Category: Physics

Here is an awesome position for one of you bigwigs out there:

The Board of Electors to the Professorship of Quantum Physics, to be held in the Department of Applied Mathematics and Theoretical Physics (DAMTP), invite applications for this Professorship, to take up appointment on 1 January 2008 or as soon as possible thereafter. Applications are welcome from persons working in the broad areas of quantum computation and quantum information theory (with these taken to include quantum cryptography and quantum communication theory). The Professor will have an outstanding international reputation in their field of research and will be expected to provide strong academic leadership in research, teaching and other activities of DAMTP.
The Chair has become vacant on the departure of the post holder, Professor Artur
Ekert, who played a leading role in establishing a successful Centre for Quantum
Computation in DAMTP housing an internationally leading research activity in quantum information science. The Department wishes to appoint a new Professor who is able to sustain this general line of research to the highest possible standards.
Further information may be obtained from the Academic Secretary, University Offices, The Old Schools, Cambridge, CB2 1TT, (email: ibise@[elephant]admin.cam.ac.uk remove the [elephant] to get the valid email), to whom a letter of application should be sent, together with details of current and future research plans, a curriculum vitae, a publications list and form PD18 with details of two referees, so as to reach him no later than 30 September 2007.
Informal enquiries about this Professorship may be directed at any time to Professor Peter Haynes, Head of the Department of Applied Mathematics and Theoretical Physics, telephone: (01223) 337862 or email: p.h.haynes@[elephant]damtp.cam.ac.uk, remove the [elephant] to get the valid email. Further information about the post and the Department may be found at http://www.damtp.cam.ac.uk/.

Oh, and to translate this post across the pond, “centre”=”center” and “1 January 2008” is “January 1, 2008.”

Over at Computational Complexity, Bill Gasarch asks about some of the things he’s heard about quantum computing:

I have been told quite often that
“You don’t have to understand Quantum Mechanics to work in Quantum Computing.”
Thats a good thing since I’ve also been told
“Nobody really understands Quantum Mechanics.”
I’ve also been told
“You don’t have to have studied Quantum Mechanics to work in Quantum Computing.”
I am skeptical of that.

Which reminds me of story about how I first tried to learn quantum theory. When I was growing up we belonged to a science book club. Most of the books we ordered where the fairly standard popular science kind of books. But there were more technical books available and I had already read a lot of popular science on quantum theory, so I decided that I wanted to get a real textbook on quantum theory.
So I ordered up this textbook and dived right in. Now the first thing this book talks about is the ultraviolet catastrophe and Planck’s solution to this problem (of course this is a made up history: Planck wasn’t trying to solve the ultraviolate catastrophe when he derived his theory of quanta.) And in this problem one of the essential points was that if you took this equation that had a symbol like [tex]$$int $$[/tex] and turned it into a symbol like [tex]$$Sigma$$[/tex], then you could avoid this catastrophe. Now I knew what the latter meant, a sum, but I had no clue what that first symbol was. But I did know a chemistry teacher who had gone to Berkeley, so I thought he would know. So I went and showed him the book, and he said “Oh! That’s an integral symbol.” And then he told me that I would have to learn Calculus to understand what this meant. Really! You have to understand calculus to learn quantum theory. Well that was a setback. (Luckily our local library had a calculus book, which I promptly checked out and learned calculus from. Ah, those were the days. BTW, a math teacher I had in high school claimed he could teach his eight year old calculus.)
Okay, so now you’re saying, “Get to the point Dave!” And certainly most of you might guess that the point I’m trying to make is that you don’t need calculus to learn quantum computing (true) or that you don’t need to know quantum physics to learn quantum computing (note I said “physics” here.). Of course the later is true, you could pick up Nielsen and Chuang and learn quantum computing without ever solving a particle in a box problem in quantum physics. But why would you want to do this? When you really care about learning something, it’s not about what you do or don’t need to begin learning, it’s about trying to grab ahold of as much information and having as much fun as possible. For example, you could turn this question around and ask, “Do you need to have taken a course in computational complexity in order to do quantum computing?” The answer is, I think (no wait, I know from personal experience!), “no.” But why would you not want to learn about P, NP, PP, BPP, EXP, etc. (and the new complexity class MIT. By the way MIT is contained inside of CIT. I have a proof of this, but it doesn’t fit in the margins of this blog.)? So while I think it is certainly true that you could learn quantum computing without taking a course (or learning on your own) in quantum physics, why in the world would you want to do this? Why not learn as much as you can about both “quantum” and about “computing”? This doesn’t guarantee success or anything, but I can guarantee you that it would benefit your soul (and it might even lead to things like physicists designing algorithms where scattering off a tree solves the NAND tree problem.)
(The main point of Bill’s article, of course is to ask whether quantum physicists should learn quantum computing, to which I refer the reader to Scott Aaronson’s answer in the comment section of the post.)

So suppose you are thinking of building an ion trap quantum computer. Since putting all your ions in one trapping region is just silly when you get to large numbers of ions, you are led to think about using multiple ion traping regions. Then you are led to two possible design choices: whether to move the ions between these regions or whether you can couple the ions to flying qubits like photons which can then couple back to ions. If you’re going to do the former, you are quickly led to the notion that it is better to have the qubits packed pretty densely, both for reasons of not wanting a gigantic quantum computer, but also for reducing the amount of moving you’ve got to do. So making small traps is of great import and a lot of groups have been doing just that, with trap sizes a few tens of microns in size.
But, uhoh, what happens if you make your traps smaller? Well one effect is that the heating rate of your ions goes up. This heating is believed to come from fluctuating patch potentials on the trap surface. This noise scales roughly like one over the size of your trap to the fourth power, and so you can imagine that going to small traps might make for some rough heating. And indeed for the small traps you’d like to use for ion trap quantum computers, this heating becomes pretty unbeareble. That is at room temperature it becomes unbareable. But there is evidence that this heating effect is thermally activated which is a fancy way of saying that if you cool your trap down this heating starts to go away. Indeed, Chris Monroes’ group has cooled a needle trap (think two ball point pens pointing at each other) to 150 Kelvin and seen a surpression in this heating rate. Indeed, one of my favorite lines at FTQC II was when Chris Monroe said of this [tex]${1 /d^4}$[/tex] heating that “we’ll get rid of it before we understand it.”
And indeed, today on the archive there is even more good news for getting rid of this noise. In 0706.3763 the ion trapping group of Isaac Chaung lab at MIT describes experiments with a surface electrode trap which has been cooled down to near 4 Kelvin:

0706.3763
Title: Suppression of Heating Rates in Cryogenic Surface-Electrode Ion Traps
Authors: Jaroslaw Labaziewicz, Yufei Ge, Paul Antohi, David Leibrandt, Kenneth R. Brown, Isaac L. Chuang
Dense arrays of trapped ions provide a promising avenue towards large scale quantum information processing. However, the required miniaturization of ion traps is currently limited by sharply increasing decoherence at sub-100 um ion-electrode distances. We characterize heating rates in cryogenically cooled surface-electrode traps, with characteristic sizes ranging from 75 um to 150 um. At 6 K, the measured rates are suppressed by 7 orders of magnitude below room temperature values, and are two orders of magnitude lower than previously published data. The observed heating depends strongly on thermal processing of the trap, which suggests further improvements are possible.

Yep they get 7 orders of magnitude slower heating in their trap by cooling from room temperature to 6 Kelvin (which, I guess, is near 4 Kelvin 🙂 ) And two orders of magnitude lower than for previously ion traps. Interestingly the effect depends on how the trap is made. Shiny traps are bad, apparently 🙂
So is the future of ion trap quantum computing to work at 4 Kelvin? Or might understanding the properties of the annealing of the trap which lead to [tex]$1/d^4$[/tex] heating suppresion give clues as to how to design room temperature traps without this heating present?