Thursday, July 30, 2009

Public Lecture

Last night I gave a public lecture in Aspen CO which went over pretty well. It will be streamed tonight at 9pm mountain time, and you can download the whole thing here anytime thereafter (maybe even before). I encourage even the most science-phobic to try watching it (you know who you are).

After the lecture (off camera) I got a lot of good questions from random audience members who were too shy to ask in front of the entire room. There was one question that really stumped me: Does anyone know of a good book about quantum computation for the lay-person? Eddie Farhi (who introduced the lecture) is a mainstay of the field of quantum computation, and he couldn't think of one either. There is a book by Seth Lloyd, but I don't know what it contains (or if I approve of it).

Added: Two minor techincal problems with the video (1) you could not see the laser pointer at all, so when I pointed and said "this thing", you have no idea what I am pointing out. (2) in a few places the resolution of the video is not as good as one would like. Despite these problems, I am reasonably happy with the outcome.
Monday, July 27, 2009

TV Interview

On this web site I am interviewed as a preview for my public lecture on wednesday. I haven't watched the interview yet.

I apologize in advance for the shirt. Due to travel difficulties (getting to be a theme in my life), I literally walked off the airplane and into the tv studio.

The file takes a long time to download on my computer -- maybe it will work better on yours. I think it is also being streamed tonight at 8:30PM mountain time here.

Added: I just watched the interview.. and after being horrified by my shirt as well as my big hiking boots, I actually kind of liked it.

Added: Jeremy Bernstein pointed out to me that it was more von Neumann who wrote down all the pieces of the modern computer, not Alan Turing. Although Turing still gets credit for the concept of the Turing machine.
I carry my passport around with me a lot because I travel so much. But when I am at home at Oxford, I stash it in a “secure location” in my house. (I believe Dick Cheney may be hiding there too). Where is this “secure location”? In a drawer built into the bottom of my bed. (I blogged about this bed previously here and here).

Anyway, as I was finishing my preparations to go on the road for the next month, with only about half an hour before I had to be on the bus to heathrow, I went to retrieve my passport, and mysteriously the drawer was stuck shut. I tried yanking a bit. No dice. I tried prying a bit. Nothing. Screwdriver. Nada. Hammer. Zip.

This bed was an assembly-yourself deal, so I start disassembling the entire bed piece by piece. In twenty minutes, I am sweating profusely (literally and figuratively) and I have the bed disassembled down to its constituent minimal pieces. But I still can’t get into the damn drawer.

Cost of bed-frame: 300 £. Cost of plane ticket: 600 £. Time remaining: about 10 minutes.

Out comes ye’ olde saw. In five minutes, I have my passport, and I have a great big hole in the base of my bed with a lot of sawdust on my bedroom floor.
Sadly I missed the bus, but caught one only half an hour later. Unfortunately, this was not good enough. I ended up missing my airplane by precisely one minute. Actually, this is the first time I have missed an airplane by being late to the airport. I’m usually one of those people who gets there four hours in advance just to be careful. Exactly one hour before the flight was scheduled to leave, they officially closed the flight and even one minute later, I could not convince the folks at Continental to let me on (I suspect they had oversold it; and closing it promptly was a cheap way to fix the problem – I noticed that most other flights to the US did not close until 20 minutes before the flight).

[Here’s something to think about. When I am one minute late for the plane, I get bumped. If the plane is one minute late.. how can I bump them? ]

Added: I managed to get a flight only four hours later. Exceedingly annoying, but not tragic. Not sure what I am going to do about the bed though.
Tuesday, July 21, 2009

Comment Policy

Those of you who have been leaving comments on my blog... please keep doing so! And all you stalkers are encouraged to start doing so.

You may have noticed, however, that comments are not being posted immediately. This is because I am now examining the comments before they get posted to avoid spam postings. Be assured, however, that anything you want to say will be posted, so long as it is not overly offensive and does not contain an advertisement for Viagra or similar. Those of you who want to read ads for Viagra or similar, I'm sure you will be able to find many many of them elsewhere on the web.
Monday, July 20, 2009

Physics Report from Dresden

The TOPO09 conference in Dresden last week (I dare you to find me in the group photo on that page... like where's waldo) was full of extremely good talks. I missed the first day of the conference, so I can’t comment on that. But in the remainder of the conference, there were two topics that stood out as themes: topological insulators and 5/2 physics.

Topological Insulators:

As everyone knows there are metals and there are insulators. In metals, the electrons can move around and conduct electricity, whereas in insulators, they can’t. When you have an insulator, the electrons can be non-mobile for any one of a number of reasons . But at some level their lack of mobility always comes down to the same type of story: The electrons fill up all of the low energy orbitals (like orbitals in an atom) – and the next lowest energy orbital is at some higher energy (i.e., there is an energy gap). So in order for an electron to move around, it needs to first jump up to a higher orbital, which at low temperatures, it cannot do (room temperature can be “low” compared to the relevant energy spacing).

Now consider the simplest possible model of an insulator: A simple crystal, with no disorder, and non-interacting electrons, such that all the low energy orbitals (bands) are filled and there is a gap to the next lowest state for the electrons (this is known as a band insulator). Since the physics of insulators was first described about 80 years ago, it was assumed that all such insulators are more or less the same. Well, we have recently discovered that they are not all the same. Some fundamentally new physics can occur when the constituent chemical elements of the insulator are down near the bottom of the periodic table.

What happens near the bottom of the periodic table is that relativity becomes important. (HUH?). Yes, that’s right – Einstein. Electrons in heavier elements are moving “faster” than electrons in the lighter elements. When they move fast enough you have to think about relativistic effects, and one of the first relativistic effect is that the spin of the electron and the orbital motion of the electron become coupled (so-called spin-orbit coupling --- in fact, you can argue that the existence of electron spin is relativistic in the first place). If the spin-orbit coupling is strong enough, the insulator can develop some completely new properties. However, unless you know what to look for, you won’t notice that it has changed, because it is still an insulator. I suppose this is why it took 80 years for us to figure out that all insulators are not the same.

Among the cool properties these new “topological” insulators is that they have conducting surface states that cannot be eliminated with any amount of disorder or structuring of the surface. This is extremely unusual and leads to all sorts of new possibilities.

At the Topo09 conference last week, Charlie Kane, a prof at UPenn, gave a super talk about these new topological insulators and all the cool stuff you can do with them. Kane is perhaps the person most credited with figuring out that this whole new class of materials exists. However, one of the other people who is highly credited for developing this new field is Rahul Roy – a lowly graduate student at the time he started making important contributions to the young field. I've managed to recruit Rahul to come to Oxford for a postdoc next year. I hope to work with him a lot on this new and exciting field.

5/2 physics:

I’ve blogged about this before. here and here, so I won’t belabor the point. But in brief: there are an increasing number of people discussing whether the new experiments on the “5/2 quantum Hall interferometer” is really showing evidence of a new type of particle – the nonabelion. I won’t say that I know what to make of the data – I certainly don’t know. The one thing I do think, however, is that the “orthodox” interpretation – what people want to see – is probably not what is going on. There are just too many problems with the story. In fact, the more I look at the data, the more nothing seems to fit. Add to this issue that the data is pretty ratty to begin with, and I think the evidence for the orthodox theory starts to look vanishingly small. I don't think the data is just noise though -- so something interesting is happening. But I think it will take a lot of headscratching to figure out what it is though -- and probably a whole lot more data.
Saturday, July 18, 2009

Travel: Things that Make You Go “Huh?”

There were thunderstorms across Germany yesterday so my plane out of Dresden was delayed. The Lufthansa people in Dresden more or less promised that I would still make my connection in Munich, but there was a huge backup at passport control to get into the “international” part of the airport and (even pushing my way to the front) I ended up missing the connection. The Lufthansa people did manage to get me booked on a plane early this morning and found me a hotel for overnight.

To get out of the airport to go to the hotel, however, I had to go back through passport control --- essentially re-entering Germany from the international terminal of the airport. Going through the passport control the bundespolizei looked at the stamp in my passport and asked “You left Germany today… and now you are coming back into Germany?”. I explained that I had missed my international connection and now I was going to a hotel for the night. Well, apparently such a thing had never happened before because the bundespolizei had to get on the phone with four of his managers and I stood waiting at the passport control booth for about half an hour. Apparently this was cause for great concern because it was absolutely verboten to leave and enter again so quickly.

Am I completely wrong to think that this is completely absurd? Wouldn’t you expect that such a situation must happen hundreds, if not thousands, of times per day?

I’m thankful only that I am not from a country that requires a visa to visit Germany. No doubt they would have thrown me back to international territory to sleep on the floor.


My first visit to Dresden was in 1990 (the same summer as my first visit to Oxford). This was very shortly after the fall of the Berlin wall and the city was not used to many visitors. My memory of the city was that it had not remotely recovered from the firebombing of WWII which literally wiped it off the map in one night. Forty-five years later, there were still large sections that were still simply rubble. And much (if not most) of the construction and reconstruction since the war were hideous eastern-block buildings.

Since the reunification of Germany, the real reconstruction of Dresden has begun. The historic churches and palaces of Dresden have been slowly reassembled with combinations of new stones, and old stones recovered from the rubble. Although the reconstruction is far from finished, the city is already quite beautiful now. One can almost imagine what it looked like in the 1800s when it was the center of power in Saxony.
I was on the airplane from London to Munich (and on to Dresden) and the stewardess came around taking drink orders. The two Brits sitting next to me both ordered tomato juice, and it sounded good to me, so I ordered one too. But when I ordered it, I had a moment of panic. I ordered “tomAHto juice” not “tomAYto juice”. The first chink in the armor has appeared. Next thing you know, I’ll be eating clotted cream and liking it.
Thursday, July 9, 2009

A pun and a clarification

If you know the rules of bowling, you will remember that the most you can score in one game is 300 points. Well, there is an old joke about a guy who “bowled three hundred and won” (it’s a pun, it doesn’t work well written: 301 versus 300 and won). This type of linguistic confusion appears to be the source of a rumor that was circulating in the physics community about the exciting data I mentioned last week. To clarify: Eva Andrei observed quantum hall effect in graphene at filling fraction 3 AND at filling fraction 1/3. She did not observe quantum hall effect at filling fraction 10/3 = 3 AND 1/3.

No Satisfaction

There are some computational problems in computer science which are known as NP complete problems. One famous example is the traveling salesman problem, where a map is handed to you and you try to find the shortest continuous path that visits a particular set of N cities. (Strictly speaking the NP-complete version is determining whether there exists a path shorter than a certain length). There are quite a few problems in this computational complexity class, and it turns out that if you manage to solve one of them, you can essentially solve them all. It is not known, as you make the problem bigger, whether the difficulty of solving these problems grows polynomial with the size of the problem (ex, with the size of N above) or exponentially with the size of the problem. In fact, this question is viewed as so important that there is a million dollars waiting at the Clay mathematics institute for anyone who can prove either that it is polynomial or it is not.

A few years ago, a group of physicists at MIT proposed that a quantum computer might be able to solve such NP complete problems in polynomial time (In fact, this would not win them the million dollars by the precise definition of the challenge set out by the Clay institute, but it is still extremely interesting). Their scheme was to find pose an NP-complete problem such that it is essentially equivalent to finding the ground state (lowest energy state) of an appropriately designed quantum system. Then the way they want to actually find this ground state in practice would be to deform the parameters of the system smoothly until it becomes a simple quantum system where the ground state is already known – then initialize the quantum system in this known ground state – then adiabatically (slowly) deform the system back to the system we are interested in. Now according to the so-called adiabatic theorem of quantum mechanics, if a system is put in its ground state and the system is deformed sufficiently slowly, it will always remain in its ground state. So at the end of the process, we are in the ground state of the system we want, and we have our solution.

Well, unfortunately, there is a catch. The catch is the words “sufficiently slowly”. The time scale that determines what “sufficiently slowly” means is the lowest energy of the first excited state at any point along the path between the initial and final state of the system. If this first excited state happens to come down to be exponentially close to the ground state, then one needs to deform the system exponentially slowly to stay in the ground state and the process will take an exponentially long time.

Since the proposal of this scheme many (if not most) of the people in my community have assumed that this catch is precisely what ruins this idea completely. Nonetheless, the idea has been floating around for quite a while now and the issue still had not been nailed down very well. This week at the INSTANS conference in Amsterdam, Boris Altshuler gave a pretty solid looking proof that this “catch” does indeed happen and such a computer would take exponentially long to finish. To do this, he focused on the the so-called 3-satisfiability (or 3-satisfaction) problem, which is one of the famous NP-complete problems. And since all the NP-problems are essentially equivalent to each other, this seems to kill the idea at last.

[ For the experts (and very briefly) the issue really boils down to whether there is any gap opened up by anticrossings between multiple low energy solutions. But since for NP problems the different low energy solutions are extremely different from each other, there is exponentially low tunneling between them, hence no gap opens up. ]

So it seems that the quantum computer will not be getting any satisfaction this way. No no no. Hey hey hey. That’s what I say. It can’t get no…
Sunday, July 5, 2009

Pretty Town -- Overrun with Tourists

My first visit to Oxford was in the summer of 1990. I was playing tourist that summer and after seeing the sights in London I took a train up to Oxford, hoping that it was far enough away from the big city that I’d be able to hitchhike from there up to Edinboro. I kept a journal of my travels that year and I happened to unearth it just recently. My comment on Oxford was “Pretty town – overrun with tourists” -- an accurate description for this time of year.

Admittedly, Oxford is a nice town to see, with all of its beautiful ancient architecture and historic sites. But now with the added draw of “Harry Potter Tourism” (as well as “Golden Compass Tourism”, “CS Lewis Tourism”, “JRR Tolkien Tourism” no, we don’t have hobbits, “Inspector Morse Tourism”, and the list goes on), it seems the natives (meaning me) can’t catch a quiet moment in the summer. It reminds me a bit of being in New York – but without the subway.

Anyway, on my visit in 1990, I only stayed in town for a mere few hours (during which I ran into no less than three other fellows students who were also playing tourist that summer – such was the density of American tourists at that time). Mainly my time here was spent trying to figure out how to get out of Oxford and on my way north. I remember asking around as to where a good place would be to hitch a ride. No one seemed to know. Eventually, some rather shabby looking homeless guy named Nick told me that I should walk to the ring-road north of the city and flag a lorry from there. My memory of the long walk to the ring road (carrying a very heavy pack) does not completely match up with the geography of the city. Probably my memory is faulty after almost 20 years. Or maybe the weight of the pack prevented me from seeing it correctly in the first place. Nonetheless, if I had followed Nick’s directions properly, the path would have taken me right past both the Physics department and Somerville College, my current homes.

On my long walk out of town, I remember stopping to watch a cricket game on the way. Probably this was at the pitch north of Somerville College on Woodstock road. The game absolutely mystified me then – and I understand only a tiny bit more about it now --- except to say that the games go on really forever and are even more boring than baseball (if such a thing is possible).

Finally, being completely exhausted from carrying my pack (which I later would reduce substantially by throwing “unnecessary” things out), I did catch a ride out of town from a random Oxonian – actually, a physicist as luck would have it. If I am not mistaken, this very physicist is now emeritus faculty at University of Sheffield, not so far from here. Hopefully soon I will have a chance to visit Sheffield and I intend to take him out to dinner to thank him for his kindness almost 20 years ago.

I did not return to Oxford until the fall of 2007.