Quantum Computing steady advances

I have been meaning to take a morning and read through the literature and try and come up with a back of the blog coverage of what I see out there. If anything I want to clarify my own ideas on the state of the field. There is a flurry of research and what I am reading is way past the theoretical predictions of 5 years ago where quantum computing was an idea expressed in pen and paper. Today experiment seems to be leading the theoretical way and opening new venues of research. Here are some highlights.

1/ Spintronics in carbon nano-tubes

Covered in Nature March 27 2008 issue.

Spintronics is the field that uses the spin of electrons to represent information rather than their charge. Already is used in production in large capacity hard drive heads and was rewarded with the Nobel prize in 2007 (Fert and Grunberg). Spintronics are being investigated in exotic geometries like nanotubes. By applying a Coulomb blockade along the axis of the tube, we can create an energy space where basically ONE electron or ONE hole can exist and we can track those energy state (see picture). The notable result here is that these experiments show that the spin is HIGHLY COUPLED with the orbital movement. Meaning that we now have a way to manipulate the spin by manipulating the movement of the electron, by interacting with the charge. That means there is an easy way to set the bits of information in spin. We can read and write spin by electrical manipulation alone.

2/ Long coherence/fast read in Diamonds

Using the good old geometry of carbon in diamonds and ultrafast laser (I can't find the article in my piles of papers), experiments have demonstrated read times that are well below the decoherence time. Diamonds offer a good medium to create entanglement, the basis for quantum computation by putting in close contact the spin carrying electrons. The problem is that the quantum information is encoded in the superimposition of states. In quantum physics this can be represented by its wave function |psi> = a|A> + b|B> in simple cases, where |A> and |B> are the individual states and a and b the relative weights of the states. Remember that in q-physics things exist in probabilistic superimposition at the same time. They are not one or the other, they are both. The degeneration of this state, also known as the "wave function collapse" leads to classical observations. The collapse is also known as "decoherence" and the quantum information disappears on the time-scale of decoherence. Here with femtosecond pulses of laser light they are able to observe the superimposition (measure) and extract the quantum information by light means. In other words reading time << decoherence time, paving the way for a practical application in diamond.

3/ Silica on Silicon quantum wave guides
Covered in this morning's Science May 2nd 2008.


Fig. 1. Silica-on-silicon integrated quantum photonic circuits. (A) A directional coupler, which can be used as the building block for integrated photonic quantum circuits by replacing the bulk BS. (B) The modeled transverse intensity profile of the guided mode superimposed on the waveguide structure. (C) Design of the integrated two-photon CNOT quantum logic gate.

This is really pretty neat. Using conventional fabrication techniques on silicon, a lab from Bristol UK demonstrates quantum coupling of photons in waveguides. They build a logical gate (C) and measure the results. Looks like a winner to me. Cheap, reliable, in line with current fabrication techniques. Less out there than the 2 previous results but exciting because of that 'pedestrian approach'.

While those computers won't be on our IT departments any time soon, I need to start looking at the cool software for those devices. I am thinking a kick ass financial markets modeler as the interactions are highly coupled and dynamic and probabilistic. If only I could find the Hamiltonian for the S&P 500!!!. Bring it on.