Probing site-resolved correlations in a spin system of ultracold molecules

Synthetic quantum systems with interacting constituents play an important role in quantum information processing and in elucidating fundamental phenomena in many-body physics. Following impressive advances in cooling and trapping techniques, ensembles of ultracold polar molecules have emerged as a promising synthetic system that combines several advantageous properties. These include a large set of internal states for encoding quantum information, long nuclear and rotational coherence times and long-range, anisotropic interactions. The latter are expected to allow the exploration of intriguing phases of correlated quantum matter, such as topological superfluids, quantum spin liquids, fractional Chern insulators and quantum magnets. Probing correlations in these phases is crucial to understand their microscopic properties, necessitating the development of new experimental techniques. Here we use quantum gas microscopy to measure the site-resolved dynamics of quantum correlations in a gas of polar molecules in a two-dimensional optical lattice. Using two rotational states of the molecules, we realize a spin-1/2 system where the particles are coupled via dipolar interactions, producing a quantum spin-exchange model. Starting with the synthetic spin system prepared far from equilibrium, we study the evolution of correlations during the thermalization process for both spatially isotropic and anisotropic interactions. Furthermore, we study the correlation dynamics in a spin-anisotropic Heisenberg model engineered from the native spin-exchange model using Floquet techniques. These experiments push the frontier of probing and controlling interacting systems of ultracold molecules, with prospects for exploring new regimes of quantum matter and characterizing entangled states useful for quantum computation and metrology

Microscopy of quantum correlations in an ultracold molecular gas

Ultracold molecules are a promising platform for quantum simulation of many-body physics due to their long-range dipolar interactions, rich set of internal states, and long coherence times. However, their complexity also makes it challenging to detect individual molecules and achieve control over their quantum states and interactions. In this thesis, I describe the development of a novel apparatus for performing quantum simulation experiments with ultracold NaRb molecules that addresses these challenges. We first form NaRb molecules in a well-defined quantum state in an optical lattice by associating atom pairs from ultracold gases of Na and Rb. Single molecules are then detected on individual sites via a high-resolution imaging system using a technique sensitive to their rotational state. We also describe the capabilities of the apparatus to control the dipolar interactions with AC and DC electric fields.Using this molecular quantum gas microscope apparatus, we measure the dynamics of site-resolved quantum correlations between the molecules due to their dipolar interactions. By using microwaves to address a two-level subspace of the rotational manifold of the molecules, we realize a spin-exchange model where the spins are coupled via the dipolar interactions. We prepare the synthetic spin system in an out-of-equilibrium state with a quench, and measure the evolution of spin correlations as the quantum system thermalizes. In addition, we demonstrate control over the dipolar interactions between the molecules by tuning their spatial anisotropy. Finally, we use Floquet driving to engineer a spin-anisotropic Heisenberg model from the native spin-exchange model. These experiments expand the capabilities of ultracold molecules for studying problems in quantum magnetism and quantum many-body physics more broadly. For example, future work could explore microscopic correlations in the dipolar Hubbard model, or characterize entangled states of interacting polar molecules relevant for quantum metrology

Energy-participation quantization of Josephson circuits

International audienceAbstract Superconducting microwave circuits incorporating nonlinear devices, such as Josephson junctions, are a leading platform for emerging quantum technologies. Increasing circuit complexity further requires efficient methods for the calculation and optimization of the spectrum, nonlinear interactions, and dissipation in multi-mode distributed quantum circuits. Here we present a method based on the energy-participation ratio (EPR) of a dissipative or nonlinear element in an electromagnetic mode. The EPR, a number between zero and one, quantifies how much of the mode energy is stored in each element. The EPRs obey universal constraints and are calculated from one electromagnetic-eigenmode simulation. They lead directly to the system quantum Hamiltonian and dissipative parameters. The method provides an intuitive and simple-to-use tool to quantize multi-junction circuits. We experimentally tested this method on a variety of Josephson circuits and demonstrated agreement within several percents for nonlinear couplings and modal Hamiltonian parameters, spanning five orders of magnitude in energy, across a dozen samples

Topology-optimized dual-polarization Dirac cones

We apply a large-scale computational technique, known as topology optimization, to the inverse design of photonic Dirac cones. In particular, we report on a variety of photonic crystal geometries, realizable in simple isotropic dielectric materials, which exhibit dual-polarization and dual-wavelength Dirac cones. We demonstrate the flexibility of this technique by designing photonic crystals of different symmetry types, such as ones with four-fold and six-fold rotational symmetry, which possess Dirac cones at different points within the Brillouin zone. The demonstrated and related optimization techniques could open new avenues to band-structure engineering and manipulating the propagation of light in periodic media, with possible applications in exotic optical phenomena such as effective zero-index media and topological photonics