Last week I attended a workshop sponsered by DARPA on “Scalable Quantum Information Processing via Error Control.” The idea behind the workshop was to bring together theorists who know something about error control with experimentalists who know all about different proposed quantum computing implementations and examine the feasibility of each of the different implementations in light of the requirements which arise from error control. It’s been a while sense I attended a workshop with “real physicists” (a.k.a. experimental physicists and their physics calculating theorist brethren.) The implementations covered were “Ions and Neutrals”, “Superconducting”, “Spins in GaAs”, “Spins in Si, Si/Ge”, “NMR”, “Linear Optics”, and “Electrons on Helium.” DARPA has recently said that a new program “FoQuS” will be starting and will be narrowing down the field of DARPA funding for the different implementations. Needless to say this has caused a lot of stress on those currently receiving DARPA funding.
Here are some of my observations from the workshop.
Ion traps rock. If were a starting graduate student who wanted to do quantum computing and do some really rocking quantum computing experiments during my graduate career I would make a dash straight towards an ion trap quantum computing group. The era of NMR is over and a new era of Ion trap quantum computing has begun! Why do I say this at the expense of the other possible implementations? First of all, the ion trap people have already successfully show the basics of coupling and manipulating qubits and they can do these with good to great fidelity. One great advantage they have (and share with other AMO proposals) is that they can do high fidelity, fairly fast (approx 10x two qubit speed) measurements. Second, they have really nailed down exactly what is going on in their system: what are the decoherence times and mechanisms, what are the heating rates when you move ions, etc. Third, ion traps have always been questioned due to their scalability and thus there has been a lot of thought in the ion trap community about how to fabricate traps for which will realize concrete quantum computing architectures. The recent demonstration of teleportation in ion traps, I think, is the beginning of a long line of beautiful protocols for multiqubit quantum protocols.
Locality has been too often ignored. The threshold for fault-tolerance is hard to analyze when you restrict yourself to particular geometric architectures (with the important exception of calculations done with toric (surface) codes which have a local quantum structure and a nonlocal classical structure. Here the thing we would like to get around is the ancilla state preparation factories which require the ancilla states to be swapped into the surface codes: this is just the old locality issue again, but in a much more tame setting. Also the surface code setup is very nice for three dimensions, but difficult to imagine in two dimensions.) If you are going to use a concatenated structure for error correction, then you really need to sit down and think about flying qubits. If not, it seems you might be in deep trouble when trying to construct your architecture.
Fitting it all together. Many implementations will have serious technical dificulties when you try to lay the control circuitry on a realistic architecture: for some spin implementations this may cause serious problems.
Superconducing qubits visibility Superconducting qubits have this (not understood?) property that they don’t get a high visibility when they do single qubit Rabi flopping. Of course there are two reasons for this to be occuring: one is that the state was not properly prepared and the other is that the measurement is not high fidelity. Until this visibility problem is well understood, superconducting qubits may be in trouble.
When things take off? In the next few years, implementations will be working on implementing quantum error correcting techniques. So, suppose you implement the five qubit quantum error correcting code. What should we expect to see? Well since these experiments will most probably be below the memory threshold, the effect of the circuit in terms of real fidelity will be to make things worse! So we will have some years where we start measuring how badly we are making things worse. What I can imagine happening is that there is a group of people who will work on these small codes on their systems and constantly improve them until they pass the threshold. Simulatenously I expect a number of people will work on architectural scalings for the implementations. And when these two meet their goals, I’m a relative optimist that all hell will break loose and we will see quantum computing being scaling in a remarkable fashion. When, then, will breakeven be reached? And how do I say this without invoking images of fusion?
State of the implementations. My impression of the state of implementations is as follows. NMR quantum computing in liquid state has reached its terminus. Ion traps will be taking the lead where NMR quantum computing has left off. I expect the ion traps to press the next four or five years of the quantum Moore’s law. Quantum dot and superconducting qubits are at the stage where they need to pen down the characteristics of their systems. It was impressive to hear that first attempts at single shot measurements in quantum dots have achieved 64% measurement fidelity. I’m a bit worried about the Kane implementations, and I’m sure there not at the ion trap level. Thus I’d say most solid state implementations are a few years (like 3 to 5 years) behind Ion traps. Linear optics quantum computing is the wild card in the whole picture. I worry most about the requirements of state preparation in linear optics quantum computing. I worry also about the device complexity, but this doesn’t seem an insurmountable barrier because I’m not an engineer or an experimentalist. Is mode matching a killer? I know the least about electrons on Helium, but I get the impression that they are all on the cusp of demonstrating two qubit interactions. They will then have to begin the quantificiation process a la ion traps. Implementations with neutrals also fit in somewhere, but I’m not quite sure where. I have always been shocked by the lack of experimental progress in neutrals: the number of quantum optics people should have lead to some nice results by now, but I haven’t seen this (but I claim no authoritative status as I’m just a lousy uneducated theorist!)
In all it was a fun workshop. The talks were super short, but the conversations after the presentations were at times very interesting. What was amazing to see was to watch an expert in ion traps talk to an expert in superconducting qubits and other such cross disciplinary conversations. Normally these two wouldn’t give the time of day to each other, but through quantum computing there is a common language. And not just a common language, but also a common set of problems with many common solutions. Quantum computing is so multidisciplinary it is scary. But it’s also the reason it is such a beautiful and exciting field.
The Sound of Snow's Cultures Colliding
Last Thursday I went to a workshop in Chicago. Before the conference I found a cafe to work in on the campus of Loloya University Chicago. After working for a while I took a break to read a book. It was then that I realized that I was a theoretical physicist reading a book about an Indian spiritualist who rejected his role as the Theosophical messiah in a cafe whose walls were covered with pictures of the Pope.
The Economist Loves Doh
5/1-5/7 issue of The Economist.
Page 31, under the title “Our kind of Doh” we find:
‘Look at me. I’m reading The Economist. Did you know Indonesia is at a crossroads?’
While experiencing first-class travel for the first time, Homer Simpson lays out his credentials to succeed Colin Powell. The Simpsons, April 25th
On page 79 we find an article on Indonesia with the title:
Investing in Indonesia
At a crossroads
Moore's Law
Our universe is expanding. Not only that, but this expansion is probably accelerating. Now two authors, Lawrence Krauss and Glenn Starkman, have proposed that a consequence of this acceleration is that in such a universe only a finite amount of information processing can be performed: astro-ph 0404510. This means, according to the authors, that the total amount of information process can be at most 10^(120) bits. A consequence of this is that Moore’s law can last for at most 600 years in any civilization!
Data and Program
A most powerful idea in computation is that the program and the data can be one and the same. (That this distinction is often lost in higher level programming languages is a shame.) Thus a program which is some form of data can manipulate the data which is itself part of the future program. We often like to speculate that the universe may act like some form of a (possibly quantum) computer. What is interesting, then, about this “universe computer” is that it does not appear to be merging program and data in any way. The machine language of the universe does not manipulate its own program. Sure, at some higher level the universe does have the ability to manipulate its own program (for the universe does allow for us to build a computer!), but at its most fundamental level, there is not manipulation of the program. Or at least this is the fashion in which we think about the physics of the universe: there are laws which are the fundamental program and there is data which is the state of the universe. The program then acts to change this state of the universe over and over again, producing the evolution of the universe. But what if this is not the way the universe works? What if there is no distinction between the data and and the program in the universe? There can still be a state, and there can still be evolution of this state, but the laws of this evolution will depend on the past executed program. The laws in there most general form can not only change in time, but they will be a function of the past history (past program) of the universe.
Just as Reimann’s realization that we can define curved spaces without reference to any higher dimensional space into which this space curves, perhaps the lesson of computer science should be that we can think about a computer without a physical device to carry out the computation. We can think of the universe as a von Neumann computer in all its abstract glory of such a machine without actually believing that such a universe computer exists.
OK, I’ve clearly stumbled into crank land with this “creature of eager speculation.” But damn straight I’ve stumbled into crank land.
We Live in Dick's World
From Valis by Philip K. Dick:
“To fight the Empire is to be infected by its derangement. This is a paradox; whoever defeats a segment of the Empire becomes the Empire; it proliferates like a vius imposiing its form on its enemies. Thereby it becoes its enemies.”
Publish Noise
From an interview with Neal Stephenson:
Hooke, for example, when he figured out how arches work, published it as an anagram. He condensed the idea into this pithy statement: “The ideal form of an arch is the form of a chain hanging, flipped upside down.” Then he scrambled the letters to make an anagram and published it. That way, he wasn’t giving away the secret, but if somebody came along a few years later and claimed that they’d invented it, he could just unscramble what he’d published. He was establishing precedence.
How long until someone publishes a paper on the arXiv which is just noise and then, after some major result appears, says “No, I did this a long time ago, and here is the key which unscrambles my paper?
Pepper
Goodbye Pepper. You threw up on me (twice!) on the ride home from where we picked you out. You herded the washing machine and the lawn mower. You barked at everyone except family. Mostly you snored loudly. The floor will be a little dirtier, the days a little quieter, and the washing machine will be much much lonelier. RIP Pepper (1995-2004)
Silly Questions
The number one most irritating question I was asked during my faculty interviews was “what will you do if quantum computation doesn’t pan out?” At first glance this question seems perfectly valid: a department should concern itself with whether they are hiring someone whose work will quickly become irrelevant. But I’ve got news for you all, quantum computing’s not going away! Why? Well not for the reason the question askers are thinking: what happens if a quantum computer can’t be built? No, quantum information science will stick around because it has an intrinsic intellectual value. And this is what makes the question so irritating: it implies that quantum information science is a fad with no intrinsic intellectual value. Do you ask string theorists whether what they do will be experimentally testable and if not what will they do? Do you ask astrophysicists whether studying cosmology will have any significant impact on society? No. But because these are part of a long tradition of theoretical physics they are acceptable intellectual persuits, whereas quantum information science, being new and getting too much press is most definitely suspect.
There is, of course, great irony in this situation. Theoretical physics has always justified a large portion of it’s work as for the greater intellectual good (holier-than-thou-physics.) But mention the word “quantum” all of a sudden normally elitist physicists turn into engineers. The psychology behind this is pretty simple in my opinion (wait this whole thing is my opinion!): physicists have yet to actually accept quantum theory. They don’t want to think about it because it’s strange. The fact that it’s current position is basically to serve as the operating system of the universe doesn’t help at all. Because we can separate the axioms of quantum theory nicely from the physics of the fundamental forces most physicists can spend their entire life living, breathing, and calculating classically. Rather embarrasing for a group which seeks to understand the fundamentals of our universe.
Living In LA Is Like Smoking
Today I sent in my acceptance for a postdoc at the Santa Fe Insitute. 16 miles to Ski Santa Fe and 72 miles to Taos.