Bandgap Controlled in Bilayer Graphene
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also need to be able to make two qubits interact,” said Schoelkopf. “With this experiment we don’t just operate one gate; we string together 10 one-qubit gates and 2 two-qubit gates.” “Both qubits in the two-qubit gates have to work at the same time, so you have to be able to reliably make two qubits with long coherence times,” said Steve Girvin, co-author of the article and co-principal investigator. “We used a charge-based qubit, which normally would be sensitive to electrical noise. But we developed one that stays insensitive to noise for a long time, up to three microseconds.” “There’s a tension between using larger-
scale manmade systems like ours as qubits, which are easier to make, test and control, versus using individual atoms, which stay coherent longer, but are much more difficult to couple together in complex ways,” said Schoelkopf. “But there’s an advantage to using a superconducting circuit, which is all controlled electronically,” he said. “The goal is to make a scalable device, with thousands and thousands of qubits working together. This is still a long way off, but the idea of using standard integrated circuit technology makes it easier to imagine that it might be possible someday.” Although the quantum processor itself must be kept just above absolute zero in
order to maintain the superconducting properties of the circuit, DiCarlo said that the rest of the system looks like a typical processor, with only wires going into the system and wires coming out. But Schoelkopf cautions it will still be some time before solid-state quantum computers become the industry standard. “I’m relatively optimistic that we should be able to combine three or more qubits soon,” Schoelkopf said. “But to make a system which will actually perform computations on your laptop would take a thousand qubits. It’s hard to see that far into the future, but this experiment is a significant step forward.”
Creep in Concrete Occurs at the Nanoscale Level
spherical objects (64% for the lower density and 74% for high). Now, as reported in the Proceedings of the National Academy of Sciences online Early Edition the week of June 15 (DOI: 10.1073/pnas.0901033106), Ulm and Vandamme explain that concrete creep comes about when these nanometersized C-S-H particles rearrange into altered densities: some looser and others more tightly packed. They also explain that a third, more dense phase of C-S-H can be induced by manipulating the cement mix with other minerals such as silica fumes, a waste material of the aluminum industry. These reacting fumes form additional smaller particles that fit into the spaces between the nano-granules of C-S-H, spaces that were formerly filled with water. This has the effect of increasing the density of C-S-H to up to 87%, which in turn greatly hinders the movement of the C-S-H granules over time.
The researchers show experimentally that the rate of creep is logarithmic, which means slowing creep increases durability exponentially. They demonstrate mathematically that creep can be slowed by a rate of 2.6.
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