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Sussex Quantum Seminars archive

Browse our archive for previous seminars from our guest speakers.

Dr Jesús Rubio (5XÉçÇøÊÓƵ 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 5XÉçÇøÊÓƵ): 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 (5XÉçÇøÊÓƵ 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 5XÉçÇøÊÓƵ): 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 (5XÉçÇøÊÓƵ 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 (5XÉçÇøÊÓƵ 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 5XÉçÇøÊÓƵ): 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 (5XÉçÇøÊÓƵ 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 (5XÉçÇøÊÓƵ 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.