Sussex Quantum Seminars

Some of our recent guest speakers.

Dr Thomas Walker (Imperial College, Blackett Laboratory): Feshbach Resonances between Single Ions and Ultracold Atomic Gasses.

The study of ultracold atomic and molecular gases has impacts ranging from cold quantum chemistry to condensed matter physics and beyond. Feshbach resonances have been a key tool in ultracold gases of neutral atoms for several decades, allowing control over the atomic interactions and the formation of cold molecules. However, rf-induced heating in ion traps has kept them out of reach in hybrid atom-ion experiments. Recently, the ultracold regime has been reached for the first time in atom-ion systems through a combination of technical improvement and choice of species. In this talk, I will report on the observation of magnetic Feshbach resonances between a single Ba ion and an ultracold cloud of Li through the enhancement both of sympathetic cooling via elastic two-body collisions and of three-body recombination. Further, I will present recent results on the behaviour of these resonances at varying collision energies.  

Dr Aidan Arnold (University of Strathclyde): The Silver Anniversary of a Rather Cool Time.

On this day 25 years ago, in a lab at the University of Sussex, we made the coldest place in the universe for the first time in the UK - a Bose-Einstein condensate, as part of my PhD thesis. I will give a short history of my time then, and also describe what I'm up to now, in particular looking at enhanced optical geometries for atomic quantum technologies.

Dr Carlo Sias (Italian Metrological Institute): Orientational Melting of a Two-dimensional Ensemble of Charged Particles.

A system of confined charged particles undergoes crystallization at sufficiently low temperature, forming self-organized structures in which each particle is spatially localized. However, when particles in a two-dimensional plane are confined by an isotropic potential, there is no preferential orientation of the crystal, and thermal fluctuations lead to the delocalization of particles in circular trajectories. This is the so-called orientational melting of the crystal, in which the particles remain localized radially and delocalized in the angular direction. Orientational melting of a mesoscopic crystal is a change of configuration that is similar to a phase transition in a macroscopic system, but it is not universal as it depends on the specific properties of the system, e.g. the exact number of particles.

In the seminar I will report on the experimental observation and characterization of orientational melting in a two-dimensional crystal of trapped Ba⁺ ions..The specific geometry of our trap makes it possible to continuously change the arrangement of the ions from a one-dimensional string to a two-dimensional crystal while keeping the ions always in a two-dimensional plane. We observe that orientational melting occurs under conditions that strongly depend on the number of particles, and find excellent agreement with the results of a Monte Carlo simulation, which we use to estimate the temperature of the ions at which melting occurs. Additionally, we are able to locally inhibit melting by adding a single impurity with a different mass. Interestingly, for a sufficiently large number of ions two or more concentric rings are populated, and the rings can exhibit independent dynamics.

Our experiment paves the way to accessing quantum regimes for delocalized strongly-interacting particles, and in particular for the coherent control of the rotational state of the ions by entangling it to the ions’ internal state.

Professor Hartmut Grote (Cardiff University): Quantum-Enhanced Interferometry for New Physics.

Laser interferometry has recently revolutionized astronomy by introducing a new sense in the observation of the universe. We can now hear the ripples of space-time: gravitational waves.
In this talk I will give an overview of how ultra-precise laser interferometers can also be used to try to shed light on other mysteries of the universe: dark matter and the question of whether space-time is quantized at the smallest level.

Professor Hendrik Ulbricht (University of Southampton): Testing Fundamental Physics by Levitated Mechanics.

I will discuss our experiments on Levitated mechanics based on optical, Meissner and Paul trapping nano- and micro-particles in vacuum. Experiments hold promise for testing the quantum superposition principle interferometrically and non-interferometrically, will explain how that works and where we are with experiments, and emphasise the role of noise in this context. Experiments are pushed in a parameter regime where both quantum states, such as superpositions, and gravity effects are possible to be generated and detected. That low energy regime is expected to provide a new test area for experiments into the overlap of quantum mechanics and gravity in the non-relativistic regime.

Dr Joseph Goodwin (University of Oxford): Ion-cavity Node Engineering for Scalable Networked Quantum Computing.

Networked architectures provide a route to freely-scalable quantum computation with trapped ions, with entanglement between ions in remote nodes mediated by the coherent production, interference and projective measurement of single photons. Two-node networks have provided proof-of-principle demonstrations but have been limited in the rate of entanglement achieved, and many further hurdles remain on the route to large-scale networked quantum computation.To approach equivalency with local, phonon-mediated entangling operations, network photons must be generated near-deterministically and at high rate, most readily achieved via integration of a high-finesse optical cavity. The use of microcavities to enhance photon production has long been recognised as essential to success of scalable quantum networks, but the development of systems of sufficiently high performance and reliability remains a considerable challenge.

In Oxford, we are tackling this problem from multiple angles. On the one hand, we are working to improve the precision and reliability of the engineering of our traps, cavities and other subsystems. In parallel we aim to improve the intrinsic robustness of the photon production and entanglement schemes utilised, to minimise the detrimental impact of residual imperfections in system manufacture and environmental control. 

In this talk, I will review a series of investigations intended to elucidate the relationship between current cavity engineering principles and their impact on quantum network performance. The phenomena considered include spatial mode hybridisation in modestly misaligned cavities, polarisation oscillation induced by cavity birefringence and temporal mixing due to spontaneous scattering in the photon production process. I will present the conclusions which have motivated our adopted architecture and engineering approaches, and describe methods we have developed for the overall design optimisation of cavity systems for quantum networking applications.I will then illustrate our technical progress towards realising our proposed architecture, providing illustrative examples of the engineering efforts underway across our group. Here, I will focus on a selection of the approaches we are applying in the development of scalable and reliable ion trap system design, touching on methods for complex 3D trap microfabrication, UHV-integrated signal delivery and high-efficiency compact atomic sources.

Dr James Gates (University of Southampton): Developing Components for Quantum Technology Platforms at Southampton.

Quantum Technology has posed a range of unique challenges, from material science to quantum error correction and verification. Our group in Southampton has worked with many of the world's leading QT researchers to develop components and fabrication techniques to solve some of these challenges. This talk will overview some of our current work on photonic, superconducting, ion and atom trap QT platforms. Topics will include integrated holographic components for photonic delivery to atom chips, ultra-precision machining of vacuum systems and superconducting chips, and parametric generation modules for atom trap control and single-photon sources. The talk will also provide a glimpse into some of these components' commercialisation and field deployment.

Dr Peter Horak (University of Southampton): Optical Micro-Resonators with Non-Spherical Mirrors for Quantum Applications.

For many applications in quantum technology or optical sensing, strong coupling between light and small particles, or even single atoms, is highly desirable. Fabry-Perot optical resonators formed between spherical micro-mirrors create field enhancement and are routinely exploited for strong coupling of light to a particle. However, the enhancement factor is limited by geometrical restrictions: if the field is focused too strongly, the beam divergence increases and light may be lost from the resonator. We investigate new designs of such micro-resonators where the shape of the mirrors is optimized to create interference patterns inside the resonator that lead to high peak intensities at the position of the particle. We use a range of approaches, such as analytical theory, evolutionary algorithms, and machine learning, to find the best designs. Our results suggest that significant field enhancement is possible with mirror shapes that deviate only moderately from spherical shapes. These can be fabricated by modern microfabrication tools and could give rise to more precise optical sensors and faster quantum information processors.

Professor Vlakto Vedral (University of Oxford): Quantum Physics in the Macro Domain.

Many macroscopic phenomena rely on the laws of quantum physics. The solid state physics, for instance, started with the realization that both electrons and vibrations have to be treated quantum mechanically to even begin to be able to understand the thermodynamical behavior of many-body systems. A growing body of evidence now suggests that living systems too could be utilising quantum coherence, superpositions, and even, in some cases, quantum entanglement to perform some tasks with higher efficiency. However, it is an exciting open question to what degree quantum effects can be maintained and controlled at the macroscopic level. This is interesting not just for our quest to realise scalable quantum computers, but also for engineering special-purpose programmable nano-machines.

I will explain the basics of witnessing entanglement and I will put this into the context of our present understanding of macroscopic quantum phenomena. I will then present the single molecule spectroscopy experiments we are currently undertaking in our laboratory to obtain a better understanding of quantum effects in complex (bio)molecules. This will include our recent observation of the vacuum Rabi splitting in a living bacterium strongly coupled with the electromagnetic field as well as creating a hybrid superconducting-tardigrade qubit. I will also discuss how these experiments can be scaled-up, as well as how we can design artificial and hybrid biomimetic structures that capture the underlying fundamental quantum behavior of complex systems. Gravity may well be the only remaining frontier. Will quantum physics ultimately be superseded in the macro domain, or will it prove to be a universal description of all the known phenomena?

Professor Fernando L. Semiao (Federal University of ABC - Brazil): Pulse Design for Population Control in Qubits under Dephasing and Thermal Noise.

The manipulation of qubits by time-dependent fields has become a key element in quantum technologies. Despite that fact that this is a problem which dates back to the investigations on nuclear magnetic resonance in the 40's, the growing interest in quantum information during the last decades has posed new challenging questions. The manipulation of single atoms and the role of the environment on individual quantum systems are representative examples. In this talk, I will present an analytical description of the laser pulse which solves a constrained dynamics where the initial and final populations of the qubit levels are fixed a priori. This constrained dynamics is sometimes impossible, and the conditions for success are precisely identified. One of the main results is the presentation of analytical conditions for the establishment of steady states with finite coherence in the presence of dephasing and thermal noise, what might naturally find applications in quantum memories.

Professor Giacomo Scalari (Institute for Quantum Electronics at ETH Zurich): High Performance On-chip THz Frequency Combs and Detectors Based on Quantum Cascade Lasers.

Recently, THz on-chip quantum-cascade-laser (QCL) sources reached operating temperatures above 200 K, unlocking the use of compact thermoelectric coolers and offering the possibility of filling the lack sources in a spectral region were no table compact source is available. In this seminar, I will present a review of the work done at ETH on THz quantum cascade lasers operating at high temperatures and THz quantum cascade frequency comb sources. Pulsed operation up to 210 K for two-well structures at 4 THz  were obtained, enabling the QCL to be operated on a thermoelectric cooler in combination with a room temperature detector.

Quantum cascade lasers are emerging as well as powerful and compact sources for frequency comb generation in the mid-IR and THz. I will discuss THz combs with spectral coverage in excess of 1 THz centered at 3 THz and operating at 80 K . Particularly, I will present a new platform for planarized THz photonics that includes THz combs and also THz detectors exploiting regenerative amplification, together with dispersion compensated ring lasers for soliton formation. SWIFTS measurements allowing the temporal reconstruction of the THz waveforms will be presented, showing a transition from FM to AM mode-clocking as a function of RF driving.

Dr Giulia Marcucci (Apoha Ltd, London): Learning at the Edge of Chaos - the Inner Link Between Complexity, Nonlinear Waves, and Neuromorphic Computing.

Nonlinear waves' historical role in developing the science of complexity and their physical feature of being widespread in optics and hydrodynamics establish a bridge between two diverse but fundamental fields: nonlinear physics and computational science. Such a link has been opening an endless number of new research routes. Many relevant results on nonlinear waves in photonics and acoustics have assumed major significance in the foundation of new computing models.

In this seminar, I will first report my work on the control of complex nonlinear regimes through topological invariants in photorefractive crystals. Such analysis represents a groundwork for enabling nonlinear waves to do computation, a feature that arises efficiently only when the wave reservoir is at the edge of chaos. Indeed, when waves are both highly nonlinear and controllable, the wave reservoir has two essential properties: given two distinct but similar inputs, their outputs are always distinguishable but never divergent. To demonstrate it, I will present my last works on the engineering of neuromorphic computers, by wave reservoirs.

Professor Claudio Paoloni (Lancaster University): High Capacity Sub-THz Wireless Networks Enabled by Travelling- Wave Tubes.

The substantial increase of internet traffic forecasted in 5G and 6G exceeds the capacity of the actual wireless networks at sub-6 GHz and low millimetre waves. The sub-THz spectrum (90 – 300 GHz) has very wide frequency bands to support tens of gigabit per second (Gb/s) not yet exploited due the high path loss, rain attenuation and not mature technology. The low transmission power from solid state amplifiers at those frequencies is not sufficient to ensure long range with high signal-to-noise ratio for supporting high data rate. This is even more critical in case of distribution in point to multipoint by a low gain antenna. Sub-THz Traveling Wave Tubes (TWTs) have been demonstrated as a promising solution to generate transmission power at Watt level, suitable for enabling long range high capacity links.

The talk will present the latest result of a wireless system with D-band (141 – 174.8 GHz) point to multipoint and point to point distribution and G-band (275 – 305 GHz) point to point transport. A discussion at system and component level of sub-THz transmission hubs, terminals and TWTs will highlight the technology challenges of sub-THz spectrum.

Dr Giovanni Barontini (University of Birmingham): QSNET - A Network of Clocks for Measuring the Stability of Fundamental Constants.

I will discuss the QSNET project, that aims to build a network of atomic and molecular clocks in the UK to achieve unprecedented sensitivity in testing variations of the fine structure constant, α, and the electron-to-proton mass ratio, μ. This in turn will allow us to either discover that fundamental constants are actually not constant, or to provide more stringent constraints on a wide range of fundamental and phenomenological "new physics" models. These include models of dark energy, ultra-light dark matter and grand unification models. The project currently includes the National Physical Laboratory, the University of Sussex, the Imperial College London, the University of Birmingham, and several international partners. I will discuss more in detail the plans of the Birmingham node, where we are building a clock based on highly charged ions of Californium, that is expected to improve our sensitivity to variations of α by orders of magnitude.

Dr François Leo (Université Libre de Bruxelles): Temporal Solitons in Coherently Driven Active Cavities.

Temporal dissipative solitons are short optical pulses that propagate indefinitely in a resonator. They have been extensively investigated in passive resonators and lasers. In my talk, I will discuss soliton formation in a hybrid system which consists in a coherently driven active cavity pumped below the lasing threshold. I will show how this novel system allows for the formation of high power, stable soliton trains. This also opens novel avenues for the investigation of new types of solitons such as parametrically driven solitons which have the potential to be used as spins in Ising machines. I will also discuss how solitons can be harnessed for analogs of quantum effects such as Bloch oscillations.

Professor Simon Cornish (Durham University): Adventures in Quantum Science with Ultracold Polar Molecules.

Ultracold Polar molecules offer a new platform for the simulation of many-body quantum systems with long-range interactions, utilizing the electrostatic interaction between their electric dipole moments and the rich internal structure associated with the molecular rotation. Realizing long-lived, trapped samples of molecules with full quantum control of the molecular internal state is an essential first step towards building such a quantum simulator. In this talk, I will relate some of the adventures we have had en route to this ambitious goal.

I will first explain how we create ultracold gases of RbCs molecules from a mixed species atomic gas using magnetoassociation on a Feshbach resonance followed by optical transfer using stimulated Raman adiabatic passage. We then use precision microwave spectroscopy of the rotational transition to probe the rich hyperfine structure of the molecule and exploit coherent Rabi oscillations to transfer the total population of molecules between hyperfine levels. We subsequently investigate the AC Stark effect due to the trapping light in low-lying rotational levels and reveal a rich energy structure with many avoided crossings between hyperfine states. Understanding this structure allows us to trap the molecules in a range of internal states and to enhance the rotational coherence through a judicious choice of internal state and intensity. We use this capability to study the collisional lifetimes of the trapped molecules for various rotational and hyperfine states, shedding light on the sticky collision issue. Finally, I will report some recent work on engineering robust storage qubits using hyperfine states of the molecule where we observed coherence times > 6.9 s using Ramsey interferometry. As an outlook, I will outline our plans for implementing magic-wavelength optical traps to achieve similar coherence times for rotation-state superpositions and will describe new experiments to image and address single molecules in ordered arrays as a basis for quantum simulation.