Category: Quantum

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.

Suppose Alice and Bob perform a test of a Bell inequality. They start together and produce an entangled quantum state. Then the parties move apart such that when they make their measurements on the entangle quantum state they are spacelike separated and are at rest with respect to each other’s motion. Let’s suppose that Alice and Bob perform their measurements at the same time (simultaneous) according to the two party’s rest frames. Now the standard paradox arises: the correlations Alice and Bob produce cannot be explained by a local hidden variable model: one party needs to produce an outcome which is predicated on the measurement choosen by the other party. So this seems very bad: if Alice sends a signal to Bob at the speed of light, immediately as she does her measurement, then the signal will arive at Bob at a time equal to the distance between the parties divided by the speed of light. But now look at exactly just this scenario from the perspective of a party who is traveling in a reference frame which is moving with a velocity in the same direction as the separation of Alice and Bob. If this frame is moving at nearly the speed of light with respect to rest frame of the parties, then the paradox seems almost triffling: in this frame of reference, Bob would get the information immediately after he would normally need it. Extrapolating as this party’s frame’s velocity goes to the speed of light and thinking that there is a limit to how sharply we can define when a measurement occurs, we see that “There exists a reference frame in which the paradox of Bell violation for all practical purposes disappears.”
What does this tell us? Well it certainly doesn’t resolve the Bell paradox. One thing to get is that the paradox is reference dependent. OK, not too interesting either. But what I think is neat about this thought experiment is it gives us a point of little or no paradox upon which we can begin to think about how to bootstrap our way into the reference frame where there is a very measurable Bell paradox. How do we do this? Heck if I know: the point is mostly that the solution should be a SMOOTH map from this little or no paradox reference frame to the frame where the information propogation is ridiculous.

Pat Gelsinger, Intel’s CTO, on quantum computing from this interview:

For nearly a decade there has been talk about the coming quantum computing revolution, yet it seems no nearer. What is it that is causing the delay?
It will probably be talked about for another decade too. It certainly won’t be relevant for another decade. That doesn’t mean it’s bad research but it’s far from coming to the commercial sphere. We need to challenge some of the assumptions about quantum computing and what it will be used for. We’re now building 3bit quantum computers with possibly a 5bit one under construction. That’s fine, but I’m already building 64bit computers. When we get quantum to 14bit then it can be used for encryption, which is one of the key applications for it. The country that gets a quantum network first will have a real competitive advantage.

I wish I had a 3 bit quantum computer in the same sense that he has a 64 bit computer! I wonder, however, where his 14 bit claim comes from?

Only time will tell if and when the problems of building a quantum computer can be overcome….As information becomes the world’s most valuable commodity, the economic, political and military fate of nations will depend on the strength of ciphers. Consequently, the development of a fully operational quantum computer would imperil our personal privacy, destroy electronic commerce and demolish the concept of national security. A quantum computer would jeopardise the stability of the world. Whichever country gets there first will have the ability to monitor the communications of its citizens, read the minds of its commercial rivals and eavesdrop on the plans of its enemies. Although it is still in its infancy, quantum computing presents a potential threat to the individual, to international business and to global security. -Simon Singh

Here is a list of school rankings of graduate physics and computer science departments. These schools should all be doing quantum computing, No?

Here is a Powerpoint copy of the talk I gave at MIT: QIP seminar talk. If you can’t view powerpoint, here is an html version.

Much hubub is made about how uncomfortable everyone feels with quantum theory. So much hubub that there are now a plethora of different “interpretations” which are all supposed to make you feel less uncomfortable with quantum theory. There are basically two things which make quantum theory interesting: contextuality and nonlocality. The first of these is really not to disturbing. Sure, philosophically, having a realistic theory with really hidden variables (meaning you’ll never get access to them) is disturbing (why the extra structure?) but its not something which is completely incompatable with our everyday conception of reality. We operate fine when we interact with our PC’s without knowing the exact details of the currents and voltages inside of these machines (this being an analogy and not a precise comparison: of course we could go in an measure the currents and voltages and therefore understand why the hell the blue screen of death just popped up on the monitor.) Nonlocality is disturbing in a different way. It says that there is no way we can have realistic descriptions which are always local (but will be hidden, of course, as a consequence of the contextuality of quantum theory.)
OK, so here is my point. The issue of nonlocality arises due to the combination of relativity and quantum theory. It is perfectly reasonable to consider all sorts of notions of locality and then do quantum theory on them. Our notions of locality arise due to a physical theory: relativity. So perhaps the way out of the nonlocality mess is not via some hand-wavy philosophical smoothing over of emotions, but instead is due to actual real hard core physics. What physical theories can we derive which produce quantum theory? Can physics save us from the quantum quagmire?

Okay. I promise. This will be my last time travel post for a while. But interestingly, the day after my paper appeared I got a hit on this blog from an ARDA internet address. Of course if I were ARDA I would pay attention to any claims of algorithmic speedups. So if I disappear tomorrow, you’ll know what happened. Of course, this could just be my good friend Tom Scott, who does who knows what for the Army, but if it is I’d rather not know so that I can keep my paranoia at a peak level.
Note added: Wired has changed the introductory sentence of this article. Where it now says “futurist” it used to read “time travel believer” (or “time travel afficionado”…well at least it had the words “time travel” in it)

Following up on my follow up to my time travel paper, there is an article on Wired describing comments Wesley Clark made about traveling faster than the speed of light. Seems everybody’s favorite four star general believes deep down in his heart that faster than light travel is possible. My first reaction, being a scientist and all, was “Great, Clark is some kind of crazy crank!” But I thought about this a bit and I think I’ve come to completely the opposite conclusion. I mean, does anyone really think George Bush even knows that traveling faster than the speed of light does not appear to be possible according to modern physics? Second, it was impressive that Clark claimed that he has argued about this with “physicists” and realizes that his belief is nothing more than an unsupported faith. My guess is Shrub would think a physicist is someone who is good at making fizzy beverages. Not to mention Shrub and his cohorts rampant political overrunning of scientific oversight (see this page for details.)

David Deutsch emailed me with nice comments and suggestions about my just posted paper. I told him the funny story behind the paper.
A few years ago I was watching a NOVA program about time travel and closed timelike curves. On watching the program I thought: “hmm, I wonder if quantum circuits always give you self-consistent evolutions?” After thinking about the problem for a few days, I was able to hack out a proof. Only then did I decide to go look in the literature to see what others had done. There I discovered Deutsch’s 1991 paper and found the result I had hacked out. The scary thing was how close our two proof were (I seem to recall even the symbols we used were identitical!) Getting deja vu when writing a paper on time travel is really spooky.