ICAP2022

New paradigms with dipolar quantum gases: Vortices, two-dimensional supersolidity, and angular responses

Quantum physics often makes possible conceptual paradoxes that appear ephemeral to our classical intuition. A recent example is the discovery of supersolid states in ultracold dipolar gases. These states combine the properties of superfluidity with those of crystalline order. This talk traces the fundamental steps for the observation of supersolidity in two dimensions from the perspective of the Innsbruck experiments. We will discuss how a quantum gas of erbium and dysprosium atoms spontaneously breaks its translational symmetry, creating a periodic modulation of its density while globally maintaining quantum phase coherence. We will show how in our experiments it is possible to access this exotic state of matter either by cooling a thermal gas or surprisingly by heating a superfluid. Finally, we will report on the recent observation of quantized vortices in dipolar condensates.

A quantum vortex collider

Quantum vortices occur in a wide range of systems, from atomic Bose–Einstein condensates to superfluid helium liquids and superconductors. Their dynamics is associated with the onset of dissipation, which makes the superflow no longer persistent. In this work, we study the fundamental mechanisms of vortex energy dissipation by realising a versatile two-dimensional vortex collider in homogeneous atomic Fermi superfluids across the BEC-BCS crossover. We unveil vortex-sound interactions by observing the conversion of the energy of vortex swirling flow into sound energy during vortex collisions. We visualise vortices annihilating into sound waves, i.e., the ultimate outcome of small-scale vortex collisions, and we find indications of the essential role played by vortex-core-bound fermionic excitations in strongly-correlated fermion superfluids. Our programmable platform opens the route to exploring new pathways for quantum turbulence decay, vortex by vortex.

Generalized hydrodynamics in 1D Bose gases

Hydrodynamics describes long wavelength dynamics, which occurs on time scales long enough so that one can assume local relaxation of the gas. In a chaotic system, the gas is then locally described as a gas at thermal equilibrium, parametrized by three quantities, the local temperature, the local density and the local velocity. The dynamics is then captured by three equations describing the time evolution of these spatially dependent quantities. In an integrable system, the notion of relaxation is still meaningful, the relaxed state being however no longer described by only three numbers but by an infinite number of quantities, or equivalently by a function. The one-dimensional Bose gas with contact interactions is an integrable system and its relaxed state is described by the rapidities distribution. One then derives generalized hydrodynamics equations which describe the time evolution of the spatially dependent rapidity distribution. In this seminar we will present the experimental tests of the generalized hydrodynamic theory.

Interactions with pulses of quantum radiation

Photons and phonons communicate interactions between distant material quantum systems, and wave packets of radiation may act as “flying qubits” and transmit quantum states and quantum gate operations in quantum optics and quantum information technologies. A theoretical description of how a wave packet of quantum radiation interacts with a local material quantum system is, hence, crucial and necessary, but no textbook provides a general description of this elementary process! In the talk, I will show how to cast the problem in a form that permits a (simple) density matrix theory for the excitation of a general quantum system by an incident quantum pulse of radiation. This theory differs significantly from the treatment of interactions between, e.g., an atom and a single optical mode in cavity QED. In particular, our theory acknowledges the multi-mode character of the final state of the field and it permits evaluation of the quantum state of any outgoing wave packet mode. We present applications of the theory to recent experiments with atomic and superconducting systems that interact with pulses of optical, microwave and acoustic radiation.

Stabilisation of a logical grid-state qubit by laser cooling

Quantum computers will require error-correction in order to produce reliable results. At the lowest level, error-correcting codes require the encoding of information in a larger Hilbert space, such that measurements used for correction can avoid corrupting the logical information. An experimentally simple choice for this low-level encoding is to use harmonic oscillators, which have an infinite Hilbert space in a single quantum system. I will describe recent experiments using a single trapped ion in which we have realised quantum error correction on a harmonic oscillator code consisting of superpositions of displaced squeezed states which produce a grid-like periodic phase space array. Error information is periodically transferred to the internal electronic state, which is then repumped, offering an error correction cycle similar to laser cooling. We use this to perform >100 rounds of error correction, extending logical coherence of the encoded qubit by a factor of more than 3.

Atoms Interlinked by Light: Programming Interactions and Probing Entanglement

The graph of interactions in a quantum many-body system is crucial for governing the flow of information and the structure of correlations. We engineer programmable nonlocal interactions in an array of atomic ensembles within an optical resonator, where photons convey information between distant atomic spins. In our system of spin-1 atoms, the photon-mediated interactions manifest in the formation of correlated atom pairs. We verify the resulting entanglement by observations of spin-nematic squeezing. We furthermore demonstrate versatile tools for controlling the graph of interactions and the structure of multimode entanglement, including (1) programming the spectrum of a drive field and (2) supplementing global cavity-mediated interactions with local addressing. These techniques open opportunities in simulating quantum gravity, exploring quantum optimization paradigms, and generating entangled resource states for sensing and computation.

Entanglement-Enhanced Matter-Wave Interferometry in a High-Finesse Cavity

Entanglement is a fundamental resource that allows quantum sensors to surpass the standard quantum limit set by the quantum collapse of independent atoms. Collective cavity-QED systems have succeeded in generating large amounts of directly observed entanglement involving the internal degrees of freedom of laser-cooled atomic ensembles. Here we demonstrate cavity-QED entanglement of external degrees of freedom to realize a matter-wave interferometer of 700 atoms in which each individual atom falls freely under gravity and simultaneously traverses two paths through space while also entangled with the other atoms [1]. We demonstrate both quantum non-demolition measurements and cavity-mediated spin interactions for generating squeezed momentum states with directly observed metrological gain 3.4 dB and 2.5 dB below the standard quantum limit respectively. An entangled state is successfully injected into a Mach-Zehnder light-pulse interferometer with 1.7 dB of directly observed metrological enhancement. These results open a new path for combining particle delocalization and entanglement for inertial sensors, searches for new physics, particles, and fields, future advanced gravitational wave detectors, and accessing beyond mean-field quantum many-body physics. [1] Graham P. Greve*, Chengyi Luo*, Baochen Wu, James K. Thompson arXiv2110.14027, to appear in Nature

Atomic and nuclear clocks for testing fundamental physics

We use frequency comparisons between highly accurate optical clocks for tests of fundamental principles. In particular, the 171Yb+ optical clock based on an electric octupole (E3) transition between the S-ground-state and the lowest excited F-level with a radiative lifetime of 1.58 years provides a favorable combination of low systematic uncertainty and high sensitivity to potentially new physics. Using this system we have established improved limits for some violations of the Einstein equivalence principle, including the most stringent limits for temporal variations of the fine structure constant and the electron-proton mass ratio. In order to control or suppress the light shift in this optical clock based on a strongly forbidden transition we apply generalized Ramsey schemes and also the excitation of the ion in the dark center of a vortex beam. I will give an outlook on the development of a 229Th nuclear optical clock that will open new perspectives for fundamental tests in atomic and nuclear physics. We have started experiments on laser spectroscopy of trapped thorium ions at about 150 nm wavelength with the aim to observe resonant laser excitation of the 8.3 eV nuclear transition.

An improved measurement of the electron’s electric dipole moment

The observed fact that the hadronic matter in the universe is overwhelmingly composed of matter and not antimatter tells us that there is at least one origin of CP violation that is not accounted for by the standard model of particle physics. It seems likely this as-yet uncharacterized CP-violating physics will give rise to electric dipole moments (EDM) in elementary particles. At JILA we use precision spectroscopy of trapped HfF+ to search for the electron’s EDM. We anticipate that by the end of the summer we will have completed a new measurement with unprecedented accuracy. Looking five years out into the future, we think we can do yet another factor of five better in an experiment based around ThF+. We will discuss both the near-term and the longer-term project.

Many-body quantum optics in atomic arrays

Atomic arrays are a unique platform for many-body physics, enabling efficient and controllable atom-atom interactions in crystal-like, ordered ensembles. In this talk, I will go beyond Hamiltonian dynamics, and discuss out-of-equilbrium physics of dissipative origin, where interactions are mediated by electromagnetic vacuum fluctuations and photon exchange. In particular, I will focus on the most paradigmatic many-body problem in quantum optics, Dicke superradiance, where a collection of initially-inverted atoms synchronizes as they decay, emitting a short and intense pulse of light. I will show that superradiance is a universal phenomenon in ordered arrays (either of atoms in free space or coupled to one-dimensional photonic channels), and is controlled by the lattice constant and array dimensionality. Our predictions can be tested in state of the art experiments with arrays of neutral atoms, molecules, and solid-state emitters and pave the way towards understanding the role of many-body decay in quantum simulation, metrology, and lasing. [Prof. Asenjo Garcia is the 2022 laureate of the IUPAP Early Career Scientist Prize In Atomic, Molecular And Optical Physics.]

Superradiance, Superabsorption and a Photonic Quantum Engine

A superradiant state is a special superposition state of atoms capable of undergoing superradiance immediately without the usual delay involved with Dicke states. We can prepare a superradiant state in a cavity by preparing N atoms in the same superposition state of the ground and excited states. By sending atoms through a nanohole array aperture with a period equal to the atomic transition wavelength, we can impose a common phase to all individual superposition states and localize the atoms exactly at antinodes of a cavity while they traverse the cavity. Surprisingly, these correlated atoms generate superradiance in the cavity even when the mean number of intracavity atoms is much less than unity. It turns out that these time-separated atoms are correlated via the long-lived cavity field so that a single intracavity atom can undergo superradiance as if the preceding atoms outside the cavity were also in the cavity. Another interesting feature of this superradiance is that the emission is amplified like a laser but without exhibiting a threshold. Through second-order-correlation measurements, we observed that the cavity field is coherent even when the mean number of photons is less than unity, in contrast to the thermal light emitted from the usual thresholdless lasers under the same condition. The superradiant state can also be used to realize the long-sought superabsorption, the opposite of superradiance. It is well known that the bright state responsible for superradiance also has an absorption rate the same as the superradiant emission rate. This notion motivated theorists to suggest various ideas for realizing superabsorption, but none has worked so far. From theoretical considerations, we found that a superradiant state can absorb light instead of emitting light if its phase is chosen properly depending on the phase of the input field. Based on this idea, we have recently realized superabsorption by reversing the superradiance process in time. The maximum number of photons absorbed was proportional to the square of the number of atoms, proving the cooperative nature of superabsorption. In addition, the superradiant state can be used to realize a superradiant photonic quantum engine. Here, the atoms entering the cavity are our fuel and their nonzero excited state population can be associated with an effective temperature by the Boltzmann factor. Our engine operates between a thermal state and a superradiant state of reservoir at the same reservoir temperature. Photons emitted by superradiance exert radiation pressure to the cavity mirrors to perform a work. The observed efficiency of the engine was 98%. Our quantum engine can serve as a testbed for quantum thermodynamics in nanoscale systems.

Quantum optics with cold atoms trapped along nanowaveguides

Considerable efforts have been recently devoted to combining ultracold atoms and nanophotonic devices to obtain not only better scalability and figures of merit than in free-space implementations, but also new paradigms for atom-photon interactions. Dielectric waveguides offer a promising platform for such integration because they enable tight transverse confinement of the propagating light, strong photon-atom coupling in single-pass configurations and potentially long-range atom-atom interactions mediated by the guided photons. In the talk, I will present our efforts in this emerging neutral-atom waveguide-QED field of research. Using nanofiber trapped atoms, we demonstrated the capability to herald, store and read out a single collective excitation that is preferentially coupled to the guided mode. In this nanofiber setting, using a dynamically-controlled Bragg configuration, we also recently pushed the non-linearity down to the few-photon level. I will then describe our works towards stronger coupling in single pass by using photonic-crystal slow-mode waveguides with engineered dispersion. Localizing and trapping atoms in the proximity is a strong challenge and I will describe how novel structures and optical techniques can be used.

Compressibility and the equation of state of an optical quantum gas in a box

Quantum gases of atoms, exciton-polaritons, and photons provide a test bed for many-body physics under both in- and out-of-equilibrium settings. Experimental control over their dimensionality, potential energy landscapes, or the coupling to reservoirs offers wide possibilities to explore phases of matter, for example, by probing susceptibilities, as the compressibility. For gases of material particles, such studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases; for optical quantum gases, they have so far remained elusive. In my talk, I will discuss experimental work demonstrating a measurement of the compressibility of a two-dimensional quantum gas of photons in a box potential, from which we obtain the equation of state for the optical medium. The experiment is carried out in a nanostructured dye-filled optical microcavity. We observe signatures of Bose-Einstein condensation at large phase-space densities in the finite-size system. Upon entering the quantum degenerate regime, the density response to an external force sharply increases, hinting at the peculiar prediction of a highly compressible Bose gas. In other recent work, we have demonstrated a non-Hermitian phase transition of an open photon Bose-Einstein condensate, which is revealed by an exceptional point in the fluctuation dynamics.

Higgs mode in a unitary Fermi gas

Rapid quenches of the interactions in a strongly interacting Fermi superfluid can induce oscillations of the order parameter, commonly known as the Higgs amplitude mode. Here, we use Bragg spectroscopy to locally measure the time-resolved response of a Fermi gas following a quench to unitarity, directly revealing the amplitude oscillations. The normalised oscillation frequency, set by twice the pairing gap, remains relatively constant for temperatures below the superfluid transition while the oscillation amplitude displays a strong temperature dependence in good agreement with time-dependent BCS theory. The Higgs excitation decays according to a power law and we measure a damping exponent at unitarity approximately midway between the BCS and BEC limits.

Laser-driven and Magnetized Ultracold Neutral Plasmas

Ultracold neutral plasmas (UNPs), created by photoionizing laser-cooled atoms just above threshold, stretch the boundaries of neutral plasma physics towards low energies and strong Coulomb coupling (measured by Γ, the ratio of the average Coulomb interaction energy to kinetic energy). Precise diagnostics and control over plasma conditions make UNPs useful for validating plasma theory and discovering new phenomena. In this talk, I will describe several experiments with UNPs formed by photoionizing an ultracold gas of neutral strontium atoms. Laser cooling of ions in the UNP yields ion temperatures as low as 50 mK and Coulomb coupling parameters as high as Γ=11. Electrons and Sr+ ions can be magnetized with experimentally accessible fields (~100 G). I will describe the use of laser-induced fluorescence to study the expansion of an UNP created in a quadrupole magnetic field and the first evidence of magnetic trapping of a UNP. This opens new possibilities for studying transport phenomena in strongly coupled and strongly magnetized systems.

Quantum Optics based on dipolar interactions between hot atoms

Electrical dipolar interactions between Rydberg atoms are so strong that even for thermal atomic vapor the Rydberg blockade can be observed via the single photon emission of a blockaded ensemble. Additionally, the light induced dipolar interaction between two level atoms, the so-called Lorentz-Lorenz shift, can be observed in very thin cells as a 2D geometry of interacting dipoles as well as in 1D geometries, which are realized in integrated nano-photonic slot waveguides. The latter leads to a substantial and observable Purcell enhancement of the blue shift at telecom wavelengths due to the dipolar interaction. As an outlook we present the concept of an optical single thermal atom detector based on a freestanding photonic crystal cavity that enhances the atom light coupling to the strong coupling regime. These examples of thermal atoms acting as a reconfigurable strongly nonlinear medium in integrated nano-photonic circuits show that quantum optical applications on the single photon level are within reach.

A tunable spin Hamiltonian of dipolar molecules

A degenerate Fermi gas of polar molecules [1] sets the stage to explore novel molecular dynamics. An external electric field is used to tune the elastic dipolar interaction by orders of magnitude while suppressing reactive loss. Efficient dipolar evaporation leads to the onset of quantum degeneracy in two-dimensional optical traps [2]. The electric field tuning of the rotational energy also produces sharp collision resonances, giving rise to three orders-of-magnitude modulation of the chemical reaction rate [3]. The precise control of electric field facilitates the preparation and addressing of isolated, individual two-dimensional layers of molecules with a free choice of rotational states. Spin exchange between molecules of neighboring layers occurs through the long-range dipolar interaction, demonstrating quantum-state engineered stereo chemical reaction [5]. Meanwhile, these interacting molecules in 2D are used to realize a fully tunable spin Hamiltonian, with both the Ising and spin exchange interactions precisely controlled via the electric field strength, orientation, and internal spin state. This work establishes an itinerant molecular spin platform to explore phenomena such as spin entanglement and spin transport mediated by the strong and tunable dipolar interaction. References: [1] L. De Marco, G. Valtolina, K. Matsuda, W. G. Tobias, J. P. Covey, and J. Ye, “A degenerate Fermi gas of polar molecules,” Science 363, 853 (2019). [2] G. Valtolina, K. Matsuda, W. G. Tobias, J.-R. Li, L. De Marco, and J. Ye, “Dipolar evaporation of reactive molecules to below the Fermi temperature,” Nature 588, 239 (2020). [3] K. Matsuda, L. De Marco, J.-R. Li, W. G. Tobias, G. Valtolina, G. Quéméner and J. Ye, “Resonant collisional shielding of reactive molecules using electric fields,” Science 370, 1324 (2020). [4] J.-R. Li, W. G. Tobias, K. Matsuda, C. Miller, G. Valtolina, L. De Marco, R. R.W. Wang, L. Lassabliére, G. Quéméner, J. L. Bohn, and J. Ye, “Tuning of dipolar interactions and evaporative cooling in a three-dimensional molecular quantum gas,” Nature Phys. 17, 1144 (2021). [5] W. G. Tobias, K. Matsuda, J.-R. Li, C. Miller, A. N. Carroll, T. Bilitewski, A. M. Rey, and J. Ye, “Reaction between layer-resolved molecules mediated by dipolar exchange,” Science 375, 1299 (2022).

Spin dynamics of ultracold atoms in optical lattices

Ultracold atoms offer a unique platform to perform quantum simulations of quantum materials and many-body systems. When atoms with spin are arranged in an optical lattice in form of a Mott insulator, they realize paradigmatic Heisenberg spin models, where only neighboring spins interact. Until very recently, all experimental studies with cold atoms addressed the special case of an isotropic Heisenberg model. Using lithium-7 atoms and Feshbach resonances to tune the interactions, we have created spin ½ Heisenberg models with adjustable anisotropy, including the special XX-model which can be exactly solved by mapping it to non-interacting fermions. Spin transport changes from ballistic to diffusive depending on the anisotropy. Special transverse spin patterns in the form of helices are exact eigenstates of the Heisenberg model and are called phantom states since they carry momentum, but no energy. We have realized these phantom helix states and used them to accurately measure the interaction anisotropy. We find major contributions from virtual molecules and short-range off-site interactions which had not been observed before.

Coherent Light Shift on Alkaline-Earth Rydberg Atoms

Atoms presenting two valence electrons have an opportunity for optical manipulation of their Rydberg state thanks to the second (core-)electron transitions. Unfortunately, excitation of this electron then brings enough energy to ionize and auto-ionization usually takes place in less than a nanosecond, thus preventing any useful application. In order to allow such a manipulation, one could use the auto-ionization zeros observed long ago in the core-electron photo-excitation spectrum. They arize from the quantum interference between different ionization paths and can be understood within Multi-channel Quantum Defect Theory. We will present our quantitative description of both the auto-ionization spectrum and of the induced light-shift when illuminating Yb Rydberg atoms in 6sns singlet states with light close to the Yb+ 6s-6p transition. We demonstrate experimentally for the first time the ability to apply a significant light-shift while keeping auto-ionization negligible. These results give high prospects for coherent state manipulation in quantum computation or quantum simulation platforms based on divalent Rydberg atoms. This effect will contribute in long term coherence of trapped atoms which can then remain trapped in their Rydberg states. It will also allow site addressing with focussed light acting solely on the Rydberg state while leaving the ground state untouched. A proof of principle has been recently demonstrated in Princeton group in a paralell study. [1] N. H. Tran, R. Kachru, and T. F. Gallagher, Phys. Rev. A 26, 3016 (1982). [2] W. E. Cooke and C. L. Cromer, Phys. Rev. A 32, 2725 (1985). [3] K.-L. Pham, T. F. Gallagher, P. Pillet, S. Lepoutre, and P. Cheinet PRX Quantum 3, 020327 (2022) [4] A. P. Burgers, S. Ma, S. Saskin, J. Wilson, M. A. Alarcón, C. H. Greene, and J. D. Thompson PRX Quantum 3, 020326 (2022)

Probing Quantum Many-Body Dynamics with Tweezer Arrays

Atom-by-atom assembly with optical tweezers enables the generation of defect-free atomic arrays with flexible geometric arrangements. Combined with controlled excitation to Rydberg states, this has become a highly versatile platform for quantum computing, simulation, and metrology. I will review these developments with a focus on two valence electron atoms: The rich level structure of such atoms enables novel cooling, control, and read-out schemes, which we have used in demonstrations of record imaging and two-qubit entanglement fidelities for neutral atoms. Applying this high-fidelity approach to chaotic many-body dynamics, we recently uncovered the generation of random distributions of pure quantum states over a Hilbert space, emerging from either temporal sampling or partial projective measurements. Such random ensembles play an important role in quantum information science associated with device verification, supremacy tests, and quantification of complexity growth. Our work opens the door to applications of such concepts in an analog quantum simulation and many-body dynamics context; as a concrete example, I will show global fidelity estimation for highly entangled states in our Rydberg atom quantum simulator.

Quantum science with microscopically-controlled arrays of two-electron atoms

Quantum science with neutral atoms has seen great advances in the past two decades. Many of these advances follow from the development of new techniques for cooling, trapping, and controlling atomic samples. In this talk, I will describe ongoing work where we have explored a new type of atom - two-electron atoms - for optical tweezer trapping. While their increased complexity leads to challenges, these atoms also offer new scientific opportunities by virtue of their rich internal degrees of freedom. Accordingly, they have impacted multiple areas in quantum science, including quantum information processing, quantum simulation, and quantum metrology. I will report on my group’s progress in these areas.

Accurate determination of the fine-structure constant using atom interferometry

Testing the standard model requires knowledge of the parameters that scale the fundamental inter actions. Among them is the fine-structure constant, α, which characterises the strength of the elec tromagnetic interaction, and thus plays a crucial role in quantum electrodynamics calculations. Using atom interferometry to measure the quotient ℏ/mRb of the reduced Planck’s constant and the mass of a rubidium-87 atom, we obtained the most accurate determination of the fine structure constant α = 1/137.035999206(11) with a relative accuracy of 81 parts per trillion (ppt). Our ultra-stable and robust experimental setup combines a Ramsey-Bord´e interferometer based on Raman diffraction, with Bloch oscillations in an accelerated optical lattice. It allows us to achieve a record sensitivity to α of 4 × 10−11 in 48 h integration time. This enabled us to investigate experimentally several systematic effects, notably those related to wave-front distortions. A test can be performed by comparing the Penning trap measurement of the electron gyromagnetic anomaly ae to its Standard Model prediction. Using our new value of α, we observe a 1.6 σ agreement. This result is the most stringent test of the standard model in the electron sector. However a discrepancy of 5.6 σ is observed between our value and the one measured with cesium atoms by the Berkeley group. This discrepancy remains unexplained. Future measurements using a Bose-Einstein condensate and rubidium-85 atoms are in progress to investigate this issue. This work is also part of an exciting research area beyond atomic physics. One of the objectives is to explain the persistent discrepancy of 4.2 σ between the experimental value and the prediction of the standard model for the magnetic moment of the muon, by looking to see if such a discrepancy may be observed for the electron.

A precision laboratory test of the gravitational redshift at the sub-cm scale

The remarkable precision of optical atomic clocks offers sensitivity to new and exotic physics through tests of relativity, searches for dark matter, gravitational wave detection, and probes for beyond Standard Model particles. While much of optical clock research has focused on improving absolute accuracy, many searches for new physics can be performed with relative comparisons between two clocks. To this end, we have recently realized a “multiplexed” strontium optical lattice clock consisting of two or more clocks in one vacuum chamber. In comparisons between two spatially separated atom ensembles in the same lattice we observe atom-atom coherence times exceeding 26 seconds using correlated Ramsey spectroscopy and measure fractional frequency shifts at a precision below one part in 10^19. In this talk I will explain the concept and operating principles of our multiplexed optical lattice clock. I will then present recent experimental results in which we performed a novel, blinded, precision test of the gravitational redshift with an array of 5 evenly-spaced atomic ensembles spanning a total height difference of 1 cm. I will present the error budget produced from our systematic evaluation, and the recently unblinded results of the test. I will also discuss how these results can also be viewed as a proof-of-principle demonstration of relativistic geodesy at the sub-cm scale. Finally, I will discuss the outlook for future searches for new physics with our apparatus.

Ultrafast chirality: twisting light to twist electrons

I will describe several new, extremely efficient approaches to chiral discrimination and enantio-sensitive molecular manipulation, which take advantage of ultrafast electronic response [1,2]. One of them is based on the new concept of synthetic chiral light [3,4], which can be used to trigger bright nonlinear optical response in the molecule of a desired handedness while keeping its mirror twin dark in the same frequency range. The other is based on the new concept of geometric magnetism in photoionization of chiral molecules and leads to a new class of enantio-sensitive observables in photoionization [5]. Crucially, the emergence of these new observables is associated with ultrafast excitation of chiral electronic or vibronic currents prior to ionization and can be viewed as their unique signature. [1] Ultrafast chirality: the road to efficient chiral measurements, D Ayuso, AF Ordonez, O Smirnova, arXiv:2203.00580 , 2022 [2] AF Ordonez, O Smirnova, Generalized perspective on chiral measurements without magnetic interactions, Physical Review A 98 (6), 063428, 2018 [3] D Ayuso et al, Synthetic chiral light for efficient control of chiral light–matter interaction, Nature Photonics 13 (12), 866-871, (2019) [4] D. Ayuso et al "Enantio-sensitive unidirectional light bending”, Nat Commun 12, 3951 (2021) [5] AF Ordonez, D Ayuso, P. Decleva, O Smirnova “Geometric magnetism and new enantio-sensitive observables in photoionization of chiral molecules” http://arxiv.org/abs/2106.14264

Non-equilibrium dynamics and sensing with large trapped-ion crystals

I will describe experimental work measuring non-equilibrium quantum dynamics and sensing with single-plane crystals of up to 200 ions stored in a Penning trap. We engineer long range Ising interactions with a spin-dependent optical dipole force that couples the center-of-mass vibrational mode of the crystal with the collective electronic spin or qubit degrees of freedom of the ions. In a quench experiment the engineered interactions generate non-equilibrium quantum dynamics of the spin degrees of freedom, which we benchmark through measurements of out-of-time-ordered correlation functions (OTOCs) made possible by time reversing the interaction. In a second experiment I will describe how the previously mentioned spin-motion coupling can be used to generate entangled states of the ions’ spin and center-of-mass motional degrees of freedom that are sensitive to weak motional displacements. By entangling the center-of-mass oscillator with the collective spin before the motional displacement is applied and by controlling the coherent dynamics via time-reversal and a many-body echo that disentangles the spin and motion, we map the displacement into a spin rotation, avoid quantum back-action, and cancel thermal noise. We report quantum enhanced sensitivity to displacements of 8.8 dB below the standard quantum limit and 19 dB below thermal noise. The prospects of using this sensitivity to detect weak electric fields from dark matter will be discussed.

Optical atomic clocks – what challenges remain on the roadmap towards a redefinition of the SI second?

Over recent years, there has been enormous progress in the development of optical atomic clocks based on either atoms trapped in an optical lattice or single trapped ions. These optical clocks have demonstrated unprecedented frequency stability and estimated systematic frequency uncertainty, far surpassing the current generation of caesium microwave primary frequency standards, with the result that a future optical redefinition of the SI second is anticipated. However before this can happen several key challenges remain to be addressed. First of all, the uncertainty budgets of the optical clocks need to be validated through a programme of international comparisons between systems developed independently by different research groups around the world. Continuity with the current caesium-based definition must also be ensured, by performing absolute frequency measurements with as low an uncertainty as possible. And finally, we need to improve the robustness of optical clocks and automate their operation, enabling them to be operated routinely as secondary representations of the second, regularly contributing to International Atomic Time (TAI) via reporting to the International Bureau of Weights and Measures, and being used to steer the local UTC(k) time scales maintained by national timing laboratories. I will discuss recent progress towards addressing these challenges, in particular drawing on examples of NPL work performed in the European collaborative project Robust Optical Clocks for International Timescales (ROCIT).

Making optical lattice clocks compact and useful for real-world applications

An “optical lattice clock” benefits from a low quantum-projection noise by simultaneously interrogating many atoms trapped in an optical lattice. The essence of the scheme is an engineered perturbation based on the “magic wavelength” protocol, which has been proven successful up to 10-18 uncertainty. About a thousand atoms enable such clocks to achieve 10-18 stability in a few hours of operation. This superb stability is especially beneficial for chronometric leveling, which discriminates a centimeter height difference of the clocks located at distant sites by the gravitational redshift. We overview the progress of optical lattice clocks and address recent topics to explore real-world applications of the 18-digit-accurate clocks, including 1) compact optical lattice clocks under development in collaboration with industry partners, 2) demonstration of an on-vehicle clock, and 3) our challenge to downsize of the clock as well as to improve the clock stability by “longitudinal Ramsey spectroscopy” that allows continuous interrogation of the clock transition.

Towards quantum simulation of U(1) LGTs with alkaline-earth-like atoms

Well-controlled synthetic quantum systems, such as ultracold atoms in optical lattices, offer intriguing possibilities to study complex many-body problems in regimes that are beyond reach using state-of-the-art classical computations. The basic idea is to construct and use a well-controlled quantum many-body system in order to study its in- and out-of-equilibrium properties. An important future quest concerns the development of novel experimental techniques that allow us to expand the range of models that can be accessed. Simulating lattice gauge theories is a particularly promising direction, which combines a broad range of research areas including high-energy physics and topological quantum computation. One of the main challenges for quantum simulation of lattice gauge theories is to find resource-efficient implementations of the required local symmetries. Despite the recent experimental progress, there is no clear path towards implementations of lattice gauge theories in extended systems beyond one dimension or with non- Abelian symmetries. Here, I report on the development of a new hybrid optical lattice- tweezer platform that combines local state-dependent control of tunnel couplings with the unique properties of fermionic alkaline-earth-like atoms to realize U(1) lattice gauge theories coupled to matter. Moreover, the SU(N) symmetric interactions of fermionic Yb atoms pave the way towards novel schemes for the realization of more complex non-Abelian symmetries. Our scheme relies on correlated tunneling of fermionic atoms in a state-dependent programmable optical lattice. We have performed ab initio calculations of the lattice gauge theory dynamics, identifying suitable parameter regimes. On the experimental side we have performed measurements of the required laser wavelengths, which is the first step towards building the final lattice setup.

Engineering a topological gauge theory in an optically dressed Bose-Einstein condensate

Topological gauge theories describe the low-energy properties of certain strongly correlated quantum systems through effective weakly interacting models. A prime example is the Chern-Simons theory of fractional quantum Hall states, where anyonic excitations emerge from the coupling between weakly interacting matter particles and a density-dependent gauge field. While in traditional solid-state platforms such gauge theories are only convenient theoretical constructions, engineered quantum systems enable their direct implementation and provide a fertile playground to investigate their phenomenology without the need for strong interactions. In my talk, I will present the quantum simulation of a one-dimensional reduction of the Chern-Simons theory, the chiral BF theory, in a Bose-Einstein condensate. Using the local conservation laws of the theory we eliminate the gauge degrees of freedom in favour of chiral matter interactions, which we engineer by synthesising optically dressed atomic states with momentum-dependent scattering properties. This allows us to reveal the key properties of the chiral BF theory: the formation of chiral solitons and the emergence of an electric field generated by the system itself. Our results expand the scope of quantum simulation to topological gauge theories and pave the way towards implementing analogous gauge theories in higher dimensions.

Non-equilibrium control of quantum lattice systems using driving and dissipation

During the last years, time periodic driving has been established as a powerful tool for the coherent control of engineered quantum systems of atoms or photons. Prominent examples are the effective realization of strong artificial gauge fields for charge-neutral particles as well as the engineering of anomalous Floquet topological band structures without analog in autonomous quantum systems. However, in isolated systems this type of Floquet engineering inevitably eventually causes heating. A more robust scenario might be the combination of periodic driving with dissipation as it results from the coupling to an engineered environment. In this way the system will be driven into a non-equilibrium steady state, which can either be targeted to approach a low-temperature state of an effective time-independent Floquet-engineered Hamiltonian or correspond to a far-from equilibrium target state. As examples, I will discuss the stabilization of quantum-Hall states for neutral particles as well as Floquet-heating induced non-equilibrium Bose condensation in an excited state.

Testing fundamental physics using laser cooled molecules

Laser cooling of molecules is advancing rapidly. Magneto-optical traps have been developed for several species of diatomic and triatomic molecules. The molecules have been cooled to temperatures of a few microkelvin and confined in magnetic traps, optical dipole traps, tweezer traps and lattices. Collisions between the molecules, and between laser-cooled atoms and molecules, have been studied, with results suggesting that high phase space densities can be reached through sympathetic or evaporative cooling. These ultracold molecules can be used to test fundamental physics. I will present progress on two experiments in this direction. In the first, we are developing a clock based on vibrational transitions in CaF molecules confined in a magic wavelength lattice. Pure vibrational transitions have little sensitivity to external perturbations so can make highly accurate clocks. By comparing to an optical atomic clock, we aim to measure variations in the electron-to-proton mass ratio. The molecular lattice clock could also provide improved frequency standards in the infra-red. In the second experiment, we aim to measure the electron’s electric dipole moment using ultracold YbF molecules. I will present the construction of this experiment which features a slow-moving beam cooled below 100 µK in an extremely low magnetic noise environment.

Quantum gas microscopy of polar molecules

Ultracold molecules are a promising platform for quantum simulation of spin physics due to their long-range interactions and large set of internal states. To understand the complex many-body states that emerge in these systems in and out of equilibrium, new experimental techniques are needed to probe molecule correlations in the strongly-interacting regime. Here we study the site-resolved dynamics of spin correlations in a gas of ultracold 23Na 87Rb molecules in a 2D optical lattice. We first form Feshbach molecules in the lattice before transferring them to the ground state via STIRAP with 85% one-way efficiency. We operate at near-magic trapping conditions where we prepare long-lived superpositions of the ground and first excited rotational states. The molecules realize a 2D quantum XY model with long-range interactions. Using a site-resolved Ramsey interferometric technique, we detect oscillations in nearest- and next-nearest-neighbor correlations due to spin interactions. Furthermore, we apply a periodic external microwave field to engineer XXZ spin Hamiltonians with tunable anisotropies. The correlations are measured by dissociating the molecules and detecting the corresponding Rb atoms with single-site resolution using a quantum gas microscope. The techniques presented here open new doors for probing quantum correlations in complex many-body systems of ultracold molecules.

Four-second optical coherence between different atomic species, and the search for new physics with atomic clocks

The extreme precision and accuracy of today’s optical atomic clocks can be used to look for very small deviations from the predictions of the Standard Model, offering a tool to search for beyond Standard Model (BSM) physics complementary to particle accelerators. These searches are based on measuring the frequency ratio of two transitions that depend differently on interactions with BSM particles or fields. In this talk, I will present frequency ratio measurements between atomic clocks based on Al+, Hg+, Sr, and Yb atoms, and the use of these measurements to constrain the coupling of ultralight scalar dark matter candidates to the Standard Model. The precision of traditional, incoherent frequency ratio measurements and resulting constraints on BSM physics are limited by the coherence time of the lasers used to probe the atomic transitions. We have recently demonstrated a new, coherent frequency ratio measurement technique called differential spectroscopy that removes this limitation and achieved a record for the precision of frequency ratio measurements between different atomic species. I will conclude with a brief discussion of the prospects for optical clocks based on different atomic, molecular, and nuclear transitions with much higher sensitivity to BSM physics in a variety of sectors.

The proton size, the fine-structure constant and the electron electric dipole moment

Fundamental physics (including physics beyond the Standard Model) can be tested using table-top precision measurements. The talk will describe measurements of the size of the proton, the fine-structure constant and the electric dipole moment of the electron. Two recently completed measurements will be described. For the first measurement, the n=2 Lamb shift of atomic hydrogen is measured, allowing for a new determination of the charge radius of the proton. This determination is crucial to helping resolve the proton radius puzzle, in which it appeared that the proton radius took on a different value when measured with muons compared to measurements using electrons. The second measurement is of the n=2 triplet P fine structure of atomic helium, and this work is part of a program to obtain a new determination of the fine-structure constant. Both of these measurements use a new measurements technique: Frequency offset separated oscillatory fields. Finally, a new major effort (EDM^3) is starting to measure the electron electric dipole moment using polar molecules embedded into inert-gas solids.

A Landau Theory for Spin Squeezing

Quantum metrology makes use of structured entanglement to perform measurements with far greater precision than would be possible with only classically correlated particles. A paradigmatic example of such entanglement is spin squeezing, which is known to be dynamically generated by the celebrated one-axis-twisting model, corresponding to an all-to-all coupled Ising Hamiltonian. Motivated by recent advances in a variety of quantum simulation platforms, there has been tremendous interest in the possibility of generating spin squeezing via Hamiltonians which do not require all-to-all interactions. This interest has centered on a class of power-law interaction models, corresponding to long-ranged generalizations of the so-called XXZ model. We conjecture that optimal spin squeezing in such models is intimately connected to the presence of finite-temperature, easy-plane ferromagnetism, which arises from the spontaneous breaking of a continuous symmetry. In particular, we demonstrate that if the temperature of the initial product state is below the ordering temperature for an XY ferromagnet, then the system will exhibit scalable spin squeezing. I will end by discussing some recent experimental efforts toward realizing spin squeezing in two dimensional ensembles of dipolar interacting spins.

Time-Reversal-Based Quantum Metrology beyond the Standard Quantum Limit

Linear quantum measurements with independent particles are bounded by the Standard Quantum Limit (SQL) that can be overcome by inducing entanglement between the particles. In optical-transition clocks, entanglement can be induced by the collective interaction of the atoms with a cavity mode. We demonstrate cavity-induced, nearly unitary spin squeezing between the nuclear spin states of the electronic ground state of 171Yb. We transfer this entanglement onto the optical-clock transition, and demonstrate 4.4dB of spin squeezing on the clock transition, corresponding to a reduction of the clock averaging time by a factor of 2.8. However, the metrological gain from entanglement is often limited by the final state readout rather than the state preparation, especially for more complex entangled many-body states with non-Gaussian probability distributions. An alternative is to use a time-reversal protocol to amplify small displacement of the entangled state. We implement such a time-reversal protocol through a controlled sign change in many-body Hamiltonian of atomic spins coupled to an optical cavity. With this approach, we demonstrate quantum measurement with non-Gaussian states with a precision improving in proportion to the particle number (Heisenberg scaling), at fixed distance of 12.6 dB from the ultimate Heisenberg Limit. Using a system of 350 neutral 171Yb atoms, this signal amplification through time-reversed interaction (SATIN) protocol achieves an improvement of 12 dB beyond the SQL in a Ramsey sequence. We also use the time-reversed Hamiltonian to experimentally investigate the relation between quantum information scrambling, out-of-time-order correlators and metrological gain.