## Friday Schroedinger's Cat Blogging, or Arcane Gazebo Has a Day Job

He reports on it. It's really very impressive. Macroscopic quantum systems with decoherence times on the order of 0.1 microseconds--that's enough time for light to travel 100 feet.

It is, however, necessary to cool the apparatus to 0.04 degrees Kelvin above absolute zero. Don't expect to see this in your home--unless your home is a sleeping bag under a desk in *Birge*~~Birch~~ Hall, that is.

Arcane Gazebo: Publication: Flux qubits and readout device with two independent flux lines: I'm posting something closer to a layman's explanation....

"Flux qubits and readout device with two independent flux lines," B. L. T. Plourde, T. L. Robertson, P. A. Reichardt, T. Hime, S. Linzen, C.-E. Wu, and John Clarke,

Phys. Rev. B72, 060506(R) (2005)

Sort-of non-technical explanation:Many of the weirder effects of quantum mechanics don't show up in macroscopic objects due to a kind of averaging out that occurs when large numbers of atoms are involved. One of these... is [that]... if a physical system has several possible states, mathematical combinations of these states are also available to it.... [H]owever, the superposition state... [rapidly] disappear[s] due to the averaging-out process I mentioned, which is referred to as decoherence. The time required for decoherence to take place for a system like a Schrödinger's cat would be vastly shorter than any observable time.What we did in our experiments was to build a macroscopic electrical circuit that behaved like a quantum object, with decoherence times long enough that we were able to see quantum effects... a loop of aluminum, broken in three places by thin layers of aluminum oxide (which in crystal form is sapphire)... cooled to a temperature 0.04 degrees Kelvin... aluminum becomes superconducting....

Two basic states of the circuit are currents travelling around the loop, either clockwise or counterclockwise. By adjusting the magnetic field... we can make one type of current flow energetically favorable.... But we can also balance the flux so that clockwise and counterclockwise currents have the same energy... the system will naturally form coherent superposition states like CW + CCW and CW - CCW. It turns out that, while the CW and CCW states have the same energy, the combinations have different energies.... This allows us to detect the superposition by applying microwave radiation with energy equal to the energy difference between the two states; when the circuit is in the lower energy state it will absorb the radiation and switch to the higher energy. We looked at the radiation absorbed by the circuit as the magnetic flux was varied across the degeneracy point: on a plot of radiation frequency vs. magnetic flux, an incoherent system will show straight lines going down to zero when the current flows are balanced, but a coherent system will bend away from zero frequency as it forms superposition states....

Not only did we see the bending of the curve away from zero, we were able to measure the energy difference between the two coherent superposition states, based on the observation that the lowest radiation absorbed was at a frequency of 4 GHz....

[T]he [Rabi] oscillations... are significantly smaller after about 80 nanoseconds. We'd like this time to be a lot longer.... This connects to quantum computing, a hot topic in physics based on the idea that computers can do certain calculations much faster if they can manipulate quantum superpositions of numbers.... The circuit I've described here is... a flux qubit.... Several groups around the world are currently working on flux qubits, and we are not the first to achieve results like these. Our qubit is distinct from the others in a few ways: it is quite a bit larger... the magnetic field is applied to it from a coil integrated onto the chip, which is important for scalability. Whether this route will be practical for quantum computation has yet to be determined, of course, but that's more of a problem for the engineers. (Feynman would say that we are already doing engineering...)

Feynman would also say that engineering is a very good thing to do, yes?