Rapidly Quenched Kosterlitz-Thouless Superfluid Transitions

Rapidly quenched Kosterlitz-Thouless (KT) superfluid transitions are studied by solving the Fokker-Planck equation for the vortex-pair dynamics in conjunction with the KT recursion relations. Power-law decays of the vortex density at long times are found, and the results are in agreement with a scaling proposal made by Minnhagen and co-workers for the dynamical critical exponent. The superfluid density is strongly depressed after a quench, with the subsequent recovery being logarithmically slow for starting temperatures near T$_{KT}$. No evidence is found of vortices being ''created'' in a rapid quench, there is only decay of the existing thermal vortex pairs.Comment: 4 pages, 5 figures, revtex4, version accepted for PR

Vortex Core Size in 3^3He-4^4He films with Monolayer Superfluid 4^4He

The superfluid transition of $^3$He-$^4$He mixture films adsorbed on alumina powder is studied, with a $^4$He superfluid coverage near one layer. With up to 1.3 layers of $^3$He added, the transition becomes strongly broadened, indicating a linear increase in the vortex core size for $^3$He coverages below one layer. Annealing of the sample mixture at 4.2 K is found to be critically important in ensuring a homogeneous film across the porous substrate.Comment: 2 pages, 2 figures, accepted for LT22 Conference Proceedings, Physica

Refining the Spin Hamiltonian in the Spin-1/2 Kagome Lattice Antiferromagnet ZnCu3_{3}(OH)6_{6}Cl2_{2} using Single Crystals

We report thermodynamic measurements of the S=1/2 kagome lattice antiferromagnet ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$, a promising candidate system with a spin-liquid ground state. Using single crystal samples, the magnetic susceptibility both perpendicular and parallel to the kagome plane has been measured. A small, temperature-dependent anisotropy has been observed, where $\chi_{z}/ \chi_{p} > 1$ at high temperatures and $\chi_{z}/ \chi_{p} < 1$ at low temperatures. Fits of the high-temperature data to a Curie-Weiss model also reveal an anisotropy. By comparing with theoretical calculations, the presence of a small easy-axis exchange anisotropy can be deduced as the primary perturbation to the dominant Heisenberg nearest neighbor interaction. These results have great bearing on the interpretation of theoretical calculations based on the kagome Heisenberg antiferromagnet model to the experiments on ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$.Comment: 4 pages, 4 figure

Efficient many-party controlled teleportation of multi-qubit quantum information via entanglement

We present a way to teleport multi-qubit quantum information from a sender to a distant receiver via the control of many agents in a network. We show that the original state of each qubit can be restored by the receiver as long as all the agents collaborate. However, even if one agent does not cooperate, the receiver can not fully recover the original state of each qubit. The method operates essentially through entangling quantum information during teleportation, in such a way that the required auxiliary qubit resources, local operation, and classical communication are considerably reduced for the present purpose

Rapid optimization of working parameters of microwave-driven multi-level qubits for minimal gate leakage

We propose an effective method to optimize the working parameters (WPs) of microwave-driven quantum logical gates implemented with multi-level physical qubits. We show that by treating transitions between each pair of levels independently, intrinsic gate errors due primarily to population leakage to undesired states can be estimated accurately from spectroscopic properties of the qubits and minimized by choosing appropriate WPs. The validity and efficiency of the approach are demonstrated by applying it to optimize the WPs of two coupled rf SQUID flux qubits for controlled-NOT (CNOT) operation. The result of this independent transition approximation (ITA) is in good agreement with that of dynamic method (DM). Furthermore, the ratio of the speed of ITA to that of DM scales exponentially as 2^n when the number of qubits n increases.Comment: 4pages, 3 figure

Neutron electric dipole moment and dressed spin

The neutron electric dipole moment (EDM) experiment has played a unique role in examining the violation of fundamental symmetries and understanding the nature of electroweak and strong interaction. A non-zero neutron EDM is one of direct evidence for CP and T violation and has the potential to reveal the origin of CP violation and to explore physics beyond the Standard Model. A new neutron EDM experiment will be built to improve a factor of 100 by using a novel technique of ultra-cold neutrons(UCN) in superfluid 4He at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL). In the experiment, 3He in the measurement cell will be used as a neutron spin analyzer and a comagnetometer. The absorption between UCN and 3He atoms will emit scintillation light in the superfluid 4He depending on the angle between nuclear spins of two particles. Consequently, the neutron precession frequency can be derived by the scintillation light amplitude. Furthermore, the 3He precession frequency can be measured by the superconducting quantum interference device (SQUID). A dressed-spin technique will also be applied to measure the small precession frequency change due to a non-zero neutron EDM. The dressed-spin technique is used to modify the effective precession frequencies of neutrons and 3He atoms to make them equal by applying an oscillatory field (dressing field) that is perpendicular to the static magnetic field. The phenomenon of the dressed spin for 3He in a cell should be demonstrated before the proposed neutron EDM experiment. A successful measurement over a broad range of the amplitude and frequency of the dressing field was done at the University of Illinois. The observed effects can be explained by using quantum optics formalism. The formalism is diagonalized to solve the solution and confirms the data. In addition, the application of the dressed-spin technique was investigated. The modulation and the feedback loop technique should be considered with the dressed-spin technique for the measurement of the small EDM effect. The modulation of the dressing field arbitrarily changes the relative precession frequency between UCN and 3He. Through the feedback loop, the effective neutron precession frequency can be measured. The corresponding sensitivity of neutron EDM will be estimated. A future neutron EDM experiment could be improved if the dressed-spin technique can be carefully considered and applied