# Sussex Quantum Seminars

Some of our recent guest speakers.

### 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.

Watch Simon's talk here. Access passcode: z=GH42$d

### Dr Jesús Rubio (University of Exeter): Precision Matters - a Journey from Quantum Thermometry to the Quantum Estimation of Scales.

Whether quantum technologies are ultimately successful will crucially depend on our ability to perform extremely precise measurements. To achieve this, detailed knowledge of the ultimate precision limits allowed by nature is required. Additionally, an efficient and systematic procedure to connect theory-driven estimators with experimental data sets is highly desirable. During the first part of this talk I address both of these problems within the context of quantum thermometry, a framework for the precise measurement of temperatures in ultracold atom (and other) systems. It is shown that, in the absence of any prior knowledge, and using Bayesian principles, the ultimate precision limits for temperature estimation are necessarily expressed in terms of a logarithmic error. Moreover, this leads to an operational rule to map experimental data sets – of any size – to an optimal temperature reading. Its potential application in thermometric experiments is further illustrated by simulating energy and position measurement records. In the second part of this talk I perform a deeper analysis of the mathematics of quantum thermometry, decoupling the underlying estimation theory from its thermodynamic origin. I show that the primary assumption behind logarithmic errors – from where the general framework for quantum thermometry follows – is simply invariance under changes of scale. On the basis of this assumption, I derive a neat framework for the precise estimation of any quantity playing the role of a scale in physics. I conclude by arguing that the quantum estimation of scales completes a trio of theories for the most elementary quantities that one could possibly measure: phases, locations and scales.

Main reference: J. Rubio el al., Phys. Rev. Lett., 127:190402 (2021)

### Professor Kevin Weatherill (Durham University): Rydberg Quantum Technologies.

Rydberg atoms are highly excited atoms with exaggerated properties. In recent years, Rydberg atoms have emerged as a promising platform for numerous quantum technologies, ranging from quantum computation and simulation, single photon sources, RF communications and SI-traceable standards for electric fields. In this talk, I will explain how the properties of Rydberg atoms make them advantageous for such applications and I will present the results from two recent experiments at Durham. [1] High speed terahertz imaging in thermal atoms that achieves frame rates that are orders of magnitude faster than other terahertz sensors. [2] Collectively encoded qubits in cold-atom ensembles that demonstrate coherence properties that are robust to atom loss and electric field noise.

[1] L. A. Downes et al. Physical Review X. 10, 011027 (2020)

[2] N. L. R. Spong et al. Phys. Rev. Lett. 127, 063604 (2021)

### Professor Rene Gerritsma (Universiteit van Amsterdam): The Quantum Physics of Interacting Atoms and Ions.

In recent years, a novel field of physics and chemistry has developed in which trapped ions and ultracold atomic gases are made to interact with each other. These systems find applications in studying quantum chemistry and collisions, and a number of quantum applications are envisioned such as ultracold buffergas cooling of the trapped ion quantum computer and quantum simulation of fermion-phonon coupling. Remarkably, in spite of its importance, experiments with atom-ion mixtures remained firmly confined to the classical collision regime. This is because the electric traps used to hold the ions cause heating during atom-ion collisions. In our experiment, we overlap a cloud of ultracold 6Li atoms in a dipole trap with a 171Yb+ ion in a Paul trap. The large mass ratio of this combination allows us to suppress trap-induced heating. For the first time, we buffer gas-cooled a single Yb+ ion to temperatures close to the quantum (or s-wave) limit for 6Li-Yb+ collisions. We find significant deviations from classical predictions for the temperature dependence of the spin exchange rates in these collisions. Our results open up the possibility to study trapped atom-ion mixtures in the quantum regime for the first time. Finally, I will present our plans on a new experimental setup that we are currently building, in which we aim to manipulate the soundwave spectrum of large ion crystals using SLM-controlled optical tweezers. The resulting platform can be used for quantum simulation of quantum spin models.

### Professor Igor Lesanovski (Eberhard Karls Universität Tübingen): Constrained Dynamics and Electron-phonon Coupling in Rydberg Quantum Simulators.

Rydberg quantum simulators, i.e. highly excited atoms held in optical tweezer arrays, belong to the currently most advanced platforms for the implementation and study of strongly interacting spin systems. An interesting dynamical regime is reached when one atom that is brought to a Rydberg state facilitates the excitation of another nearby one. The resulting dynamics can be similar to that of epidemic spreading and also may form an ingredient for observing non-equilibrium phase transitions. In my talk I will discuss recent results concerning the analysis of constrained spin dynamics on Rydberg quantum simulators. In this context I will also focus on the inevitable coupling between Rydberg excitations and vibrational degrees of freedom which permit the engineering of exotic types of interaction.

### Dr Auro Perego (University of Aston): Gain-through-loss; Theory and Applications in Nonlinear Photonics.

Optical losses are in general considered to be a detrimental effect which reduces the efficiency of photonic devices and that hence must be avoided. Although this may be in many cases true, there are very relevant and counterintuitive situations where modes suffering optical losses are amplified in virtue of losses presence itself.

I will review our research on a still not well known and unexploited class of modulation instabilities caused by spectrally dependent losses in nonlinear optical systems with cubic and quadratic nonlinearity. I will show how energy dissipation can be engineered to design a new class of amplifiers and parametric oscillators operating without satisfying standard phase-matching conditions, and tuneable repetition rate optical frequency comb sources in normal dispersion Kerr resonators. The universality of the dynamical process underpinning this loss-enabled behaviour makes its observation possible even in other nonlinear systems beyond photonics.

### Andrew Groszek (Newcastle University): Large Scale Flows in Two-Dimensional Quantum Turbulence.

Non-equilibirum interacting systems can evolve to exhibit large-scale structure and order. In two-dimensional turbulent flow the seemingly random swirling motion of a fluid can evolve towards persistent large-scale vortices. To explain such behaviour, Lars Onsager proposed a statistical hydrodynamic model based on quantised vortices. Our work provides an experimental confirmation of Onsager's model. We drag a grid barrier through an oblate superfluid Bose-Einstein condensate to generate non-equilibrium distributions of quantised vortices. We observe signatures of an inverse energy cascade driven by the "evaporative heating" of these vortices, which leads to steady-state vortex configurations characterised by negative absolute temperatures. We measure these temperatures directly using our recently developed thermometry technique for two-dimensional superfluid turbulence. Complementary observations of negative temperature vortext states have also recently been presented in a similar experiment by Gauthier et al.

### Dr Rachel Godul (National Physics Laboratory): Optical Atomic Clocks for Testing Fundamental Physics.

For thousands of years, man used the rotation of the Earth as a reference for timekeeping. But the modern world requires far greater precision and the advent of atomic clocks has enabled fractional uncertainties in the realisation of the second to improve by many orders of magnitude. With ever improved accuracy in measurements of time and frequency, it also becomes possible to test fundamental physics at previously inscrutable levels. This talk will present measurements from optical clocks at the National Physics Laboratory that have been used to search for time-variation of fundamental constants and also to place constraints on couplings between Dark Matter and the fields of the Standard Model.

### Ben Sauer (Imperial College): Direct Laser Cooling of Molecules.

Direct laser cooling techniques that have been developed for atoms can also be applied to certain molecules. Physicists at Imperial College have been workng on cooling of CaF and YbF; diatomic molecules for which quasi-closed transitions can be found. Ben will review the techniques they use and discuss the challenges compared with laser cooling of atoms. He'll discuss their results and in particular how their laser cooling techniques will enable the next generation of experiments to measure the electron electric dipole moment.

### Dr Ben Lanyon (University of Innsbruck): Light-matter Entanglement over 50km of Optical Fibre.

When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology. Envisioned quantum networks use light to distribute entanglement between their remote matter-based quantum nodes. In this short talk, I will present our observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of optical fibre: two orders of magnitude further than the state of the art and a practical distance to start building large-scale quantum networks. Our methods include an efficient source of ion-photon entanglement via cavity-QED techniques (0.5 probability on-demand fibre-coupled photon from the ion) and a single photon quantum frequency converter to the 1550 nm telecom C band (0.25 fibre-coupled device efficiency). Modestly optimising and duplicating our system could allow for 100 km-spaced ion-ion entanglement at rates over 1 Hz. Our results therefore show a path to entangling remote registers of quantum-logic capable trapped-ion qubits, and the optical atomic clock transitions that they contain, spaced by hundreds of kilometres.

### Professor Rainer Blatt (University of Innsbruck): The Quantum Way of Doing Computations.

Since the mid-nineties of the 20th century, it became apparent that one of the centuries' most important technological inventions, computers in general and many of their applications could possibly be further enhanced by using operations based on quantum physics. Computations, whether they happen in our heads or with any computational device, always rely on real physical devices and processes. Data input, data representation in a memory, data manipulation using algorithms and finally, data output, require physical realizations with devices and practical procedures. Building a quantum computer then requires the implementation of quantum bits (qubits) as storage sites for quantum information, quantum registers and quantum gates for data handling and processing as well as the development of quantum algorithms. In this talk, the basic functional principle of a quantum computer will be reviewed. It will be shown how strings of trapped ions can be used to build a quantum information processor and how basic computations can be performed using quantum techniques. The quantum way of doing computations will be illustrated with analog and digital quantum simulations. Ways towards scaling the ion-trap quantum processor will be discussed.

### Ana Rakonjac (Durham University): Towards Atom Interferometry with Bright Solitary Matter Waves.

Performing interferometry with atoms instead of light holds great promise as the basis for a new generation of sensors. Bose-Einstein condensates (BECs) are excellent candidates for atom interferometry due to their wave-like nature, superfluid properties, and the ability to manipulate them coherently. Many interferometry protocols have been proposed theoretically, and there is presently much experimental effort in implementing novel interferometry schemes, as well as miniaturising atom interferometers for practical use as sensing devices. One such scheme makes use of bright solitary matter waves, or solitons, formed by manipulating interatomic interactions in BECs to create long-lived compact matter wavepackets. Solitons can propagate macroscopic distances without dispersion, making them ideal candidates for use in a number of interferometer geometries. A crucial component of any interferometer is a coherent beam splitter. For solitons, a beam splitter can be formed by a narrow repulsive barrier, where a soliton incident on the barrier is split into two. After allowing the two daughter solitons to oscillate in a weak harmonic potential, the solitons recombine on the same barrier. In the appropriate regime, the recombination is coherent, with any phase accumulation along one path resulting in a population difference in each path after recombination. In practice, there are other factors that influence the outcome. In this talk, I will discuss our efforts to implement a proof-of-principle atom interferometer using bright solitary matter waves and the experimental challenges involved.

### Brianna Heazlewood (University of Oxford): Cold Ion-neutral Reactions in Coulomb Crystals.

Studying reactions between two unstable species has been an ongoing experimental challenge. I will present our new approach to this old problem: combining a source of cold radicals with Coulomb-crystallised ions held within an ion trap. Studying ion-radical collisions in this way offers a number of advantages, including the ability to detect reactions with high sensitivity and excellent control over the reaction conditions. Preliminary studies involving charge exchange between Coulomb-crystallised ions held within a linear Paul ion trap and cold neutral molecules will be presented. For quantitative analysis, a mass-sensitive detection method is adopted – with the ejection of all ions onto an external detector at a selected time. This time-of-flight mass spectrometry (ToF-MS) approach removes ambiguity about the identities of dark ions: both the masses and relative numbers of all trapped species at the point of ejection can be ascertained directly from the ToF trace. Combining ToF-MS detection capabilities with real-time imaging of the Coulomb crystal enables one to examine both the kinetics and the dynamics of ion-neutral reactions. Our source of cold radical species, a Zeeman decelerator, will be described and the clever (in my opinion) way that we gain optimal performance will be explained. Finally, I will present our designs for a combined Zeeman decelerator-ion trap apparatus, and discuss our progress to date.

### Vera Schafer (University of Oxford): Fast Entangling Gates with Trapped Ions.

Trapped ion qubits are one of the most promising candidates for scalable quantum computing. Entangling gates with trapped ions achieve higher fidelities than in any other system, but are typically performed in an adiabatic regime, where the motional frequencies of the ions in the trap limit the gate speed. Many schemes have been proposed to overcome these limitations, but have only now been successfully implemented. We use amplitude-shaped cw-pulses to perform entangling gates significantly faster than the speed limit for conventional gate mechanisms. At these gate speeds, the motional modes are not spectrally isolated, leading to entanglement with both motional modes sensitively depending on the optical phase of the control fields. We demonstrate gates with fidelity F = 99.8% in 1.6 µs - over an order of magnitude faster than previous trapped ion gates of smiliar fidelity. We also perform entanglement generation for gate times as short as 480 ns - this is below a single motional period of the ions.

### Martina Knoop (CNRS/Université d'Aix-Marseille): Precision Spectroscopy with Large Ion Clouds.

Trapped ions are at the heart of many spectacular advances in recent years, in particular regarding high-resolution spectroscopy. They serve as elementary bricks in atomic clocks, quantum information, and the verification of the variation of fundemental constants. Due to the advanced control of the internal and external degrees of freedom of stored ions, they are perfectly adapted to be used as model systems for quantum and classical problems. Laser cooling and individual addressing, detection, and manipulation of single atoms have paved the way to a large variety of important applications and fundamental experiments. I will present some key experiments based on recent work that is done in my group in particular with large ion clouds. I will insist on the recent realisation of a three-photon coherent population trapping protocol. I will discuss precision and robustness of this technique and also the technical aspect of phase-locking three independent lasers by means of an optical frequency comb.