The Quantum Pontiff

Okay, so keeping running notes on friendfeed isn't going to work for me. Just too hard to do this and make a readable record. Really we should just be taping the talks.

Summary of day one below the fold (this may be a bit off as this is being written a day later.)

Jack Harris, Optomechanical systems

Papers: arXiv:0811.1343, arXiv:0707.1724.

Jack talked about his cool work coupling optical systems to mechanical systems. Take a cavity and stick a mechanical system (a dielectric membrane of thickness 50nm and quality factor of about a million) into it. Jack showed how you could cool the center of mass motion of the mechanical system to 7 mK. Note that other modes may still be hot! Further it looks like it should be possible to get to the ground state of this mode. Jack also showed how to get a coupling which was quadratic in the position variable. This should allow for quantum non-demolition measurement of the position. A trick used here was to use avoided crossing of higher modes of the cavity to get larger quadratic coupling.

Malcolm Carroll, Development of a Silicon Physical Qubit and Single Logical Qubit Design

Malcolm talked about Sandia's attempt to build a Silicon double quantum dot qubit. In the double quantum dot qubit, the qubit states are the singlet and one of the triplet states. This qubit was demonstrated by the Marcus lab at Harvard in GaAs. One issue in GaAs is that the qubit couples to nuclear spins...so move to Silicon where this is not a problem. The problem then is that Silicon is not as well behaved. In particular surface effects are a real problem. Malcolm showed how they can get around some of this disordering by doing lots of material engineering and also by making smaller dots (smaller dot = less change of the bad stuff being around.)

Malcolm also talked about Sandia's work on building a quantum error correcting architecture for their double quantum dot qubits. This involves a lot of heavy duty thinking about laying things out for their qubit, and worrying about schedule constraints based upon this layout, and how to integrate this all in a cryo system. A main result of this study was that if they wanted their first level of error correction to show an improvement they need to use dynamic decoupling.

Thaddeus Ladd, Recent Progress in Quantum Computing with Optically Controlled Semiconductors

Thaddeus talked about four different subjects. One was how to rapidly initialize a single spin qubit in a InAs quantum dot using ultra-fast laser pulses. This was able to get high polarization in 100ms. Another topic was performing fast optical gates on these systems. A third topic was how to generate indistinguishable single photons from two separate ZnSe/ZnMgSe quantum wells.

Haitao Quan, Quantum Fidelity and Thermal Phase Transitions

Paper: arXiv:06012007,

Haitao talked about the fidelity approach to understanding quantum phase transitions. My understanding is that to study quantum phase transitions (where you are varying a parameter and watching how the phases change) instead of looking at typical things like the specific heat, one looks at the fidelity of the ground state (at zero temperature) under perturbation. That is, if |gs(x)> is the ground state and x is the control parameter, then you examine . Haitao talked about this approach and also the equivalent case where you working with a nonzero temperature system. In that later case you can talk about the Uhlmann fidelity and you can perturb not just the control parameter but also the temperature. Haitao showed how the fidelity approach works very well for a lot of quantum phase transitions but not so well for others and suggested some ways to fix these latter cases.