Observation of Quantized Conductance in Neutral Matter

In transport experiments the quantum nature of matter becomes directly evident when changes in conductance occur only in discrete steps, with a size determined solely by Planck's constant h. The observations of quantized steps in the electric conductance have provided important insights into the physics of mesoscopic systems and allowed for the development of quantum electronic devices. Even though quantized conductance should not rely on the presence of electric charges, it has never been observed for neutral, massive particles. In its most fundamental form, the phenomenon requires a quantum degenerate Fermi gas, a ballistic and adiabatic transport channel, and a constriction with dimensions comparable to the Fermi wavelength. Here we report on the observation of quantized conductance in the transport of neutral atoms. We employ high resolution lithography to shape light potentials that realize either a quantum point contact or a quantum wire for atoms. These constrictions are imprinted on a quasi two-dimensional ballistic channel connecting two adjustable reservoirs of quantum degenerate fermionic lithium atoms. By tuning either a gate potential or the transverse confinement of the constrictions, we observe distinct plateaus in the conductance for atoms. The conductance in the first plateau is found to be equal to 1/h, the universal conductance quantum. For low gate potentials we find good agreement between the experimental data and the Landauer formula, with all parameters determined a priori. Our experiment constitutes the cold atom version of a mesoscopic device and can be readily extended to more complex geometries and interacting quantum gases.Comment: 7 pages, 4 figure

Confinement induced molecules in a 1D Fermi gas

We have observed two-particle bound states of atoms confined in a one-dimensional matter wave guide. These bound states exist irrespective of the sign of the scattering length, contrary to the situation in free space. Using radio-frequency spectroscopy we have measured the binding energy of these dimers as a function of the scattering length and confinement and find good agreement with theory. The strongly interacting one-dimensional Fermi gas which we create in an optical lattice represents a realization of a tunable Luttinger liquid.Comment: 4 page

State Capacity and Public Debt: A political economy analysis

High public debt combined with low capacities of the state to raise taxes and to support markets can put even developed countries into turmoil. However, the existing political economy literature of state capacity, pioneered by Besley and Persson (2009), does not investigate the interaction of these capacities with public debt. This paper studies the incentives behind raising debt and building state capacity in an integrated analytical framework. We examine the impact of political stability, cohesiveness of institutions, and income fluctuations on the political outcome, while allowing for sovereign default. We investigate when public debt and state capacity investments move in the same or opposite directions in response to exogenous parameter changes. This allows us to show when a state will simultaneously accumulate high public debt and invest only little in its capacities to raise taxes and to support markets, leading to a positive probability of sovereign default