Videos

Thermodynamics has shed light on engines, efficiency, and time’s arrow since the Industrial Revolution. But the steam engines that powered the Industrial Revolution were large and classical. Much of today’s technology and experiments are small-scale, quantum, far from equilibrium, and processing information. Nineteenth-century thermodynamics needs re-envisioning for the 21st century. Guidance has come from the mathematical toolkit of quantum information theory. Applying quantum information theory to thermodynamics sheds light on fundamental questions (e.g., how does entanglement spread during quantum thermalization? How can we distinguish quantum heat from quantum work?) and practicalities (e.g., quantum engines and the thermodynamic value of coherences). I will overview how quantum information theory is being used to revolutionize thermodynamics in quantum steampunk, named for the steampunk genre of literature, art, and cinema that juxtaposes futuristic technologies with 19th-century settings.

Implementation of advanced Quantum Technologies might benefit from the remarkable quantum properties shown by molecular spin systems based on the coordination bond. The versatility of the molecular approach combined with rational design has recently boosted the operativity temperature of molecules acting as bits of memory, otherwise known as Single-Molecule Magnets, or the coherence time of molecular spin qubits. The richness and tunability of the spectrum of spin levels make them particularly suitable for quantum error correction, while spin-spin interaction can be tuned to realize quantum gates and quantum simulators. Molecules can also be processed to be deposited on surfaces, allowing the realization of hybrid nanostructures. However, the molecular approach also poses key challenges, requiring for instance to overcome limitations such as those induced by low energy vibrational modes typical of molecular lattices. This drawback can be in part overcome by chemical design. Achieving the control of a single molecule is also challenging, requiring to couple the electric field, which can be confined at the molecular scale, with the spin degrees of freedom of the molecule. Initialization of spin systems is also an issue because only at very low temperatures does the Zeeman interaction overcome the thermal energy. Learning from nature, we are proposing to exploit chirality, and in particular spin selectivity in electron transfer processes through chiral structures, as an alternative way to spin polarize molecular systems. An overview of our recent results will be provided.

Quantum computers hold the promise of solving computational problems which are intractable using conventional methods. For fault-tolerant operation quantum computers must correct errors occurring due to unavoidable decoherence and limited control accuracy. Here, we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors. Using 17 physical qubits in a superconducting circuit we encode quantum information in a distance-three logical qubit building up on recent distance-two error detection experiments. In an error correction cycle taking only 1.1 µs, we demonstrate the preservation of four cardinal states of the logical qubit. Repeatedly executing the cycle, we measure and decode both bit- and phase-flip error syndromes using a minimum-weight perfect-matching algorithm in an error-model-free approach and apply corrections in postprocessing. We find a low logical error probability of 3 % per cycle when rejecting experimental runs in which leakage is detected. The measured characteristics of our device agree well with a numerical model. Our demonstration of repeated, fast, and high-performance quantum error correction cycles, together with recent advances in ion traps, support our understanding that fault-tolerant quantum computation will be practically realizable.

In 1998 astronomers discovered that the expansion of the universe is accelerating. Somehow, something must have made gravity repulsive on cosmological scales. This something was called dark energy; it is described by Einstein’s cosmological constant; and it amounts to about 70% of the total mass of the universe. It has been conjectured that the cosmological constant is a form of vacuum energy, but its prediction from quantum field theory has failed by many orders of magnitude. The lecture shows how a theory informed by empirical evidence on Casimir forces does produce the correct order of magnitude and agrees with astronomical data, and how subtle this is.

Individually trapped neutral atoms provide a promising platform to engineer quantum many-body systems in a controlled, bottom-up approach. They can be readily manipulated in large numbers and interact strongly when excited to Rydberg states. In this talk I will give an overview over the physics of these Rydberg atom arrays and discuss several phenomena that can be observed in these systems, as well as applications for quantum information processing. First, I will discuss many-body phenomena in and out of equilibrium. In equilibrium, Rydberg atom arrays allow to access quantum critical phenomena at transitions between disordered and ordered phases that break various spatial symmetries, and even topologically ordered states. Out of equilibrium, novel quantum many-body phenomena, such as quantum many-body scars can be observed in these systems. In the second part of this talk, I will show how these phenomena connect to quantum computation, specifically how quantum optimization algorithms can be implemented in current experiments with Rydberg atom arrays.

Scale invariance, a concept first introduced in high energy physics, has found many applications in the physics of quantum gases and fluids. It applies to Fermi gases in the unitary regime, to two-dimensional Bose gases, at least within a classical field description, and -- in a discrete version -- to the Efimov effect. In this presentation, we will describe some illustrations of this scale invariance, starting with stationary situations such as the derivation of an equation of state or the existence of solitons. We will then move on to time-dependent problems, for which scale invariance can be incorporated in a more general symmetry, conformal invariance. We will show how the "breathing mode", an emblematic consequence of this symmetry, appears and we will finally discuss the existence of breathers, specific structures with periodic time evolution, which are also consequences of conformal invariance.

Small quantum networks consisting of several nodes sharing entangled states are within reach with current and near-term technologies. They offer new possibilities for quantum information processing beyond what achievable in standard point-to-point configurations. In this talk, quantum networks are considered in the device-independent scenario where devices are seen as quantum back boxes processing classical information. We first show how the characterization of correlations in quantum networks is related to the study of causal networks. We then present several results illustrating the possibilities these networks offer in the foundations of quantum physics or for the development of quantum information technologies. In the first case, we show how real quantum theory can be falsified in a small network consisting of three observers in an entanglement swapping configuration. In the second, we discuss a proposal for the implementation long-distance device-independent quantum key distribution.

The structure of atoms, molecules and solids is mainly determined by static Coulomb forces, while the coupling to the quantized degrees of freedom of the electromagnetic field plays only a secondary role. Recently, it has been speculated that this general rule can be overcome in the context of cavity quantum electrodynamics (QED), where the coupling of dipoles to a single field mode can exceed the bare energy of the photon itself. Under these conditions, light-matter interactions become non-perturbative, as characterized by an effective finestructure constant of order unity. In this seminar I will give a basic introduction to this extreme coupling regime of cavity QED and explain how vacuum-induced many-body effects can lead to novel ground state phases in QED, which are the opposite of what has been assumed so far. Beyond a purely fundamental interest, these general mechanisms can be important for potential applications, ranging from cavity-assisted chemistry to quantum technologies based on ultrastrongly coupled circuit QED systems.

Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in high-performance computing and simulation of quantum systems, as well as in sensor technology. They can shift the boundaries of today’s systems and devices beyond classical limits and seemingly fundamental limitations. The use of complex photonic systems, which comprise multiple optical modes as well as nonclassical light, has been proposed for various quantum applications over the last decades and illustrate the versatility of photonic systems. However, their implementation often requires advanced setups of high complexity, which poses considerable challenges on the experimental side. Here we present three differing approaches to advance current experimental approaches for multi-dimensional photonic quantum systems: non-linear integrated quantum optics, pulsed temporal modes and time-multiplexing. Non-linear integrated quantum devices with multiple channels enable the combinations of different functionalities, such as sources and fast electro- optic modulations, on a single compact monolithic structure. Pulsed photon temporal modes are defined as field orthogonal superposition states and can constitute a high dimensional quantum system. They occupy only a single spatial mode and thus they can be efficiently used in single-mode fibre communication networks. Finally, time-multiplexed quantum walks are a versatile tool for the implementation of a highly flexible simulation platform with many modes and dynamic control of the underlying graph structures and coherence properties of the quantum network.

In this talk, I will give an overview of current efforts to employ classical machine learning for quantum technologies. Recent years have seen machine learning techniques like deep neural networks revolutionize many fields of science and technology. Since about 2016, they are being applied increasingly with success to challenges for quantum technologies. Examples include the use of neural networks for interpreting measurement results, generation of novel experimental setups, and deep reinforcement learning for discovering strategies in quantum control and feedback or for optimizing quantum circuits.

In September 2015 the twin `Advanced LIGO' observatories enabled the first direct detection of gravitational waves from astrophysical sources. The waves detected originated from the collision and merger of two black holes 1.3 billion light years from earth. This detection marked the start of new field of gravitational astrophysics, in the 100th anniversary year of Einstein's General Theory of Relativity. The observatories operate by using laser interferometry to search for the tiny changes in the relative positions of the suspended optics in a Michelson-type interferometer, induced by the passage of gravitational waves. This talk will describe the technologies developed to reach the threshold of first detection and outline the research directions being pursued to increase the operating sensitivity of the detectors - where quantum effects form an important noise source in the instruments.

Optically active spins in solids are often considered prime candidates for scalable and feasible quantum-optical devices. Numerous material platforms including diamond, semiconductors, and atomically thin 2d materials are investigated, where each platform brings their own advantages along with their challenges. Semiconductor quantum dots are the current state-of-the-art for optical properties such as tuneability, brightness and indistinguishability. Their nickname "artificial atom" was coined historically to highlight how similar they can be to isolated single atoms, but in fact they are far from the realisation of a simple two-level system. The inherently mesoscopic nature of a quantum dot leads to a multitude of dynamics between spins, charges, vibrations, and light. In particular, it offers a unique realisation of a tripartite interface between light, a single proxy qubit (electron spin) and an isolated spin ensemble (nuclei). Ability to control these constituents and their mutual interactions creates opportunities to realise an optically controllable ensemble of ~50,000 spins. In this talk, I will present the two-decade journey from treating the quantum dot nuclei as noise to the observation of their collective magnon modes and eventually to their tuneable quantum correlations, all witnessed via a single electron spin driven by light.

We develop a variational quantum algorithm to solve partial differential equations (PDE's) using a space-efficient variational ansatz that merges structured quantum circuits for coarse-graining with Fourier-based interpolation. Variational circuits represent symmetrical smooth functions as the ansatz, and we combine them with different classical optimizers that differ on the gradient calculation: no gradient, numerical gradient and analytic gradient. As benchmark, we show the results for the computation of the ground state of the one-dimensional quantum harmonic oscillator. In idealized quantum computers, the infidelity is of order 10^{−5} with 3 qubits, and these fidelities can be approached in real noisy quantum computers, either directly or through error mitigation techniques. However, we find that the precision is sub-par with other classical methods, suggesting the need for better strategies in the optimization and the evaluation of the cost function itself.

In an attempt to provide further insight into one of the major questions of physics beyond the standard model, highly sensitive optomechanical sensors are developed utilizing techniques from the field of atomic physics. These sensors are table-top experimental tools offering exquisite control of mechanical, rotational and electrical degrees of freedom of an optically levitated ~ng mass in vacuum, enabling unprecedented acceleration and force sensitivities for these mass scales. I will present two recent searches, the first looking for recoils from passing DM particles and the second for deviations from charge neutrality and so-called "millicharged particles". For certain, well-motivated dark matter models, these searches exceed the sensitivity of even large-scale experiments, thereby offering a complementary approach.

Optical atomic clocks which are primarily limited by the quantum projection noise can reach stability of 10^{-19} in about one hour. By Hamiltonian engineering, optical clocks with quantum entangled states can reach better stability in a shorter time, thus opening the possibility to survey physics at higher frequency range. I will report the progress of our effort on quantum-enhanced optical lattice clocks. With cavity feedback squeezing and coherent optical state transfer, we demonstrate entanglement on optical clock transition in Yb-171. We achieve a metrological improvement of 4.4 dB over the standard quantum limit (SQL). Recently, we performed a signal amplification through time-reversed interaction (SATIN) protocol achieving the largest sensitivity improvement beyond SQL in any interferometer to date at 11.8(5) dB between the ground state magnetic sublevels. Together with the future effort in improving the local oscillator laser stability, we are one step closer to the fully operational quantum-enhanced optical atomic clock.

Quantum computers promise to efficiently solve not only problems believed to be intractable for classical computers, but also problems for which verifying the solution is also considered intractable. This raises the question of how one can check whether quantum computers are indeed producing correct results. This task, known as quantum verification, has been highlighted as a significant challenge on the road to scalable quantum computing technology. We review the most significant approaches to quantum verification and comparethem in terms of structure, complexity and required resources. We also comment on the use of cryptographic techniques which, for many of the presented protocols, has proven extremely useful in performing verification. Finally, we discuss issues related to fault tolerance, experimental implementations and the outlook for future protocols.

Today, matter-wave interferometers as clocks and gravimeters allow for precision measurements of time and gravity at unprecedented level. In all these sensors, the exquisite control of both internal (electronic) and external (center of mass motion) degrees of freedom of ultra-cold atomic samples, enable us to study interactions at their most basic, quantum level, paving the way for new tests of fundamental physics. For all these applications, novel interferometric schemes based on narrow intercombination transitions of alkali-earth (alkali-earth like) atoms, has recently gained attention. In this talk, I'll review some of the most recent results obtained by our group with strontium and cadmium atoms. Furthermore, novel spin-squeezing techniques with direct application to narrow transitions will also be discussed.

Quantum computing promises to solve seemingly intractable problems. In the medium term, both optimization and chemistry will likely be the only impactful applications for this novel technology. In this talk, an overview of quantum computing in industry with a focus on optimization is presented. It is argued that quantum optimization machines are still superseded by classical approaches on CMOS for the foreseeable future. However, problem modeling as well as physics-inspired algorithms present new, potentially transformative ways to solve hard combinatorial optimization problem across industries. This is illustrated with a new hybrid algorithm for combinatorial optimization problems.

Quantum technologies have the potential to improve in an unprecedented way the security and efficiency of communications in network infrastructures. We discuss the current landscape in quantum communication and cryptography, and focus in particular on recent photonic implementations, using encoding in discrete or continuous properties of light, of central quantum network protocols, enabling secret key distribution, verification of multiparty entanglement and transactions of quantum money, with security guarantees impossible to achieve with only classical resources. We also describe current challenges in this field and our efforts towards the miniaturization of the developed photonic systems, their integration into telecommunication network infrastructures, including with satellite links, as well as the practical demonstration of novel protocols featuring a quantum advantage for a wide range of tasks. These advances enrich the resources and applications of the emerging quantum networks that will play a central role in the context of future global-scale quantum-safe communications.

The tremendous scientific opportunities presented by ultracold molecules have driven rapid progress in both the assembly of diatomic molecules from ultracold atoms and the direct cooling of diatomic and polyatomic molecules. Diatomic species have been magneto-optically trapped and sub-Doppler cooled and their collisions have been studied in several experimental systems, including magnetic traps and merged optical tweezers. The pioneering work that led advances in direct cooling began with Stark deceleration, buffer-gas cooling and loading of traps, buffer-gas beam sources, mechanical slowing, and a variety of electromagnetic trapping and cooling mechanisms. As the field of cold and ultracold molecules has grown, polyatomic molecules have attracted new focus as potential novel quantum resources that have distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, all polyatomic molecules have long-lived states arising from nuclear motion with angular momentum about the internuclear axis. These states exhibit linear, Debye-level Stark shifts at very low applied electric fields and offer distinct Stark-shifted level structures that are absent in laser-coolable diatomic molecules. These and other features in polyatomic molecules can be applied to quantum simulation, fundamental symmetry tests, searches for dark matter, and particle physics beyond the Standard Model, potentially at the 1 PeV scale. Generic classes of polyatomic molecules have been identified as amenable to laser cooling into the ultracold (~1 μK) regime. One class is that of metal oxide radicals (MOR), which includes linear, symmetric top, and asymmetric top species. In this talk I will discuss some of the past experiments that brought us to this point, and the challenges and scientific opportunities with the laser cooling of polyatomic molecules. Results on SrOH, YbOH, CaOH and CaOCH3 will be discussed, as well as preliminary work on more complex species.

The quantum optical control of solid-state mechanical devices, quantum optomechanics, has emerged as a new frontier of light-matter interactions. Objects currently under investigation cover a mass range of more than 17 orders of magnitude - from nanomechanical waveguides to macroscopic, kilogram-weight mirrors of gravitational wave detectors. Extending this approach to levitated solids opens up complete new ways of coherently controlling the motion of massive quantum objects in engineerable potential landscapes. I will discuss recent experimental advances in quantum controlling levitated solids, including demonstrations of the motional quantum ground state of optically trapped nanoparticles in a room temperature environment. I will also discuss the perspective for such experiments to create superpositions of gravitational source masses. This addresses directly one of the outstanding questions at the interface between quantum physics and gravity, namely “what is the gravitational field generated by a quantum object?”.

In Fall 2019, a team at Google made the first-ever claim of "quantum computational supremacy"---that is, a clear quantum speedup over a classical computer for some task---using a 53-qubit programmable superconducting chip called Sycamore. In Fall 2020, a group at USTC in China made a claim of quantum supremacy, using "BosonSampling" (a proposal by me and Alex Arkhipov in 2011) with 50-70 photons in an optical network. In addition to engineering, these accomplishments built on a decade of research in quantum complexity theory. This talk will discuss questions like: what exactly were the contrived computational problems that were solved? How does one verify the outputs using a classical computer? And how sure are we that the problems are indeed classically hard? I'll end with a proposed application for these sampling based quantum supremacy experiments that I've been working on: namely, the generation of certified random bits, for use (for example) in proof of-stake cryptocurrencies.

In 1976 Kroger and Reich established the existence of a low-lying, nuclear excited state in Th-229 that appeared to be accessible with laser technology. This discovery touched off a flurry of activity to perform laser spectroscopy of a nucleus, as a laser-accessible nuclear transition would provide a host of new technological and scientific applications. Despite this significant activity, to date, no one has succeeded in actually performing laser spectroscopy on the nucleus. I will discuss our efforts to do exactly this.

Light-matter interaction at the nanometer scale lies at the heart of elementary optical processes such as absorption, emission or scattering. Over the past two decades, we have realized a series of experiments to investigate the interaction of single photons, single molecules and single nanoparticles. In this presentation, I discuss recent studies, where we reach unity efficiency in the coupling of single photons to single molecules and describe our efforts to exploit this for the realization of polaritonic states involving a controlled number of molecules and photons. Furthermore, I report on efforts to exploit the phononic degrees of freedom of molecules and their environment as a quantum resource.

This Seminar is a fast journey through the build-up of quantum thermodynamics, an emerging field at the crossroad between quantum information, quantum open systems and stochastic thermodynamics. Born at the time of industrial revolution to optimize the exploitation of thermal resources, the concepts of thermodynamics have been adapted to small systems where thermal fluctuations are predominant. Extending the framework to quantum fluctuations is a great challenge of quantum thermodynamics, that opens exciting research lines e.g. measurement fueled engines or thermodynamics of driven-dissipative systems. On a more applied side, it provides the tools to optimize the energetic consumption of future quantum computers.

We address the challenge of bringing solid-state quantum emitters all to the same resonant frequency within one chip. The study of cooperative effects, and the possibility to scale up quantum photonic technologies depend on such ability. Here, we demonstrate optical frequency tuning of individual, lifetime-limited molecules by few hundred times their natural linewidth. The effect persists even after the pump laser is switched off, and is attributed to a local Stark shift associated with optically induced long-lived charge-separated states. The experimental observation is applied to independently tune five close-by molecules into resonance within twice their line-width.

The building of quantum networks is stimulating the development of multiple physical platforms and different types of encodings in a heterogeneous structure allowing full functionality. Central to this endeavour is the capability to distribute and interconnect optical entangled states relying on different discrete and continuous quantum variables. Here, we report an entanglement swapping protocol involving single-photon entanglement and hybrid entanglement between particle- and wave-like optical qubits and demonstrate the creation of hybrid entanglement heralded by a specific Bell-state measurement. This ability opens up the prospect of connecting heterogeneous nodes of a network, with the promise of increased integration and functionalities.

In this project, we explore the combination of sub-wavelength, two-dimensional atomic arrays, and Rydberg interactions as a powerful platform to realize strong, coherent interactions between individual photons with high fidelity. In particular, the spatial ordering of the atoms guarantees efficient atom-light interactions without the possibility of scattering light into unwanted directions, for example, allowing the array to act as a perfect mirror for individual photons. In turn, Rydberg interactions enable single photons to alter the optical response of the array within a potentially large blockade radius R_b, which can effectively punch a large "hole" for subsequent photons. We show that such a system enables a coherent photon-photon gate or switch, with an error scaling that is significantly better than the best-known scaling in a disordered ensemble.

An amazing level of quantum control is routinely reached in modern experiments with atoms, but similar control over molecules has been an elusive goal. A method based on quantum logic spectroscopy [1] can address this challenge for a wide class of molecular ions [2,3]. We have now realized the basic elements of this proposal. In our demonstration, we trap a calcium ion together with a calcium hydride ion (CaH+) that is a convenient stand-in for more general molecular ions. We laser-cool the two-ion crystal to its motional ground state and then drive Raman-transitions in the molecular ion, where a transition in the molecule also deposits a single quantum of excitation in the motion of the ion pair (motional "sidebands"). We can efficiently detect this single quantum of excitation with the calcium ion, which projects the molecule into the final state of the sideband transition, a known, pure quantum state. The molecule can be coherently manipulated after the projection, and its resulting state read out by another quantum logic state detection. We demonstrate this by driving Rabi oscillations between different rotational states [4, 5] and by entangling the molecular ion with the logic ion [6]. All transitions in the molecule are either driven by a single, far off-resonant continuous-wave laser or by a far-off-resonant frequency comb. This makes the approach suitable for quantum control and precision measurement of a large class of molecular ions.

The past decades has seen dramatic progress in our ability to manipulate and coherently control the motion of atoms. Although the duality between wave and particle has been well tested since de Broglie introduced the matter-wave analog of the optical wavelength in 1924, manipulating atoms at a level of coherence allowing for precision measurement has only become possible thanks with our ability to produce atomic samples of few microdegrees above absolute zero. Since the initial experiments many decades ago, the field of coherent atom optics has grown in many directions. This progress has both fundamental and applied significance. The exquisite control of matter waves offers the prospect of a new generation of force sensors of unprecedented sensitivity and accuracy, from applications in navigation and geophysics, to tests of general relativity or study of highly-entangled quantum states. The spectacular sensitivity or matter-wave interferometers can be used for very precise measurements. It is for example possible to measure the acceleration of gravity with an accuracy of 1 part per billion, the rotation of the Earth with an accuracy better than 1 millidegree per hour and detect minute changes in gravity caused by mass displacements. These devices are so precise that they are used today as reference for fundamental constants (mass, gravity), and are powerful candidates to test general relativity on ground, underground or in space. Projects are currently ongoing to verify the universality of free fall or to detect gravitational waves in a frequency range yet unreachable with current detectors. Nevertheless the future of matter-wave inertial sensors goes far beyond lab-based inertial sensors. While these experiments are typically quite large, require a dedicated laboratory, and are designed to operate well only in environments where the temperature, humidity, acoustic noise is tightly constrained, many efforts have been put in designing compact, robust and mobile sensors. The development of this technology led to a new generation of atomic sensors that have been operated in airplanes and in rockets, that are commercially available and could be the next generation of navigation unit.

The dream of the quantum engineer is to have an arbitrary waveform generator for designing quantum states and Hamiltonians. Motivated by this vision, I will report on advances in optical control of long-range interactions among cold atoms. By coupling atoms to light in an optical resonator, we engineer tunable non-local Heisenberg interactions, with applications to protecting spin coherence and to generating correlated atom pairs. I will present recent results on optically programming the graph of spin-spin couplings and the resulting correlations in an array of atomic ensembles. Our scheme allows an arbitrary magnon dispersion relation to be specified via the modulation waveform of a control laser, opening the door to studies of frustrated magnetism and enabling explorations of quantum spin dynamics in exotic geometries and topologies. I will also touch on a complementary approach of Rydberg dressing for optical control of local interactions, and discuss prospects in quantum simulation, quantum optimization, and quantum metrology.

A system of ultracold atoms in an optical lattice is an ideal quantum simulator of a strongly correlated quantum many-body system [1]. Ultracold fermions with an enlarged spin symmetry of SU(N) offer novel possibilities of quantum simulation [2]. In particular, recent theories for an SU(N) Fermi-Hubbard model predict novel quantum magnetisms. I will present our study of an SU(N=6) Fermi-Hubbard model by working with ultracold two- electron atoms of ytterbium. By developing an all-optical means of singlet-triplet oscillation, we successfully detect nearest- neighbor spin correlations in various lattice geometries. Importantly, this enlarged spin symmetry of SU(N) is a powerful tool to lower the temperature of atoms in an optical lattice, known as a Pomeranchuk cooling effect. The detailed comparison between theory and experiment allows us to infer the realization of a lowest temperature of cold-atom Fermi-Hubbard model in one dimension [3]. I will also present the experiments for realizing a novel four-body entangled state of SU(4)-singlet in a plaquette lattice configuration and the quantum magnetism in an open dissipative Fermi-Hubbard system.

Variational quantum algorithms stand as the most promising approaches towards practical applications of near-term quantum computers. However, these methodologies usually require a large number of measurements, which represents an important roadblock for future real-world applications. We introduce a novel approach to tackle this problem: a variational measurement scheme. We present an algorithm that optimises informationally complete POVMs on-the-fly in order to minimise the statistical fluctuations in the estimation of relevant cost functions. We use it in combination with the Variational Quantum Eigensolver to calculate ground-state energies of molecular Hamiltonians and show that it is competitive with state-of-the-art measurement reduction approaches. Our computational experiments further suggest a measurement scaling exponent below 2. We also highlight the potential of the informational completeness of the measurement outcomes by reusing the ground-state energy estimation data to perform reduced state tomography with high fidelity on the XX model spin chain.

It is thought that the rapid expansion of the early universe resulted in the spontaneous production of cosmological particles. The latter evolved into the patterns in the cosmic microwave background visible today. The analogue of cosmological particle creation in a quantum fluid could provide insight, but an observation was not achieved previously. This talk presents our observation of analogue cosmological particle creation in a 3-dimensional quantum fluid of light. The process is seen to be spontaneous, and in close quantitative agreement with the quantum-field theoretical prediction. We find that the long-wavelength particles provide a window to early times, and we apply this principle to the cosmic microwave background. This work introduces a new quantum fluid, as cold as an atomic Bose-Einstein condensate.

Ultracold quantum gases are excellent platforms for quantum simulation and sensing. So far these gases have been produced using time-sequential cooling stages and after creation they unfortunately decay through unavoidable loss processes. This limits what can be done with them. For example it becomes impossible to extract a continuous-wave atom laser, which has promising applications for precision measurement through atom interferometry [1]. In the first part of this talk I will present how we create continuous-wave BECs, BECs that persist in a steady-state for as long as we desire. Atom loss is compensated by feeding fresh atoms from a continuously replenished thermal source into the BEC by Bose-stimulated gain [2]. The only step missing to create the long-sought continuous-wave atom laser is the addition of a coherent atom outcoupling mechanism. In addition this BEC may give us access to interesting driven-dissipative quantum phenomena over unprecedented timescales. The techniques we developed to create the continuous source of thermal atoms are also nicely suited to tackle another challenge: the creation of a continuously operating superradiant clock [3]. These clocks promise to become more rugged or more short-term stable than traditional optical clocks, thereby opening new application areas. In the second part of my talk I will present how we are developing two types of superradiant clocks within the European Quantum Flagship consortium iqClock. The first operates on a kHz-wide transition of Sr [4] and the other on the mHz-narrow Sr clock transition

Collisions between cold molecules are essential for studying fundamental aspects of quantum chemistry, and may enable formation of quantum degenerate molecular matter by evaporative cooling. However, collisions between trapped, naturally occurring molecules have so far eluded direct observation due to the low collision rates of dilute samples. I will present our experiment where we directly observed collisions between cold, trapped molecules, achieved without the need of laser cooling. We magnetically capture molecular oxygen in a 0.8K x kB deep superconducting trap, and set bounds on the ratio between the elastic and inelastic scattering rates, the key parameter determining the feasibility of evaporative cooling. We further co-trap and identify collisions between atoms and molecules, paving the way to studies of cold interspecies collisions in a magnetic trap.

Boson sampling is a computational problem that has been proposed as a candidate to obtain an unequivocal quantum computational advantage. The problem consists in sampling from the output distribution of indistinguishable bosons in a linear interferometer. There is strong evidence that such an experiment is hard to classically simulate, but it is naturally solved by dedicated photonic quantum hardware, comprising single photons, linear evolution, and photodetection. This prospect has stimulated much effort resulting in the experimental implementation of progressively larger devices. We will review recent advances in photonic boson sampling, describing both the technological improvements achieved and the future challenges. We will discuss recent proposals and implementations of variants of the original problem, theoretical issues occurring when imperfections are considered, and advances in the development of suitable techniques for validation of boson sampling experiments.

Quantum effects can alter the dynamic and static properties of non-equilibrium systems. For example, near phase transitions they may influence emergent critical behaviour [1]. An example is the so-called contact process, which is a toy model for the spreading of an infection. This classical non-equilibrium system features a phase transition between a state where in the long-time limit the population is healthy and another state where the infection becomes endemic. Synthetic quantum systems, which are realisable with current quantum simulator platforms, permit the implementation of quantum versions of this non-equilibrium process. For example, the quantum contact process [2]- where infection spreading is coherent and not probabilistic - displays a phase transition just like its classical counterpart, but with modified critical behavior.
In my talk I will discuss how quantum generalisations of cellular automata [3] may serve as a platform that permits to systematically explore the impact that quantum effects have on non-equilibrium processes. These synthetic systems - whose entire space-time structure is encoded in a single quantum state - can be implemented on current Rydberg quantum simulators and allow a systematic inclusion of quantum effects, such as entanglement. In some limit, these systems map onto the so-called Domany-Kinzel cellular automata [4] and thereby establish a natural link to a classical non-equilibrium setting.

The assumption that physical systems relax to a stationary state in the long-time limit underpins statistical physics and much of our intuitive understanding of scientific phenomena. For isolated systems, this follows from the eigenstate thermalization hypothesis. When an environment is present the expectation is that all of phase space is explored, eventually leading to stationarity.
In this talk, we will identify and discuss simple and generic conditions for dissipation to prevent a quantum many-body system from ever reaching a stationary state [1]. We go beyond dissipative quantum state engineering approaches towards controllable long-time non-stationary dynamics typically associated with macroscopic complex systems. The resulting coherent and oscillatory evolution constitutes a dissipative version of a quantum time-crystal.
We will show how such dissipative dynamics can be engineered and studied with fermionic ultracold atoms in optical lattices using current technology. We discuss how dissipation leads to long-range quantum coherence, complexity, and η-pairing indicating a superfluid state in these setups [2] and the potential connection to driving induced superconductivity [3].

Ultracold molecules hold promise for various quantum science applications that could utilize their long-range dipole-dipole interactions and rich internal structures. Trapping and manipulating molecules in optical tweezers offer many advantages due to their high level of controllability. In this talk, I will discuss our recent results in forming a single NaCs molecule in its rovibrational ground state starting from a pair of atoms in an optical tweezer. This is achieved by first magnetoassociating into a Feshbach molecule and then applying a Raman pulse which gives rise to coherent Rabi oscillations between the ground state. Our work opens up exciting possibilities with fully quantum-state-controlled molecules in optical tweezer arrays.

Quantum chemical simulation is a challenging task for classical computers due to the rapid growth of information with system size. Quantum computing techniques may alleviate this issue, but to date demonstrations have been limited to static properties of small systems. We show how analog quantum simulation can be used to study chemical dynamics governed by vibronic coupling Hamiltonians. Our approach uses an optimal, linear mapping of vibrational modes and electronic states of the molecule onto bosonic modes and internal states of the simulator. In addition, we show how our approach readily extends to large, open-quantum systems, all using existing technology.

Nonlocality in networks with independent entanglement sources has only been experimentally verified in simple tripartite networks, via the violation of bilocality inequalities. Here, by using a scalable photonic platform, we implement star-shaped quantum networks consisting of up to five distant nodes and four independent entanglement sources. We exploit this platform to violate the chained n-locality inequality and thus witness, in a device-independent way, the emergence of nonlocal correlations among the nodes of the implemented networks. These results open new perspectives for quantum information processing applications.

Quasicrystals are an intriguing form of condensed matter that is not periodic, but nonetheless long-range ordered. They can be described as self-similar, fractal structures containing more than one type of unit cell, similarly to the celebrated Penrose tiling. Despite them being long-range ordered like a crystal, many foundational concepts of periodic systems such as Bloch waves or Brillouin zones are not applicable. This peculiar situation enables new physics including fractal band structures, many-body localization, phasonic degrees of freedom, and an intriguing direct link to higher dimensions.
I will present our experimental realization of an eightfold symmetric optical quasicrystal for ultracold atoms and demonstrate how matterwave diffraction directly reveals the self-similar fractal nature of this potential and realizes continuous quantum walks in up to four synthetic dimensions. I will also present the localization transition in these potentials and close with an outlook on realizing many-body localization and the so-far elusive 2D Bose glass in these potentials.

The distribution of entanglement between the nodes of a quantum network will allow new advances e.g. in long distance quantum communication, distributed quantum computing and quantum sensing. On the ground, quantum information can be distributed across the nodes using single photons at telecommunication wavelengths traveling in optical fibers. To bridge distances much longer than the fiber attenuation length, quantum repeaters can be used. The nodes of a quantum network are matter systems that should efficiently interact with quantum light, allow entanglement with photons (ideally at telecommunication wavelengths) and serve as a quantum memory allowing long-lived and faithful storage of (entangled) quantum bits. In addition, for efficient distribution of entanglement, the nodes should allow multiplexed operation and ideally enable quantum processing capabilities between stored qubits. While several candidates are actively investigated, none so far regroups all the desired functionalities, highlighting the need for hybrid quantum networks connecting disparate quantum nodes with complementary capabilities. In this talk, after introducing the context I will describe our recent progress towards the realization of quantum repeater nodes with multiplexed ensemble-based quantum memories, using cryogenically cooled rare-earth ion doped solids and laser-cooled cold atomic gases. I will also describe our efforts to distribute quantum information between disparate quantum nodes and to scale up quantum network links including light-matter and matter-matter entanglement experiments. In particular, I will report a recent experiment demonstrating entanglement between remote multiplexed solid-state quantum memories, heralded by a telecom photon. Finally, I will explain our current work to build quantum processing nodes using e.g. single rare-earth ions in microcavities or ensembles of cold Rydberg atoms.

The Gamma Factory (GF) is an ambitious proposal developed as part of the CERN Physics Beyond Colliders program to operate the LHC as a novel kind of a light source that can produce up to ~10¹⁷ photons per second with energies of up to ~400 MeV. The key idea is to scatter laser photons off of partially stripped ultrarelativistic ions. The GF would enable a number of hitherto impossible experiments and can be thought of not only as a unique photon source, but also as a giant ion trap for precision physics. The first step in practical realization of the GF is a full-featured proof-of-principle experiment at the SPS that is currently being considered by CERN.

Rydberg atoms has emerged as a versatile platform for different applications in quantum technology from computing and simulation to sensing and imaging [1]. In this talk, I will focus on one of the many Rydberg projects in Durham. In our Rydberg quantum optics experiment, we store optical photons in a cold atomic ensemble in the form of Rydberg polaritons. In recent work, we have looked at the potential of Rydberg polaritons in the context of quantum information. We show that Rydberg polaritons have a number of attractive features. In particular, the combined atomic and photonic character of the polariton [2] allows coherent mapping between static and flying qubits, the large dipole moments between Rydberg states enables fast single qubit rotations, and as the quantum information is shared amongst many atoms, there is an in-built robustness to atom loss [3]. Finally, the robustness of collectively-encoded Rydberg qubits to environmental noise is considered.

Circuit quantum electrodynamics, in which microwave cavity modes are coupled to “artificial atoms” realized with Josephson junction qubits, has allowed for a variety of investigations in quantum optics and quantum information. In recent years, our team at Yale has focused on a hardware efficient approach, where high-Q microwave cavities serve as quantum memories. When dispersively coupled to transmon qubits, quite complex non-classical states can be created in these cavities, and operations between cavities can be enacted through parametric driving or other means. For instance, we have recently shown high-quality cavity-cavity swaps via a beam-splitter or conversion operation, single and two-mode squeezing, and engineered cross and self Kerr interactions. Finally, one can perform strong projective measurements of the photon number, the photon parity, or indeed any other binary-valued operator within the multi-dimensional Hilbert space. This system therefore has all of capabilities of linear optical systems, but with the addition of deterministic state preparation, measurement, and nonlinear interactions. One way to employ these capabilities is to directly simulate problems which are “naturally” bosonic in nature, to calculate vibronic spectra, Franck-Condon factors, or nonlinear molecular dynamics. I will present preliminary work towards using this platform as a novel but programmable simulator for small molecules.

During the last ten years, a fast-growing scientific activity has been devoted to understanding, controlling and using the dynamical degrees of freedom of levitated solid-state objects in vacuum. Levitated nanoparticles can move, rotate and fall. They also have solid-state internal degrees of freedom. At the interface between the fields of cavity optomechanics and atomic and molecular optics, the rich physics of levitated nanoparticles provide unique opportunities for applied and fundamental research. In this talk, we will first motivate and review the research activities in this field. We will then focus on two recent results in our theory group. First, we will discuss how the center-of-mass motion of a micromagnet can be coupled to its internal acoustic phononic modes. This offers a new cooling method and a tool to probe and control the internal physics of an isolated micromagnet. Then, we will discuss how to optimally control the harmonic potential of a levitated nanoparticle to quantum-delocalize its center-of-mass motional state. This has applications for testing quantum mechanics at large scales, enhancing sensitivities of force sensors, and boosting the entangling rate of two weakly interacting nanoparticles.

Control refers to the ability to steer a dynamical system using external fields; quantum control does so by exploiting quantum coherence, i.e., the wave nature of matter. One way to think of it is in terms of constructive and destructive interference between different quantum pathways, all connecting the same initial and final states. I will illustrate the concept of pathway interference using the photoionization of chiral molecules, i.e., molecules with a left-handed or right-handed nuclear scaffold, as example. The ionizing field may be tailored to minimize or maximize the signature of molecular handedness in the photoelectron spectrum, using interference between pathways probing different intermediate states.
The essential elements of quantum physics, quantum coherence and entanglement, are not only the agents of quantum control, they are also at the core of emerging quantum technologies such as quantum-enhanced sensing or quantum information processing. I will discuss how quantum control allows to identify fundamental performance bounds and derive protocols to reach these performance bounds in realistic models for basic building blocks of quantum-enhanced sensing and quantum information processing.

The extraordinary advances in quantum control of matter and light have been transformative for precision measurements enabling probes of the most basic laws of Nature to gain fundamental understanding of the physical Universe. Exceptional versatility, inventiveness, and rapid development of precision experiments supported by continuous technological advances and improved theory give a high chance for paradigm-shifting discovery. The development of atomic clocks with systematic uncertainties in the 10^-18 range enables searches for the variation of fundamental constants, dark matter, and violations of Lorentz invariance. I will give an overview of dark matter searches with clocks including prospects for significantly improved sensitivity with highly charged ions and a nuclear clock. At the end, I will also introduce our new online portal for high-precision atomic data and computations.

Gauge theories are the cornerstone of our understanding of fundamental interactions among particles. Their properties are often probed in dynamical experiments, such as those performed at ion colliders and high-intensity laser facilities. Describing the evolution of these strongly coupled systems is a formidable challenge for classical computers, and represents one of the key open quests for quantum simulation approaches to particle physics phenomena. In our work, we show how recent experiments done on Rydberg atom chains naturally realize the real-time dynamics of a lattice gauge theory at system sizes at the boundary of classical computational methods.

Understanding the behavior of an impurity strongly-interacting with a Fermi sea is a long-standing challenge in many-body physics. For a single impurity, a first-order transition is predicted between polaronic quasiparticle and dressed molecular ground states. However, at realistic conditions, the fate of this transition is still unknown. We study imbalanced ultracold Fermi gas with a novel high-sensitivity Raman spectroscopy technique to isolate the quasiparticle contribution and extract physical parameters. We observe a reduction of the quasiparticle weight with increasing interaction and support this finding with a theoretical model of thermally occupied quasiparticles, revealing a smooth transition with a coexistence region.

With quantum computers on the brink of outperforming their classical counterpart, it becomes increasingly important to find ways of verifying that these devices indeed perform as advertised. We approach this task by challenging independent quantum processors with seemingly random sampling problems of different sizes, which are linked through the principles of measurement-based quantum computing. This enables efficient cross-checking of multiple quantum processors, as well as internal consistency checks of individual quantum processors. The protocol is hardware agnostic and requires no classical simulation as we demonstrate using five state-of-the-art quantum processors.

Perfect diamond is transparent for visible light but there are famous diamonds, such as the famous Oppenheim Blue or the Pink Panther worth ten's of millions of dollar, which have intense colour. An important source of colour in diamond are lattice defects which emit and absorb light at optical frequencies and may indeed possess a non-vanishing ground state electronic spin. Unlike atomic physics systems, which are operated under extreme conditions to ensure isolation from their environment, colour centers in diamond are by their very nature in direct contact with the environment that is constituted by uncontrolled phonon, spin and charge degrees of freedom that form part of the host material. As a result they suffer from environmental noise. In order to address this challenge a combination of material science to improve the hardware properties and of quantum control methods to further decouple the solid state qubits from their environment while maintaining desired interactions that are needed to achieve the ultimate goal of practically usable coherent quantum dynamics. In this lecture I will explore the physics of one of these defects, the nitrogen vacancy center, and show how we can manipulate its electronic spin to develop quantum simulators, nanoscale quantum sensors and sources of nuclear hyperpolarisation. Applications of such devices range from sensing in biology to medical imaging.

According to textbook quantum mechanics, a measurement of the position of an object imposes a random quantum back action kick to its momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. As a consequence, and in accordance with the Heisenberg uncertainty principle, this leads to the so-called standard quantum limit on the precision of sensing of position, forces and fields. In this talk I will present the ideas and experimental results for measurement of motion of a mechanical oscillator with, in principle, unlimited precision. This is achieved by measuring the motion in a special reference frame with an effective negative mass. Entanglement of the system of interest and the reference frame leads to cancellation of the quantum back action of the measurement which is the origin of the standard quantum limit. Applications to force sensing, and gravitational wave detection will be discussed.

Dipolar interactions are fundamentally different from the usual van der Waals forces in real gases. Besides the anisotropy the dipolar interaction is nonlocal and as such allows for self organized structure formation [1]. Similar to the Rosensweig instability in classical magnetic ferrofluids self-organized structure formation was expected. However on the mean-field level such a transition is instable due to the diluteness of the gaseous sample. In contrast to these predictions in 2015 we could observe the formation of a stable droplet crystal and found that this unexpected stability is due to beyond mean-field quantum corrections of the Lee-Huang-Yang type. When arranged in a 1D array also phase coherence between the droplets was observed, which was first evidence for a supersolid state of matter. Upon crossing the transition to the dipolar supersolid a Goldstone mode appears, which we have observed. The existence of this mode proofs the superfluid stiffness or the so-called phase rigidity of this new state of matter. Recently we have also measured the static structure factor across the transition which allows to show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, and that the supersolid state supports both superfluid as well as crystal phonons. A recent review on the discovery of quantum dropets and dipolar supersolids can be found in Ref. [2]. At the end of the talk I will quickly introduce a new approach to quantum gas microscopy making use of a pulsed ion microscope with resolution below 200 nm, very fast time resolution, large field of view and depth of field and the possibility to image in 3D [3].

Quantum optical systems with cold atoms and ions provides one of the best ways to build controllable quantum many-body systems as quantum computers and quantum simulators. Here we report on recent developments in building, and in particular programming quantum simulators based on trapped ions as intermediate scale quantum devices. Our discussion will focus on hybrid classical-quantum scenarios: here the quantum part is the generation of highly entangled states on the quantum device in quench dynamics, which is combined with a classical post processing of measurement data, possibly run in a feedback loop with the quantum device. Examples highlighting these developments include the implementation of self-verifying variational quantum simulations, illustrated here by computing the ground state and quantum phase transition of a Schwinger Model as 1D QED. In addition, we develop and demonstrate a ‘randomized measurement toolbox’, allowing to access in experiments quantities like Renyi entanglement entropies and — as ongoing research — measure the entanglement spectrum, i.e. ‘seeing the Schmidt decomposition live’ in quench dynamics from an initial product state towards thermodynamic equilibrium.

We will discuss the recent advances involving programmable, coherent manipulation of quantum many-body systems using atom arrays excited into Rydberg states. Specifically, we will describe our recent technical upgrades that now allow the control over 200 atoms in two-dimensional arrays. Recent progress involving the realization of exotic states of matter, exploration of their non-equilibrium dynamics as well as realization and testing quantum optimization algorithms using such systems will be discussed.

Precision spectroscopy is a driving force for the development of our physical understanding. However, only few atomic and molecular systems of interest have been accessible for precision spectroscopy in the past, since they miss a suitable transition for laser cooling and internal state detection. This restriction can be overcome in trapped ions through quantum logic spectroscopy [1]. Coherent laser manipulation originally developed in the context of quantum information processing with trapped ions allows us to combine the special spectroscopic properties of one ion species (spectroscopy ion) with the excellent control over another species (logic or cooling ion). The logic ion provides sympathetic cooling and is used to control and read out the internal state of the spectroscopy ion. In my talk I will provide an overview of different implementations and applications of quantum logic spectroscopy to investigate previously inaccessible species such as molecular ions [2] and highly charged ions [3]. Spectroscopy of these species may reveal physics beyond the standard model, such as new force carriers or scalar fields that are dark matter candidates and could induce a variation of fundamental constants, or appear as nonlinearities in isotope shift spectroscopy.

Computational problems may be solved by realizing physics systems that can simulate them. Here we present a new system of up to >1000 coupled lasers that is used to solve difficult computational tasks. The well-controlled dissipative coupling anneals the lasers into a stable phase-locked state with minimal loss, that can be mapped on different computational minimization problems. We demonstrate this ability for simulating XY spin systems and finding their ground state, for phase retrieval, for imaging through scattering medium and more.

More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream is a reality in laboratories around the world. This has become possible by using complex experimental setups of thousands of optical elements, which allow atoms to be cooled to Nanokelvin temperatures, where they almost come to rest. Recent experiments with quantum gas microscopes allow for an unprecedented view and control of such artificial quantum matter in new parameter regimes and with new probes. In our quantum gas microscope experiments, we can detect both charge and spin degrees of freedom simultaneously, thereby gaining maximum information on the intricate interplay between the two in the paradigmatic Hubbard model. In my talk, I will show how we can reveal hidden magnetic order, directly image individual magnetic polarons or probe the fractionalisation of spin and charge in dynamical experiments. For the first time we thereby have access to directly probe non-local ‘hidden’ correlation properties of quantum matter and to explore its real space resolved dynamical features also far from equilibrium.

Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Therefore, using controllable cold atomic systems to shed light on the organizing principles and universal behaviors of dynamical quantum matter is highly appealing. One emerging paradigm is the dynamical phase transition (DPT) characterized by the existence of a long-time-average order parameter that distinguishes two non-equilibrium phases. I will report the observation of a DPT in two different but complementary systems: a trapped quantum degenerate Fermi gas and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.

This talk will present our effort to control and use the dipole-dipole interactions between cold atoms in order to implement spin Hamiltonians useful for quantum simulation of condensed matter situations [1]. We trap individual atoms in arrays of optical tweezers separated by few micrometers. We create almost arbitrary geometries of the arrays with unit filling in two and three dimensions up to about 70 atoms. To make the atoms interact, we either excite them to Rydberg states or induce optical dipoles with a near-resonance laser.
We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipole-dipole interaction. This interaction realizes the XY spin model and leads to the hopping a spin excitation from a site to another. We use this interaction to study elementary excitations in a dimerized spin chain featuring topological properties (Su-Schrieffer-Heeger model). We have observed the edge states in the topological condition. We probed the regime beyond the linear response by adding several excitations, which act as hard-core bosons [2].
With optical dipoles, we explore light scattering in one dimensional chains of atoms. This system realizes a dissipative spin model, which could find applications in quantum optics to generate optical non-linearities and non-classical states of light [3].

An ultrafast laser emits vastly multimode light over a broad spectral band, a.k.a. the optical frequency comb (OFC), but the emission happens but one photon at a time, if in a stimulated manner, and no entanglement is created in the light. Changing the gain medium from linear (one-photon) to nonlinear (two-photon) yields an optical parametric oscillator which features massively multipartite entanglement of the OFC modes, as demonstrated experimentally by our group and others. This entanglement can then be exquisitely tailored to cluster states with specific graphs, in particular the two-dimensional ones that are universal for measurement-based, one-way quantum computing. It is worth noting that this requires only sparse experimental resources that are highly compatible with integrated optics, thereby paving the way to the realization of practical quantum computers.

The metric system began with the French revolution, with the lofty ideal that measurements would be tied to the size of the earth, universally available to all. Soon, practical considerations required units of length and mass based on unique physical artifacts, a near-antithesis to universal availability. Now we are experiencing the greatest revolution in measurement since the French revolution, a revolution rooted in the atomic and quantum view of nature, again offering universal availability. The definitions of the kilogram, ampere, kelvin, and mole were all changed on 20 May 2019, and are now based on chosen and fixed values for Planck’s constant, the quantum of electric charge, Boltzmann’s constant, and Avogadro’s number. I will explain how this is possible, why it was necessary, and speculate about future changes in the SI. In this context I will also discuss the role of precision measurement in the history and future of quantum physics.

The interaction of a single-mode light field with a single atom or an ensemble of atoms can be described by a simple Hamiltonian and has been extensively studied. Nonetheless, the vector properties of light in conjunctions with the multilevel structure of real atoms and their collective response result in rich and surprising physics. In our group, we investigate this subject matter using nanophotonic components, such as subwavelength-diameter optical fibers and whispering-gallery-mode resonators, for interfacing light and atoms. I will present three effects that we observed in experiments with these systems and that go beyond the standard description of light-matter coupling. First, transversally confined light can locally carry transverse spin angular momentum, which leads to propagation direction-dependent emission and absorption of light. Second, when imaging an elliptically polarized emitter with a perfectly focused, aberration-free imaging system, its apparent position differs from the actual position. Third, an ensemble of atoms can change the photons statistics of light transmitted through the ensemble. There, depending on the number of coupled atoms, a collectively enhanced nonlinearity leads to pronounced photon bunching or anti-bunching.

Since the beginning of the millennium, physicists know how to generate pulses of light of attosecond duration [1], thus gaining access to this incredibly short time scale. In this presentation, we will show how attosecond pulses bring new light on ultrafast electron dynamics in atomic photoionization. We use attosecond pulse trains together with a weak infrared probe to measure both amplitude and phase of photoionization matrix elements. Our method, which combines high temporal and spectral resolution [2], allows us to gain new insights on photoionization dynamics, including electron correlation and spin flip induced by spin-orbit interaction. In another experiment, we characterize an electron wavepacket near an autoionizing resonance in helium using a Wigner representation [3], and retrieve the corresponding time-dependent density matrix.

Quantum degenerate gases of polar molecules provide a new platform for quantum science [1]. A Fermi gas of KRb molecules is fully thermalized with atom-molecule interactions and characterized using thermometry based on suppressed density fluctuations [2]. To demonstrate the full potential of strong dipolar interactions in the molecular gas, we apply external electric fields to explore the exciting interplay between molecular interaction dynamics and dissipation. By confining KRb to two dimensional optical traps with a perpendicular electric field [3], we demonstrate greatly enhanced elastic collisions with strong suppression of inelastic loss, with their ratio reaching 100. The favorable 2D dipolar interactions have led to rapid thermalization and evaporation of molecules.

Measuring the transport properties of a system connected to reservoirs is one of the most common and most useful probe of the properties of a solid. Besides its practical interest transport in quantum systems poses fundamental and challenging theoretical questions, since it is one of the simplest realizations of an out of equilibrium phenomenon. I will review these issues, in particular in the case of one- and quasi-one (e.g. ladders) systems. In such systems we know that interactions lead to unusual ground states [1-2] and remarkable properties such as spin-charge decoupling, which of course has strong consequences for transport properties, in particular decoupling charge and spin transport [3]. I will connect these theoretical questions with experiments done in the context of cold atomic systems that provide novel ways to probe such physics [4].

The human fascination for fractals dates back to the time of Christ, when structures known nowadays as a Sierpinski gasket were used in decorative art in churches. Nonetheless, it was only in the last century that mathematicians faced the difficult task of classifying these structures. In the 80’s and 90’s, the foundational work of Mandelbrot triggered enormous activity in the field. The focus was on understanding how a particle diffuses in a fractal structure. However, those were classical fractals. This century, the task is to understand quantum fractals. Last year, we experimentally realized a Sierpinski gasket using a scanning tunneling microscope to pattern adsorbates on top of Cu(111) and showed that the wavefunction describing electrons in a Sierpinski gasket fractal has the Hausdorff dimension d = 1.58 [1,2,3]. However, STM techniques can only describe equilibrium properties.
Now, we went a step beyond and using state-of-the-art photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].

Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I focus on the latter topic where we work toward a detailed microscopic picture of molecules transforming from one species to another and reveal several surprises along the way. By preparing quantum-state-selected KRb molecules at a temperature of 500 nK, we observed reactions proceeding through a long-lived intermediate, which provides a handle to steer with light the reaction pathway away from its natural course. Despite the long lifetime that might allow thermalization, our measurements indicate that ergodicity does not hold for all degrees of freedom.

How does a quantum system interact with a travelling pulse of quantum radiation, prepared, e.g., in a number state or a coherent state of light? You may think that this problem has been text book material for decades along with detailed solutions for the case of simple, few level systems. But, in fact, it has not. While crucial for multiple effects in quantum optics and for the entire concept of flying and stationary qubits, quantum optics textbooks do not provide a formal description applicable to this foundational and elementary interaction process. After the introduction of a new (and simple) theoretical formalism that, accounts for the interaction of travelling pulses of quantized radiation with a local quantum system, I shall discuss applications of the theory to quantum pulses of optical, microwave and acoustic excitations and show examples of relevance to recent experiments with qubits and non-linear resonators.

When confining photons in semiconductor lattices, it is possible to strongly modify their physical properties and explore the physics of a variety of Hamiltonians. Photons can behave as finite or even infinite mass particles, photons can propagate along topological edge states without back scattering, photons can become superfluid and behave as massive interacting particles. These are just a few examples of exotic properties that we can imprint into quantum fluids of light in semiconductor lattices. Such manipulation of light presents not only potential for applications in photonics, but great promise for fundamental studies of driven dissipative systems. After a detailed introduction to quantum fluids of light, I will illustrate the variety of physical systems we can emulate with this photonic platform by presenting some recent experiments related to quasi-crystals, helical photons, and photonic graphene. Perspectives in terms of quantum correlations will be discussed.

It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity. Surprisingly, though, a deep understanding of the mechanisms behind this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single, isolated atom is known to have a giant optical response, with a resonant scattering cross section that far exceeds its physical size.

Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and near-field interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strong-disorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and near-field interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one near-resonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for real-life solids.

Several systems have been investigated in the last couple of decades as possible platforms for the realization of a quantum computer. Among the different systems examined, trapped-ion systems have thus far demonstrated the highest fidelity quantum gates and very long coherence times. However, scaling trapped-ion quantum computers to large numbers of qubits has proven to be a difficult problem. In this talk I will review the quantum tool-box of trapped-ion quantum computing and discuss the coherent control techniques that were developed in recent years that render trapped-ion quantum gates robust against errors and allow for quantum computing on long chains of ions.

Many-body systems are very hard to simulate due to the explosion of parameters with the system size. Quantum computers can help in this task, although one may need scalable systems, something that is out of reach in the short run. An attractive alternative is provided by analog quantum simulators which, even though they are not universal, they can still be tuned to study interesting problems. Atoms in optical lattices seem to be ideally suited for that task. Most of the proposals of such simulators have focused so far on condensed matter or high energy physics problems. In this talk I will show how one can extend the range of problems to other scenarios, especially to quantum chemistry.