Global symmetries of quantum Hall systems: lattice description

I analyze non-local symmetries of finite-size Euclidean 3D lattice Chern-Simons models in the presence of an external magnetic field and non-zero average current. It is shown that under very general assumptions the particle-vortex duality interchanges the total Euclidean magnetic flux Phi/(2 Pi) and the total current I in a given direction, while the flux attachment transformation increases the flux in a given direction by the corresponding component of the current, Phi -> Phi+4 Pi I, independently of the disorder. In the language of 2+1 dimensional models, appropriate for describing quantum Hall systems, these transformations are equivalent to the symmetries of the phase diagram known as the Correspondence Laws, and the non-linear current-voltage mapping between mutually dual points, recently observed near the quantum Hall liquid-insulator transitions.Comment: 14 pages, 3 ps figures include

Quantum degenerate Bose-Fermi mixtures on 1-D optical lattices

We combine model mapping, exact spectral bounds, and a quantum Monte Carlo method to study the ground state phases of a mixture of ultracold bosons and spin-polarized fermions in a one-dimensional optical lattice. The exact boundary of the boson-demixing transition is obtained from the Bethe Ansatz solution of the standard Hubbard model. We prove that along a symmetry plane in the parameter space, the boson-fermion mixed phase is stable at all densities. This phase is a two-component Luttinger liquid for weak couplings or for incommensurate total density, otherwise it has a charge gap but retains a gapless mode of mixture composition fluctuations. The static density correlations are studied in these two limits and shown to have markedly different features.Comment: 4 pages, 4 figure

Dynamically corrected gates for qubits with always-on Ising couplings: Error model and fault-tolerance with the toric code

We describe how a universal set of dynamically-corrected quantum gates can be implemented using sequences of shaped decoupling pulses on any qubit network forming a sparse bipartite graph with always-on Ising interactions. These interactions are constantly decoupled except when they are needed for two-qubit gates. We analytically study the error operators associated with the constructed gates up to third order in the Magnus expansion, analyze these errors numerically in the unitary time evolution of small qubit clusters, and give a bound on high-order errors for qubits on a large square lattice. We prove that with a large enough toric code the present gate set can be used to implement a fault-tolerant quantum memory

Universal set of Dynamically Protected Gates for Bipartite Qubit Networks II: Soft Pulse Implementation of the [[5,1,3]] Quantum Error Correcting Code

We model repetitive quantum error correction (QEC) with the single-error-correcting five-qubit code on a network of individually-controlled qubits with always-on Ising couplings, using our previously designed universal set of quantum gates based on sequences of shaped decoupling pulses. In addition to serving as accurate quantum gates, the sequences also provide dynamical decoupling (DD) of low-frequency phase noise. The simulation involves integrating unitary dynamics of six qubits over the duration of tens of thousands of control pulses, using classical stochastic phase noise as a source of decoherence. The combined DD/QEC protocol dramatically improves the coherence, with the QEC alone responsible for more than an order of magnitude infidelity reduction.Comment: 12 pages, 9 figure

Holons on a meandering stripe

To study a possible effect of transverse fluctuations of a stripe in a 2D antiferromagnet on its charge dynamics, we identify elementary excitations of a weakly doped domain wall in the Hubbard model. Hartree-Fock numerics and analysis of fermion zero modes suggest that for U>=3t charged excitations are mobile holons, Q=1, S=0. Each holon resides on a kink in the position of the domain wall. We construct a simple model in which transverse stripe dynamics is induced solely by motion of the holons. In the absence of spin excitations (spinons, Q=0, S=1/2), stripe fluctuations DO NOT suppress a tendency to form a global charge-density order.Comment: REVTeX, 4 two-column pages, 4 eps figures with epsf.st

Refocusing of a qubit system coupled to an oscillator

Refocusing, or dynamical decoupling, is a coherent control technique where the internal dynamics of a quantum system is effectively averaged out by an application of specially designed driving fields. The method has originated in nuclear magnetic resonance, but it was independently discovered in atomic physics as a ``coherent destruction of tunneling''. Present work deals with the analysis of the performance of ``soft'' refocusing pulses and pulse sequences in protecting the coherence of a qubit system coupled to a quantum oscillator.Comment: 2.5pages Conference proceedings for Nanostructures: Physics and Technology, Novosibirsk (2007). Macros file nano2cmr.sty include

Higher-dimensional quantum hypergraph-product codes

We describe a family of quantum error-correcting codes which generalize both the quantum hypergraph-product (QHP) codes by Tillich and Z\'emor, and all families of toric codes on $m$-dimensional hypercubic lattices. Similar to the latter, our codes form $m$-complexes ${\cal K}_m$, with $m\ge2$. These are defined recursively, with ${\cal K}_m$ obtained as a tensor product of a complex ${\cal K}_{m-1}$ with a $1$-complex parameterized by a binary matrix. Parameters of the constructed codes are given explicitly in terms of those of binary codes associated with the matrices used in the construction.Comment: 6 pages, no figures. In version 2, a hole in the proof and several typos are correcte

Network of edge states: random Josephson junction array description

We construct a generalization of the Chalker-Coddington network model to the case of fractional quantum Hall effect, which describes the tunneling between multiple chiral edges. We derive exact local and global duality symmetries of this model, and show that its infrared properties are identical to those of disordered planar Josephson junction array (JJA) in a weak magnetic field, which implies the same universality class. The zero frequency Hall resistance of the system, which was expressed through exact correlators of the tunneling fields, is shown to be quantized both in the quantum Hall limit and in the limit of perfect Hall insulator.Comment: revtex, 4 double-column pages, 2 ps figures included with epsf. To be published in PRL. Changes include: clarified the concept of duality, added one reference, and substantially rewritten the ending of the manuscrip

Universal set of scalable dynamically corrected gates for quantum error correction with always-on qubit couplings

We construct a universal set of high fidelity quantum gates to be used on a sparse bipartite lattice with always-on Ising couplings. The gates are based on dynamical decoupling sequences using shaped pulses, they protect against low-frequency phase noise, and can be run in parallel on non-neighboring qubits. This makes them suitable for implementing quantum error correction with low-density parity check codes like the surface codes and their finite-rate generalizations. We illustrate the construction by simulating quantum Zeno effect with the $[[4,2,2]]$ toric code on a spin chain

Algebraic bounds for heterogeneous site percolation on directed and undirected graphs

We analyze site percolation on directed and undirected graphs with site-dependent open-site probabilities. We construct upper bounds on cluster susceptibilities, vertex connectivity functions, and the expected number of simple open cycles through a chosen arc; separate bounds are given on finite and infinite (di)graphs. These produce lower bounds for percolation and uniqueness transitions in infinite (di)graphs, and for the formation of a giant component in finite (di)graphs. The bounds are formulated in terms of appropriately weighted adjacency and non-backtracking (Hashimoto) matrices. It turns out to be the uniqueness criterion that is most closely associated with an asymptotically vanishing probability of forming a giant strongly-connected component on a large finite (di)graph.Comment: 16 pages, 3 figures. In v5, significant revision of text, expansion of uniqueness discussion, vertex connectivity and infinite graph bound