Interesting People mailing list archives
Re: quantum computer announcement]
From: David Farber <dave () farber net>
Date: Mon, 5 Feb 2007 06:05:52 -0500
Begin forwarded message: From: Rod Van Meter <rdv () tera ics keio ac jp> Date: February 4, 2007 10:09:27 PM EST To: David Farber <dave () farber net> Subject: [Fwd: [IP] quantum computer announcement] Reply-To: rdv () tera ics keio ac jp This is the D-Wave guys, and for those of you who don't know, what they are working on is Adiabatic Quantum Computing. I'm very fuzzy on AQC, but it's not a gate model of programming the device -- you essentially set it up and let it cool into the solution state, something like a literal quantum analog version of simulated annealing. Proving that this device works using truly quantum effects will be very tricky. D-Wave itself hasn't published much in the literature, but they have a little. They also have a number of reasonably well respected names among their advisers (who presumably stand to benefit economically if this thing works). D-Wave has a lot of VC money, and presumably those VCs are interested in reality; if the device doesn't work, they won't make any money. Their existing device is 16 qubits, Josephson junction-type devicesbuilt using relatively standard VLSI techniques and operated at 4K, liquid
helium temperatures (I didn't understand the bit about why it doesn't have to be an even lower, millikelvin operation, but that's a minor point). My guess is that they are unable to demonstrate quantum entanglement across all 16 qubits in a way that would satisfy the major journals (in fact, if they could do it across *three*, that would be more than anyone else for the specific type of qubit they use, and the current record for any type of qubit that I know of for any type of qubit is 12 qubits). They claim they will hit 1,000 qubits by the end of 2008, and the basic device is certainly plausible -- all you have to do is stamp out more qubits on the chip. There is a lot of system engineering to do to control that many qubits; at the moment, each one takes several U of rack-mount equipment to control, and several I/O pins on the chip. Moving much of that on-chip is the obvious goal, and is possible, butit's work. (This, plus the basic fact that Josephson junction (JJ) devices
are VERY fast, are two big pluses of this approach, from a systems point of view.) The harder part will be making quantum states survive long enough, and work well enough, that 1,000 qubits represents a *useful* device. Decoherence, or the lifetime of the quantum state, is a MAJOR problem for the JJ devices. What D-Wave is demoing is *quadratic* speedup on an NP-complete search problem. This is valuable, and in accordance with what we know about quantum computing and complexity, but not the Holy Grail of quantum computing. No one will pay them money to solve 16-bit NP optimization problems, but people will well before they hit 1,000 qubits, if it all works. They also claim to be working on various simulations of quantum systems using their device, which *does* have the potential to do New Science with relatively modest-sounding systems, and is one of the best potential uses of a quantum computer. They claim that they have X axis and Z axis control of individual qubits (you need to be able to reach any point on the unit sphere from any other), and a ZZ coupling between pairs of qubits. They also say that they need an XZ coupling to reach fully general QC, and that will be tricky. But my understanding of the theory is that what they have should be sufficient; there is probably some more technical constraint that isn't explicitly in the announcement -- like they can only do X on odd-numbered qubits and Z on even-numbered ones, or somesuch. There is a good discussion on Dave Bacon's blog, as well: http://dabacon.org/pontiff/?p=1427 Anyway, the device looks impressive, and may represent a major step forward. Hard to know for sure at this stage. Good luck to the D-Wave guys, and congrats on their achievements so far! --Rod P.S. To toot my own horn for a second, Mark Oskin and I had a paper in JETC last year on the systems issues in building large-scale quantum computers. It's available on my web page at http://www.tera.ics.keio.ac.jp/person/rdv/quantum/index.html and may make those with a computer systems bent make more sense of the quantum computing systems. Personally, I'd rather be building these systems than thinking about how to build them, but one step at a time... ------------------------------------------- Archives: http://archives.listbox.com/247/ Powered by Listbox: http://www.listbox.com
Current thread:
- Re: quantum computer announcement] David Farber (Feb 05)