## QIP – Post 4 (Wednesday recap)

Since it’s already Thursday, I want to give a recap of Wednesday.  I got back to my apartment rather late last night and didn’t get a chance to do the recap (more on why I was late in a bit).

The morning was kicked off by a pretty cool talk given by André Chailloux on quantum coin flipping motivated by the idea that Alice and Bob don’t trust each other (there must be an opportunity for a joke in here somewhere…).  The interesting thing is that their construction is entirely classical except for the presence of a black box.  Given all the other interesting aspects of the results, this seemed the most interesting to me since it seems to say something about how the universe works, i.e. just how that black box interacts with the classical parts is, in essence, the quantum-classical boundary.  At any rate, they have a strong coin flip protocol that matches Kitaev’s bound with a probability of cheating of $z=2-\sqrt{2}$. Regarding their black box, it is a bit like a back-and-forth teleportation scheme in a way. Their methods used point (as in points on a graph) games as an example and what they were doing had many elements of a directed graph (so there are connections to some of Rob Spekkens’ work in here – in fact they interpret their results in terms of the Spekkens-Rudolph protocol). In this midst of the talk, young André had a “Jan-Åke Larsson” moment when his girlfriend Skyped him (Jan-Åke’s wife did the same thing in the midst of an APS talk he gave in Denver several years ago).

Andreas Winter (who shall forever after be referred to as “The Chairman” – you had to be there), gave a nice talk on highly entangled states that, oddly, offer absolutely no secrecy. Aside from some computer problems (note that all the non-Macs have had trouble with the display connection), The Chairman’s main question was if there was a “bound” key and the answer is that, yes, there is an asymptotically bound one. His results were broken into both symmetric (bosonic) and antisymmetric (fermionic) parts and he focused his talk on the latter. He had some great pictures tossed in (including a Platypus – gotta love the Platypus).

Thomas Vidick followed with a talk about improved extractors for bounded quantum storage which is another interesting cryptography thing related to adversaries attempting to extract keys (oh those nasty adversaries). He was able to prove the security of his work using a proof by contradiction (I love seeing undergraduate stuff pop up in these talks).

I didn’t write down notes on most of Carolin Lunemann’s talk, but it was more cryptography stuff and sounded pretty cool. Stephanie Wehner followed with another talk on two-person cryptography (no Eve is present – essentially, do Alice and Bob trust one another?). In such a scheme, one might imagine Bob as an ATM machine and Alice is the customer. Now, on the one hand, Bob (the ATM) could be malicious, i.e. someone may have hacked in or maybe it’s not even a real ATM. So Bob could be distrustful. On the other hand, Alice might not be who she claims to be and thus she could be distrustful as well. Wehner’s group essentially attacks this really fascinating problem via another quantum bounded storage model.

Vincent Nesme finished up the morning with a talk on unitarity and causality (yeah, now we’re getting somewhere, heh heh heh – or not depending on whether we’re causal – ok, nevermind). In any case, his talk contrasted a structuralist approach with an axiomatic one. In fact, his approach reconciled the two approaches to QCA. In the process he invoked index theory which is something I know almost nothing about. But I think I want to know more since he had an interesting take on the relation between causality and graphs. I need to ponder it some more before commenting in detail on it. The one thing that confused me a bit was his non-symmetric light cone. I wasn’t sure what his horizontal axis was since, in normal Minkowski space, it shouldn’t be non-symmetric but he didn’t seem to specify a different value for that axis and he wasn’t working in curved space (at least he didn’t seem to indicate he was). Anyway, the quantum case had a different (more symmetric) light-cone and he noted that this was the cost of apparently superluminal transmission of information.

Now, in the afternoon, Suze Gildert and I headed out (in a black Mercedes E-Class!) to find four things – at least one of Einstein’s houses, the grave of James Joyce, the grave of Wolfgang Pauli, and the nation of Lichtenstein. We found all but Lichtenstein. If I were any nuttier than I am, I would SWEAR it doesn’t exist (a conspiracy of cartographers?). At least, the Swiss don’t really seem to acknowledge its importance as a country. It turns out we turned around 4 miles from the border and never once saw a sign indicating we were even close. Avis didn’t give me a map and the GPS system in the car was surprisingly bad considering it was a Mercedes. But I refuse to leave until I’ve been to Lichtenstein, so I might skip some of tomorrow’s sessions in order to get there. Regarding the others, I will have photos in due course. Pauli’s was pretty cool in the sense that it probably gets very few visitors. In order to find it, I had to ask a couple of guys who worked for a local floral place and were putting flowers on a recent grave. Neither spoke English and one (oddly the older one) had never heard of Pauli, but the younger one knew exactly what I was talking about (dumb American that I am – I really do wish the US did a better job of language education) and led us straight to it. Of course, Heinz Hopf (of Hopf algebra fame) is buried right next to Pauli. Joyce’s grave was very nice too. There’s a nice statue of him next to it. Across the street happened to be the world headquarters of FIFA (that’s soccer’s international organization). Again, picture will be forthcoming.

Unfortunately I missed the rump session and thus Todd’s talk on umbrellas and – apparently – quantum animal husbandry. Perhaps Todd can enlighten us by linking to his talk…