The Chicago Quantum Exchange and the University of Chicago held the first Quantum Creators Prize Symposium on September 30, 2021, to recognize the achievements of early-career researchers in the broad areas of quantum information science and engineering. It was held in person at the David Rubenstein Forum in Chicago as part of the 2021 Chicago Quantum Summit.
Meet the 2021 Quantum Creators Prize Winners
Probing many-body noise in a strongly interacting two-dimensional dipolar spin system
Fully characterizing quantum many-body states is a hard computational problem, requiring in general an exponential number of single-particle-resolved measurements. Often, however, one would simply like to efficiently extract target information from a many-body system without resorting to prohibitively expensive or impossible methods; for example, single-spin measurements are inaccessible for ensembles of solid-state defects. Following the rich tradition of magnetic resonance spectroscopy, one can instead consider the noise generated by a many-body spin system, as measured via the decoherence of a probe qubit. I will discuss what we can learn from the decoherence dynamics of such a probe, in the context of experimentally characterizing both static and dynamical proper.es of strongly- interacting magnetic dipoles. The experimental platform consists of two types of spin defects in diamond: nitrogen- vacancy (NV) color centers (probe spins) and substitutional nitrogen impurities (many-body system). I will show how signatures of the many-body system's dimensionality, dynamics, and disorder are naturally encoded in the functional form of the NV's decoherence profile. This information can be applied to directly measure the two-dimensional nature of a nitrogen delta-doped diamond sample, and to probe the correlation time of the many-body spin system.
Strongly interacting electrons in synthetic superlattices
Synthetic superlattice systems or moiré materials have recently emerged as a promising new platform for studying strongly interacting electron systems. In the past few years these two-dimensional van der Waals materials were found to host a plethora of interesting quantum phases of matter, such as e.g. superconductors, correlated insulators, orbital magnets (with or without a quantized anomalous Hall effect) and fractional Chern insulators. In this talk I will review some of the recent progress in our theoretical understanding of one of the most studied moiré materials, namely Magic-Angle Twisted Bilayer Graphene (MATBG). I will explain how the ground state of the interacting MATBG Hamiltonian at certain electron fillings can be obtained almost exactly in the strong-coupling limit, and how this leads to the physical picture of MATBG as a ‘generalized quantum Hall ferromagnet’. In the final part of the talk, I will discuss the importance of strain in driving MATBG from the strong-coupling regime to the intermediate-coupling regime and how this modifies the low-energy physics.
Probe of Band Structure Singularities with a Lattice-Trapped Quantum Gas
One type of ultracold-atom quantum simulator is formed by using lasers to generate a spatially periodic optical potential and allowing ultracold atoms to evolve within the potential. Such a simulator is a powerful experimental tool that provides insight into the properties of crystalline solids. Important crystalline solid properties, such as electrical resistivity and optical absorption, are set by the crystal’s energy band structure. However, it is not only the band structure that determines the properties of a crystal. The band structure may have points where two or more bands are degenerate in energy, and where the wave function used to describe the system becomes ill-defined (i.e., singular). This means that the local geometry and global topology of the space in which the wave functions live are important for explaining material properties (e.g., quantum Hall effects and orbital magnetism). In this talk I will describe our recent experimental studies of band-structure singularities, performed by investigating a non-Abelian transformation produced by transport of atoms directly through singular points. We produce a Bose-Einstein condensate and load it into one band of an optical honeycomb lattice, before accelerating the atoms along a quasi-momentum trajectory that enters, turns, and then exits the singularities at linear and quadratic touching points of the band structure. From measurements of the band populations after transport we identify topological winding numbers for these singularities to be 1 and 2, respectively.
Doping a Chiral Spin liquid towards Topological Superconductivity and critical theories
This talk will mainly discuss theories of deconfined transitions from chiral spin liquids (CSLs) to d+id superconductors (SC). On triangular lattices, two distinct CSLs are clarified. Both states are described within the Abrikosov fermion representation of spins, and the effect of doping can be accessed by introducing charged holons. Doping one kind of CSL, which has been numerically found, leads to 2 scenarios: 1) if holons condense, a chiral metal with enlarged unit cell and finite Hall conductivity is obtained. 2) The internal magnetic flux adjusts with doping and holons form a bosonic integer quantum Hall (BIQH) state. Remarkably, the latter phase is identical to a d+id superconductor. In this case the Mott insulator to superconductor transition is associated with a bosonic variant of the integer quantum Hall plateau transition. In the 2nd part, distinct ways of connecting CSL to d+id SC on square lattices are proposed. We briefly discuss a related duality between the critical theories, and discuss the symmetries and operator mapping of the critical theory.
Pathways toward unconventional light-induced states in quantum materials
Phase transitions instigated by an intense laser pulse usher in a new era for manipulating states of matter in quantum materials. The ability to selectively couple light to different degrees of freedom in a solid – such as charge, spin, lattice, and orbital – offers the possibility of generating unconventional phases through nonthermal means and controlling their macroscopic properties on femto- to picosecond timescales. In this talk, I will focus on three recurring themes in the study of nonequilibrium phase diagrams of quantum materials: (i) phase competition, (ii) electronic correlations, and (iii) defect generation. More specifically, an ultrashort light pulse can (i) perturb the subtle energy balance between proximal ground states, (ii) modify the Coulomb interaction by exciting free carriers, and (iii) induce topological structures that give rise to novel metastable states. These points will be discussed using examples from charge-density-wave compounds, which serve as a model system for other correlated materials to illustrate various light-matter interaction pathways.
Induced superconductivity in fractional quantum Hall
Quantum computing promises transformative societal impact but faces a grand challenge: realization of fault-tolerant qubits in a scalable architecture. An inherent fault tolerance may be achieved by topological quantum computing hypothesized more than two decades ago, whose experimental pursuit has been initiated by the more recent discovery of topological insulators and following proposals for hybrid approaches involving superconductors. Common to all such hybrid approaches pursued experimentally is noninteracting charge carriers. However, universal topological quantum computing requires interacting hybrids, for example those based on fractional quantum Hall state. We have coupled a graphene-based high-mobility heterostructure to a niobium nitride superconductor, in which superconductivity and robust fractional quantum Hall coexist. The narrow superconductor geometry allows for pairing between quantum Hall edges, which converts electrons to Andreev holes. The probability of hole conversion as a function of integer and fractional fillings, magnetic field and temperature provides evidence for spin-orbit coupling, topological superconducting gap, and superconducting pairing of fractional charges. These results provide a route towards universal topological quantum computing.
Rare-earth ion qubits in optical resonators: a platform for quantum networks and nuclear spin physics
Abstract not available.
Provably efficient machine learning for quantum many-body problems
Classical machine learning (ML) provides a potentially powerful approach to solving challenging quantum many-body problems in physics and chemistry. However, the advantages of ML over more traditional methods have not been firmly established. In this work, we prove that classical ML algorithms can efficiently predict ground state properties of gapped Hamiltonians in finite spatial dimensions, after learning from data obtained by measuring other Hamiltonians in the same quantum phase of matter. In contrast, under widely accepted complexity theory assumptions, classical algorithms that do not learn from data cannot achieve the same guarantee. We also prove that classical ML algorithms can efficiently classify a wide range of quantum phases of matter. Our arguments are based on the concept of a classical shadow, a succinct classical description of a many- body quantum state that can be constructed in feasible quantum experiments and be used to predict many properties of the state. Extensive numerical experiments corroborate our theoretical results in a variety of scenarios, including Rydberg atom systems, 2D random Heisenberg models, symmetry- protected topological phases, and topologically ordered phases.
Designing molecular color centers for quantum information science
The development of quantum technologies, such as quantum computing, communication, and sensing, relies on exquisite control over materials design and fabrication. As a result, substantial effort has been devoted to designing, creating, and controlling quantum bits (qubits). Thus, “building” spin-based qubits with atomic precision represents a powerful approach to develop and optimize these systems. Synthetic chemistry offers such an approach to create designer qubits, providing impeccable control over both physical and electronic structure of the qubit. However, in contrast to other qubit platforms, molecular qubits typically lack general mechanisms to achieve optical-spin initialization and readout. To overcome this limitation, we aim to imbue molecules with the optical addressability exhibited by solid-state color centers. We first outline the criteria required to replicate the electronic structure that enables optical addressability in molecular systems. In doing so, we target organometallic, pseudo-tetrahedral chromium compounds in strong ligand field environments. The combination of a strong ligand field environment and near tetrahedral symmetry around these d2, Cr4+R4 compounds, where R = o-tolyl, 2,3-dimethylphenyl, and 2,4-dimethylphenyl, generates a spin-triplet ground state with the desired optical-spin interface. We then demonstrate optical initialization, coherent spin manipulation, and optical readout for these Cr4+ compounds. Through ligand functionalization and substitution, we highlight both the impact of minor environmental modifications on the resulting spin and optical properties as well as the chemical handles that we may tune to optimize our molecular design. Our results illustrate that these synthetically flexible candidate qubits offer a platform to realize bespoke optically addressable molecular color centers. Moreover, this highly collaborative work demonstrates the need for cross-disciplinary approaches to address current and future challenges in quantum information science.
Imaging clock shifts in a Fermi-degenerate gas of strontium
Over the past decade, Alkaline-earth optical lattice clocks have far surpassed SI-defining caesium fountain clocks in terms of both accuracy and stability. Their exponential growth has been enabled by ever increasing atom numbers and atom-light coherence times.
With the goal of continuing this trend, we have developed a platform capable of achieving state-of-the-art coherence times (> 10s) at unprecedented atomic densities (~10^13/cm^3) by trapping a Fermi-degenerate gas of strontium in a state-independent, three-dimensional optical lattice. This talk will focus on recent efforts to improve the duty cycle of, and measure novel clock shifts in such an apparatus. We report on the creation of a spin polarized quantum gas of fermions (T/T_F < 0.2) in 2.5 s through highly efficient laser and evaporative cooling, and state-selective optical trapping. Clock shifts unique to such an apparatus include those arising from multi-body (N>2) collisional processes and long-range electric-dipole interactions. Spatially resolving the atomic response, as a function of the local density, enables the measurement of such shifts beyond what is possible through relying on the shot-to-shot stability of the local oscillator.
Line-graph-lattice models and materials
The geometric properties of a lattice can have profound consequences on its band spectrum. For example, geometric frustration and symmetry constraints can give rise to dispersionless and topologically nontrivial bands, respectively. Line-graph lattices have been known to exhibit the former; recently, we have shown how their flat bands may also exhibit the latter and host fragile topology. Given that line-graph lattices arise naturally from lattices of microwave resonators, this theoretical work informs experimental studies with superconducting circuits. At the same time, line- graph lattices may be present within real materials. This possibility has motivated a high-throughput search for line- graph-lattice crystalline structures within a database of inorganic stoichiometric materials.
Toric code topological order in Rydberg atom arrays: a BEC of quantum strings
Recent decades have led to a deep theoretical understanding of topological order---quantum phases of matter characterized by emergent anyons and long-range entanglement. However, their physical realization and detection has remained a very challenging endeavor. In this talk, I will show how Rydberg atoms placed on a two- dimensional ruby lattice naturally realize one of the most paradigmatic types of topological order, known as the toric code. The key concept is that the long-range Ising model describing this system leads to an effective dimer model on the kagome lattice. This gives a direct way of probing the presence of topological order: one can interpret the desired phase as a Bose-Einstein condensate of string operators measuring the dimers and their quantum coherence. I will discuss how these nonlocal order parameters are directly measurable in this cold atom set-up, as has been demonstrated in a recent implementation of this proposal.
Ultracold Complex Polyatomic Molecules
The exquisite control over quantum systems achievable at ultralow temperatures has been a hallmark of modern atomic and molecular physics. For example, by rapidly and repeatedly scattering laser photons, individual neutral atoms have been loaded into long-lived optical traps for quantum computation and simulation applications. Diatomic molecules have also been isolated in optical tweezers, motivated by theoretical proposals that their inherently strong couplings may enhance quantum computing schemes. Compared to these systems, polyatomic molecules offer qualitatively unique vibrational and rotational motions that enable new opportunities in physics, chemistry, and quantum technology. For instance, all polyatomic molecules have long-lived states with angular momentum arising from nuclear motion. These states offer distinct level structures including Debye-level Stark shifts at low applied electric fields. To fully leverage these features often requires ultracold temperatures, but the structural asymmetries present in molecules were previously thought to limit laser cooling to a select group of small (diatomic) species. Excitingly, recent theoretical and experimental work has begun to show that, in fact, many classes of complex molecules can be tamed using standard methods such as rapid photon cycling and laser cooling. At the same time, polyatomic molecules present some distinct challenges that require an experimental toolbox blending the deep cooling enabled by photon cycling with the efficiency afforded by Sisyphus-type forces. In this talk, we will present recent experimental work establishing some of these tools. In addition, we will discuss theoretical results showing how these methods may be extended to complex polyatomic species, including chiral molecules and molecules containing aromatic rings. We will draw on examples spanning linear (CaOH, YbOH), nonlinear symmetric (CaOCH3), and asymmetric (CaSH, SrOC10H7) molecules. Though early in the development of this field, it appears we are entering a fascinating new era of molecular physics: one in which "designer" molecules can be tailored for specific experimental tasks without sacrificing the benefits of laser-based quantum control.
Design and synthesis of new nickelate superconductors using molecular beam epitaxy
Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new ‘cuprate-like’ superconducting materials. One prime materials candidate has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd0.8Sr0.2NiO2. Undoped NdNiO2 belongs to a series of layered square-planar nickelates with chemical formula Ndn+1NinO2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here, using reactive oxide molecular beam epitaxy, which provides atomic level layer-by-layer control of thin film synthesis, we design and synthesize the quintuple-layer (n = 5) member of this series, Nd6Ni5O12. This layered compound Nd6Ni5O12 is a unique lower-dimensional version of NdNiO2, as it comprises NdO2 spacer layers that sandwich every five layers of NdNiO2. Importantly, Nd6Ni5O12 achieves optimal cuprate-like electron filling (3d8.8) without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd6Ni5O12 interpolates between cuprate-like and infinite-layer nickelate-like behavior. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality. In this talk, using nickelates to exemplify, I will additionally discuss molecular beam epitaxy (MBE) as a technique that can be generally harnessed in the synthesis of new quantum materials and superconductors.
Transition from an atomic to a molecular Bose–Einstein condensate and quantum many-body chemistry
Molecular quantum gases (that is, ultracold and dense molecular gases) have many potential applications, including quantum control of chemical reactions, precision measurements, quantum simulation and quantum information processing. For molecules, to reach the quantum regime usually requires efficient cooling at high densities, which is frequently hindered by fast inelastic collisions that heat and deplete the population of molecules. Here we report the preparation of two-dimensional Bose–Einstein condensates (BECs) of spinning molecules by inducing pairing interactions in an atomic condensate near a g-wave Feshbach resonance. The trap geometry and the low temperature of the molecules help to reduce inelastic loss, ensuring thermal equilibrium. From the equation-of-state measurement, we determine the molecular scattering length to be +220(±30) Bohr radii (95% confidence interval). Our work demonstrates the long-sought transition between atomic and molecular condensates, the bosonic analogue of the crossover from a BEC to a Bardeen−Cooper−Schrieffer (BCS) superfluid in a Fermi gas. I’ll also talk about our recent study on the chemical reaction process in ultralow temperature regime where atoms in the BEC participate collectively to form the molecules. This is reflected in a coherent oscillation in the molecule number, in contrast to a monotonic growth to a saturated value in the high temperature regime.