearly shows the beginning of an era constructing nanoscopic structures by regulating fundamental forces.

2 April 2019 - Ryan Willets (Geonium Chip group): Single Microwave Photon Detection.

Ryan will start with an introduction to Penning Traps and move on to describe quantum-non-demolition measurements of multiple single microwave photons. While these measurements have been proven with larger Penning Traps, they have yet to be attempted with a scalable Penning Trap.

26 March 2019 - Tim James (Quantum Systems and Devices group): The Two Colour Magneto Optical Trap.

Tim will explore how a second cooling frequency can improve the number of atoms trapped in a magneto-optical trap (MOT). He'll explain what a "two colour" MOT is and how it differs from the "single colour" MOT. Using experimental results, he'll cover some basic theoretical ideas around the two colour MOT.

5 March 2019 - Juan Sebastian Totero Gongero (Emergent Photonics Laboratory): Complexity-driven Photonics: Collective Interactions and Spontaneous Synchronisation in Nonlinear Photonic Systems.

Complex systems are ensembles of randomly interconnected elements where mutual interactions enable unexpected dynamics and behaviours. These systems are abundant in our daily experience: typical examples are the human brain, society, biological ecosystems, and finance. In the last century, researchers from different disciplines have investigated the fundamental properties of complex systems, unveiling fascinating and counterintuitive dynamics. Due to its ultrafast time scale, nonlinear photonics has recently emerged as an ideal playground to harness these complex interactions to develop advanced devices. In this presentation, I will introduce some of the concepts underlying the field of complexity-driven photonics. In particular, I will discuss how in our laboratory we are employing the "spontaneous synchronisation" of interacting optical waves to design ultrafast pulsed lasers, quantum information sources and brain-inspired optical computing devices.

26 February 2019 - Seokjun Hong (Ion Quantum Technology group): Microfabrication of Surface Ion Trap Chips.

Seokjun will start with a brief introduction of qubit platform and the history of ion trap structures. He will move on to focus on the detailed microfabrication processes required for advanced surface ion traps.

5 February 2019 - Harry Bostock (Ion Quantum Technology group): Quantum Sensing using Trapped Ions.

Much has been seen of trapped ions as a method for quantum computing, but one exciting new application for this technology is using them for radio frequency, microwave, and static field sensing. Harry will show how trapped ions can be used as quantum sensors and how they have the potential to outclass both well-established classical sensors and other quantum sensors.