New directions with old collections: changing existing exhibitions for new purposes.

Paper presented at the Wits History Workshop: Myths, Monuments, Museums; New Premises? 16-18 July, 199

Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars

The Hipparcos Intermediate Astrometric Data for carbon stars have been reprocessed using an algorithm which provides an objective criterion for rejecting anomalous data points and constrains the parallax to be positive. New parallax solutions have been derived for 317 cool carbon stars, mostly of types R and N. In this paper we discuss the results for the R stars. The most important result is that the early R stars (i.e., R0 - R3) have absolute magnitudes and V-K colors locating them among red clump giants in the Hertzsprung-Russell diagram. Stars with subtypes R4 - R9 tend to be cooler and have similar luminosity to the N-type carbon stars, as confirmed by their position in the (J-H, H-K) color-color diagram. The sample of early R-type stars selected from the Hipparcos Catalogue appears to be approximately complete to magnitude K_0 ~ 7, translating into a completeness distance of 600 pc if all R stars had M_K= -2 (400 pc if M_K= -1). With about 30 early R-type stars in that volume, they comprise about 0.04% (0.14% for M_K= -1) of the red clump stars in the solar neighborhood. Identification with the red clump locates these stars at the helium core burning stage of stellar evolution, while the N stars are on the asymptotic giant branch, where helium shell burning occurs. The present analysis suggests that for a small fraction of the helium core burning stars (far lower than the fraction of helium shell-burning stars), carbon produced in the interior is mixed to the atmosphere in sufficient quantities to form a carbon star.Comment: 11 pages, 6 figures, A&A Latex. To appear in A&

Optimized pulses for the control of uncertain qubits

Constructing high-fidelity control fields that are robust to control, system, and/or surrounding environment uncertainties is a crucial objective for quantum information processing. Using the two-state Landau-Zener model for illustrative simulations of a controlled qubit, we generate optimal controls for \pi/2- and \pi-pulses, and investigate their inherent robustness to uncertainty in the magnitude of the drift Hamiltonian. Next, we construct a quantum-control protocol to improve system-drift robustness by combining environment-decoupling pulse criteria and optimal control theory for unitary operations. By perturbatively expanding the unitary time-evolution operator for an open quantum system, previous analysis of environment-decoupling control pulses has calculated explicit control-field criteria to suppress environment-induced errors up to (but not including) third order from \pi/2- and \pi-pulses. We systematically integrate this criteria with optimal control theory, incorporating an estimate of the uncertain parameter, to produce improvements in gate fidelity and robustness, demonstrated via a numerical example based on double quantum dot qubits. For the qubit model used in this work, post facto analysis of the resulting controls suggests that realistic control-field fluctuations and noise may contribute just as significantly to gate errors as system and environment fluctuations.Comment: 38 pages, 15 figures, RevTeX 4.1, minor modifications to the previous versio

Rarefied Hypersonic Flow About A Flat-Ended Circular Cylinder

The rarefied Mach 25 flow about a flat-ended cylinder was calculated using Bird's Direct Simulation Monte Carlo technique. Collisions between diatomic molecules were calculated using three rotational-translational energy exchanges models: (1) Pullin's approximate classical model (2) phenomenological (Borgnakke-Larsen) model with the exchange factor depending on the collision energy (variable phi model) (3) and hybrid classical phenomenological exchange model. Two scattering potentials were used to represent nitrogen: (1) the inverse 13.5 power potential and (2) the Morse potential. Simulation results for each collision model were found to be in good agreement with electron beam density surveys in the stagnating flow ahead of the cylinder. Heat transfer to the blunt front face of the cylinder was found to be sensitive to the collision model used, but all were within the statistical scatter of published data. It was suggested that measurements of temperature profiles in the stagnation region would allow better evaluation of the collision models

Time-optimal CNOT between indirectly coupled qubits in a linear Ising chain

We give analytical solutions for the time-optimal synthesis of entangling gates between indirectly coupled qubits 1 and 3 in a linear spin chain of three qubits subject to an Ising Hamiltonian interaction with equal coupling $J$ plus a local magnetic field acting on the intermediate qubit. The energy available is fixed, but we relax the standard assumption of instantaneous unitary operations acting on single qubits. The time required for performing an entangling gate which is equivalent, modulo local unitary operations, to the $\mathrm{CNOT}(1, 3)$ between the indirectly coupled qubits 1 and 3 is $T=\sqrt{3/2} J^{-1}$, i.e. faster than a previous estimate based on a similar Hamiltonian and the assumption of local unitaries with zero time cost. Furthermore, performing a simple Walsh-Hadamard rotation in the Hlibert space of qubit 3 shows that the time-optimal synthesis of the $\mathrm{CNOT}^{\pm}(1, 3)$ (which acts as the identity when the control qubit 1 is in the state $\ket{0}$, while if the control qubit is in the state $\ket{1}$ the target qubit 3 is flipped as $\ket{\pm}\rightarrow \ket{\mp}$) also requires the same time $T$.Comment: 9 pages; minor modification