A new platform for quantum science: programmable arrays of single atoms inside an optical cavity

– 02/16 – 3:30pm – Gant West, GW-002 –

Abstract: Recently, programmable arrays of single atoms have emerged as a leading platform for quantum computing and simulation with experiments demonstrating control over hundreds of atoms [1]. Interfacing an atom array with a high-quality optical cavity promises even greater control and new capabilities. By coupling atoms to an optical cavity, we can more efficiently collect light from each atom improving detection. In addition, an optical cavity can be used to efficiently entangle many atoms in a single step relying on a novel technique called counterfactual carving [2]. I will describe our progress towards the goal of detecting and correcting errors on a register of Rubidium atoms selectively coupled to a large-waist optical cavity. Beyond detecting errors, applying corrections requires real-time feedback, and I will present a simple experiment demonstrating that fast feedback on microsecond timescales can already improve measurement fidelity. Finally, I will describe our accidental realization that we can use our cavity to directly observe collisions between pairs of trapped atoms in real time.

[1] Dolev Bluvstein et al. “Logical quantum processor based on reconfigurable atom
arrays”. In: Nature (Dec. 2023).
[2] Joshua Ramette et al. “Counter-factual carving exponentially improves entangled-state fidelity”. In: Arxiv preprint arXiv:2401.11407 (2024).

On noise in swap ASAP repeater chains: exact analytics, distributions and tight approximations

– 11/07 – 12:00pm – HBL Instruction 1102 –

Abstract: Losses are one of the main bottlenecks for the distribution of entanglement in quantum networks, which can be overcome by the implementation of quantum repeaters. The most basic form of a quantum repeater chain is the swap ASAP repeater chain. In such a repeater chain, elementary links are generated and swapped as soon as two adjacent links have been generated. As each entangled state is waiting to be swapped, decoherence is experienced, lowering the fidelity of the state. The aim of this project is to understand the total amount of decoherence experienced. We find analytical expressions for the average noise and its distribution for a small number of links. Furthermore, by exploiting tools from analytic combinatorics we find exponentially tight approximations on the average noise. Finally, we also use methods from statistical physics to numerically calculate quantities of interest for the inhomogeneous case. Our tools can be used to understand and optimize the performance of near-term quantum communication systems.

Bio: Dr. Kenneth Goodenough is a postdoctoral researcher under Don Towsley at the University of Massachusetts, Amherst. During his PhD with David Elkouss at QuTech he has worked on near-term repeater schemes, and afterwards focused on distillation and error correction. Currently he is interested in understanding the mathematical structures behind noisy quantum systems.

Quenched random-mass disorder in the critical Gross-Neveu-Yukawa Models

– 11/03 – 1:00pm – S213K –

Abstract: In the clean limit, continuous symmetry-breaking quantum phase transitions in 2D Dirac materials such as graphene and surfaces of 3D topological insulators are described by (2+1)D critical Gross-Neveu-Yukawa (GNY) models. In this talk, I will present our results of the study of the effects of quenched random-mass disorder, both short- and long-range correlated, on the universal critical properties of the Ising, XY, and Heisenberg GNY models. The problem was studied via the application of the replica renormalization group combined with a controlled triple epsilon expansion below four dimensions. Among interesting results, we find new finite-disorder quantum critical and multicritical points and an instance of the supercritical Hopf bifurcation in the renormalization-group flow, which is accompanied by the birth of a stable limit cycle corresponding to discrete scale invariance.

Time permitting, I will lay out a picture of possible percolation of the topological phase in the ferroelectric superconductors subjected to magnetic field.

Real space diagnosis of bulk topological invariants

– 09/29 – 12:00pm – S213 K –

Abstract: Significant progress has been made to identify materials hosting topologically non-trivial band structures. Much of this progress is due to advancements in analysis of symmetry indicators. Despite this progress, only a handful of high-quality topological materials are under experimental control. Recent theoretical developments have shown the existence of exotic topological phases invisible to symmetry eigenvalues in experimentally relevant materials, requiring direct identification of the bulk topological invariant. I will present newly developed real-space tools for assignment of the bulk invariant in such systems. It is shown that these methods can be implemented in the presence of disorder and correlations. In addition, these methods can be incorporated into high-throughput DFT studies in the limit of weak correlations, revealing non-trivial topology in multiple experimentally synthesized materials.

Vortex Excitations of Dirac Bose-Einstein Condensates

– 12/08 – 11:00am – S213 K / Zoom –

Abstract: We explore vortices in non-equilibrium Dirac Bose-Einstein condensates (Dirac BEC) described by a stationary Dirac Gross-Pitaevskii equations (GPE). We find that the multi-component structure of Dirac equation enables the difference in phase winding of two condensates with respective phase winding number differing by one, \(\ell_{a} – \ell_{b} = \pm 1\). We observe three classes of vortex states distinguished by their far-field behavior: A ring soliton on either of the two components in combination with a vortex on the other component, and, in the case of strong inter-component interactions, a vortex profile on both components. The latter are multiple core vortices due to the phase winding difference between the components. We also address the role of a Haldane gap on these vortices, which has a similar effect than inter-component by making the occupation on either sublattice more costly. We employ a numerical shooting method to reliably identify vortex solutions and use it to scan large parts of the phase space. We then use a classification algorithm on the integrated wavefunctions to establish a phase diagram of the different topological sectors.

arXiv:2202.07594

Hopfions in Condensed Matter: Anisotropic Heisenberg Magnets

– 11/30 – 2:00pm – S117 –

 

Abstract:

Nontrivial topological defects such as knotted solitons called hopfions have been observed in a variety of materials including chiral magnets, nematic liquid crystals and even in ferroelectrics as well as studied in other physical contexts such as Bose-Einstein condensates.  These topological entities can be modeled using the relevant physical variable, e.g., magnetization, polarization or the director field.  Specifically, we find exact static soliton solutions for the unit spin vector field of an inhomogeneous, anisotropic three-dimensional (3D) Heisenberg ferromagnet and calculate the corresponding Hopf invariant H analytically and obtain an integer, demonstrating that these solitons are indeed hopfions [1]. H is a product of two integers, the first being the usual winding number of a skyrmion in two dimensions, while the second encodes the periodicity in the third dimension. We also study the underlying geometry of H, by mapping the 3D unit vector field to tangent vectors of three appropriately defined space curves. Our analysis shows that a certain intrinsic twist is necessary to yield a nontrivial topological invariant (linking number). Finally, we focus on the formation energy of hopfions to study their properties for potential applications.  

[1] R. Balakrishnan, R. Dandoloff, and A. Saxena, arXiv:2202.07195  

Short bio: Avadh Saxena is Group Leader of the Condensed Matter and Complex Systems group (T-4) at Los Alamos National Lab, New Mexico, USA where he has been since 1990.  He is also an affiliate of the Center for Nonlinear Studies at Los Alamos. His main research interests include phase transitions, optical, electronic, vibrational, transport and magnetic properties of functional materials, device physics, soft condensed matter, non-Hermitian quantum mechanics, geometry, topology and nonlinear phenomena/materials harboring topological defects such as solitons, polarons, excitons and breathers. He is an Affiliate Professor at the Royal Institute of Technology (KTH), Stockholm, Sweden and holds adjunct professor positions at the University of Barcelona, Spain, Virginia Tech and the University of Arizona, Tucson. He is Scientific Advisor to National Institute for Materials Science (NIMS), Tsukuba, Japan. He is a Fellow of Los Alamos National Lab, a Fellow of the American Physical Society (APS), and a member of the Sigma Xi Scientific Research Society, APS and American Ceramic Society (ACerS). Contact him at: avadh@lanl.gov