Number squeezed and fragmented states of strongly interacting bosons in a double well

We present a systematic study of the phenomena of number squeezing and fragmentation for a repulsive Bose-Einstein condensate (BEC) in a three dimensional double well potential over a range of interaction strengths and barrier heights, including geometries that exhibit appreciable overlap in the one-body wavefunctions localized in the left and right wells. We compute the properties of the condensate with numerically exact, full dimensional path integral ground state (PIGS) Quantum Monte Carlo simulations and compare with results obtained from using two- and eight-mode truncated basis models. The truncated basis models are found to agree with the numerically exact PIGS simulations for weak interactions, but fail to correctly predict the amount of number squeezing and fragmentation exhibited by the PIGS simulations for strong interactions. We find that both number squeezing and fragmentation of the BEC show non-monotonic behavior at large values of interaction strength a. The number squeezing shows a universal scaling with the product of number of particles and interaction strength (Na) but no such universal behavior is found for fragmentation. Detailed analysis shows that the introduction of repulsive interactions not only suppresses number fluctuations to enhance number squeezing, but can also enhance delocalization across wells and tunneling between wells, each of which may suppress number squeezing. This results in a dynamical competition whose resolution shows a complex dependence on all three physical parameters defining the system: interaction strength, number of particles, and barrier height.Comment: 33 pages, 21 figures. Submitted for publication in Physical Review

Stable quantum memories with limited measurement

We demonstrate the existence of a finite temperature threshold for a 1D stabilizer code under an error correcting protocol that requires only a fraction of the syndrome measurements. Below the threshold temperature, encoded states have exponentially long lifetimes, as demonstrated by numerical and analytical arguments. We sketch how this algorithm generalizes to higher dimensional stabilizer codes with string-like excitations, like the toric code.Comment: 11 Pages, 7 Figure

Relaxation dynamics of the toric code in contact with a thermal reservoir: Finite-size scaling in a low temperature regime

We present an analysis of the relaxation dynamics of finite-size topological qubits in contact with a thermal bath. Using a continuous-time Monte Carlo method, we explicitly compute the low-temperature nonequilibrium dynamics of the toric code on finite lattices. In contrast to the size-independent bound predicted for the toric code in the thermodynamic limit, we identify a low-temperature regime on finite lattices below a size-dependent crossover temperature with nontrivial finite-size and temperature scaling of the relaxation time. We demonstrate how this nontrivial finite-size scaling is governed by the scaling of topologically nontrivial two-dimensional classical random walks. The transition out of this low-temperature regime defines a dynamical finite-size crossover temperature that scales inversely with the log of the system size, in agreement with a crossover temperature defined from equilibrium properties. We find that both the finite-size and finite-temperature scaling are stronger in the low-temperature regime than above the crossover temperature. Since this finite-temperature scaling competes with the scaling of the robustness to unitary perturbations, this analysis may elucidate the scaling of memory lifetimes of possible physical realizations of topological qubits.Comment: 14 Pages, 13 figure

Hybridization of Lepomid sunfishes by use of cryopreservation

The goal of this project was to integrate cryopreservation into the hybridization of sunfishes. The first objective was to evaluate the conditioning of sunfishes to spawn in cages in warmwater ponds. Bluegill and green sunfish were stocked in cages in ponds heated to 27 C by geothermal water. Fish in heated ponds did not spawn on artificial nests in cages. Fish were also brought into the laboratory for artificial spawning. There were 22 spawns yielding 47 ± 19% (mean ± SD) fertilization. The second objective was to develop methods for the refrigerated and frozen storage of sperm of bluegill Lepomis macrochirus and green sunfish Lepomis cyanellus. Sperm were stored in Hanks’ balanced salt solution prepared at 300 mOsmol/kg, and activated with solutions prepared at 80 mOsmol/kg. Sperm stored at 4 C maintained motility for as long as10 d, but the optimum use of sperm was on the day it is collected. Cryopreserved sperm should be thawed in a 40 C water bath for 7 s. The motility of cryopreserved sperm lasted for less than 1 min. Cryopreserved sperm of bluegill and green sunfish were able to fertilize green sunfish eggs. The third objective was to improve methods of cryopreservation of sperm of bluegill and coppernose bluegill Lepomis macrochirus purpurescens to produce hybrids with green sunfish. Sperm were exposed to five concentrations of five cryoprotectants and motility was estimated at 10 min intervals for 30 min. There were concentration-dependent effects on motility over time. One concentration of each cryoprotectant was used for cryopreservation. Sperm cryopreserved with methanol had the highest post-thaw motility while sperm cryopreserved with propylene glycol yielded the highest fertilization. Sperm of bluegill and coppernose bluegill were cryopreserved and used to fertilize eggs of the same green sunfish. There were no significant differences between the subspecies for motility before freezing (P = 0.4704), post-thaw motility (P = 0.5841), fertilizing ability (P = 0.1351) and hatch (P = 0.1133)

Stroboscopic Generation of Topological Protection

Trapped neutral atoms offer a powerful route to robust simulation of complex quantum systems. We present here a stroboscopic scheme for realization of a Hamiltonian with $n$-body interactions on a set of neutral atoms trapped in an addressable optical lattice, using only 1- and 2-body physical operations together with a dissipative mechanism that allows thermalization to finite temperature or cooling to the ground state. We demonstrate this scheme with application to the toric code Hamiltonian, ground states of which can be used to robustly store quantum information when coupled to a low temperature reservoir.Comment: 5 pages, 2 figures. Published versio

Criteria for dynamically stable decoherence-free subspaces and incoherently generated coherences

We present a detailed analysis of decoherence free subspaces and develop a rigorous theory that provides necessary and sufficient conditions for dynamically stable decoherence free subspaces. This allows us to identify a special class of decoherence free states which rely on incoherent generation of coherences. We provide examples of physical systems that support such states. Our approach employs Markovian master equations and applies primarily to finite-dimensional quantum systems