Author: dabacon

Turns out that of the mother and father protocols which have helped illuminate the field of quantum information, only the mother is necessary: quant-ph/0606225:

Title: The mother of all protocols: Restructuring quantum information’s family tree
Authors: Anura Abeyesinghe, Igor Devetak, Patrick Hayden, Andreas Winter
We give a simple, direct proof of the “mother” protocol of quantum information theory. In this new formulation, it is easy to see that the mother, or rather her generalization to the fully quantum Slepian-Wolf protocol, simultaneously accomplishes two goals: quantum communication-assisted entanglement distillation, and state transfer from the sender to the receiver. As a result, in addition to her other “children,” the mother protocol generates the state merging primitive of Horodecki, Oppenheim and Winter, a fully quantum reverse Shannon theorem, and a new class of distributed compression protocols for correlated quantum sources which are optimal for sources described by separable density operators. Moreover, the mother protocol described here is easily transformed into the so-called “father” protocol whose children provide the quantum capacity and the entanglement-assisted capacity of a quantum channel, demonstrating that the division of single-sender/single-receiver protocols into two families was unnecessary: all protocols in the family are children of the mother.

Yesterday I was at Bell Labs for a one day meeting on quantum computing. Bell Where, the young ones ask? You know, the place where the transistor was invented! (Can you name the three who won the Nobel prize in physics for the invention of the transistor? How many people that you meet walking down the street can name any of these three?)
Amazingly this meeting was covered by local media: see here. Any investors might be interested in the last few paragraphs of the article:

At least one audience member was impatient.
Jan Andrew Buck heads Princeton Group International, which backs biotech ventures. He said he is itching for a bare-bones quantum computer for plotting complicated routes and schedules.
“I think I can get a squeaky, scratchy quantum computer to market in two or three years,” Buck said. All he needs, he said, are investors with deep pockets and short deadlines.

Now I consider myself an optimist, but I think Jan Andrew Buck has just out optimismed even my cheery outlook.
Back to the topic at hand, the meeting was fun! The first two talks, by David DiVincenzo and Isaac Chuang, were interesting in that they both made some arguments about whether the “sea of qubits” type architecture for a quantum computer is really feasible. Loosely, the idea of a sea of qubits is to have, say, a two dimensional dense grid of qubits which you can control with nearest neighbor interactions. One difficulty with this approach is that if you have a dense sea of qubits, it is hard to imagine how to get all of the elements you need to control these qubits from a classical controller outside of the quantum computer to each individual qubit. This is particulary worrisome for some solid state qubits, where high density is often needed in order to get controllable strong two qubit interactions (like say in some quantum dot approaches), but applies to many other types of implementations as well. David DiVincenzo talked about work he performed with Barbara Terhal and Krysta Svore on threshold for two dimensional spatial layouts (see quant-ph/0604090.) Because the cost of this spatial layout was not huge, along with his work with a particular implementation of a superconducting qubit at IBM, David reconsidered, in his talk, whether the sea of qubit was really that bad of a problem. He concluded by discussing how perhaps techniques developed in making three dimensional circuitry could be used to overcome the sea of qubits problem. Ike, on the other hand, talked about the issues of designing a quantum computing made out of ions, where the issue of getting your classical control may not be as severe (other talks focused on the MEMS mirror arrays which will be used to control, in parallel, many thousands of ion trap qubits.) Ike was one of the original people to point out the difficulties in the “sea of qubits” ideas, and I can’t help but think the reason he started working on ion traps and not solid state implementations was in some part motivated by this problem.
To me, the debate about what the architecture for a future quantum computer will look like is very intersting. Mostly because this debate has to do, I think, with quantum computing people taking very seriously what “scalable” means. I personally can’t stand the word “scalable.” Why? Well mostly because it is put in front of every single proposal in which the authors can reasonably imagine some far fetched way to scale their proposed quantum system up. Call me jaded. But what is fun to watch is that, now that there is serious discussion of many qubit quantum computers, the real difficulties of scalbility are beginning to emerge. Scalability is about money. Scalability is about ease. Scalability is about architecture. Which physical implementation will scale up to our future quantum computer and what will the architecture of this computer look like? Depending on the day you ask me you’ll get a different answer. Which, I suppose, is one of the reasons why I remain but a lowly theorist…too scared to jump on the bandwagon I trust the most.