Quantum Bayesian Networks

December 31, 2010

In Love With XY-pic Software

Filed under: Uncategorized — rrtucci @ 5:25 pm

I’m currently trying to write a paper on quantum Bayesian networks, the structures which gave this blog its name, and which have interested me for a long time. My huge interest in QB nets drove me to write a Mac application called Quantum Fog that implements them. That was more than ten years ago, and since then, I’ve learned a lot more about QB nets, especially about their connection to quantum information theory, which is the reason why I’m trying to write a new paper about them.

Anyhow, I decided that writing such a paper would be much less laborious if I could draw figures of directed graphs directly in LaTex. (quantum as well as classical Bayesian networks are directed acyclic graphs). As usual, the LaTex gods have not disappointed me. It turns out that drawing directed graphs in LaTeX is very easy using XY-pic.

I just love XY-pic and that’s why I’m writing this blog post, to rave about it. XY-pic was written by Kristoffer H. Rose, Ross Moore and collaborators to draw figures of commutative diagrams in category theory. It has also been used very successfully to draw figures in “automata theory, algebra, neural networks, topology (knots and braids), database theory, chemistry, and genealogy”. The very useful and popular package Q-circuit for drawing quantum circuits in LaTex is implemented using XY-pic commands.

For those already familiar with LaTex (one of the most beautiful and useful and influential scientific softwares in the history of mankind, comparable in importance to the invention of the printing press), learning to do basic stuff with XY-pic can be done in a day. The tug (TeX users group) page for XY-pic will lead you to some nice tutorials with examples. You can also do a Google Image search for “xy-pic and latex”, and find examples that way.

If you too need a way of drawing directed graphs in LaTex, and you’ve never used xy-pic before, you may find it useful to see in this blog post a very simple example. Mind you, XY-pic can be used to draw much more sophisticated diagrams, but I hereby present as a simple example, the famous (among Bnet aficionados) Bayesian network called the “Chest Clinic”. The code in this file produced the following figure:

December 29, 2010

Save RINNO From Extinction

Filed under: Uncategorized — rrtucci @ 4:27 pm

Quantum Information Theorists use the term P.O.V.M., an acronym that stands for the unfathomable, impossible to parse, jumble of words: Positive Operator Valued Measure. I’m part of a group that advocates that this term be abolished, and be replaced with the more cuddly acronym R.I.N.N.O which stands for: Resolution of Identity by Non Negative Operators.

If you would like to join us in our campaign to promote the preservation of the word RINNO, check out “A Petition to the American Physical Society for the Creation of a New Acronym”

December 27, 2010

12 o’clock/3 o’clock Convention

Filed under: Uncategorized — rrtucci @ 8:49 am

My “code generator” computer programs output several types of text files. Among them is one type which I call a “Picture File”, which is an ASCII picture of a quantum circuit (an example can be found here). In such pictures, time flows downward, and each row (i.e., line) represents a different gate. Suppose the rightmost qubit is called 0, and there are 4 qubits called 3, 2, 1, 0. Then a single line of a Picture file looks like this

3- – -2- – -1- – -0

where the numbers are the names of the qubits and should be replaced by a gate symbol, like “H” for Hadamard gate, for instance.

In my LaTex papers, I draw the circuits so that time flows horizontally, rather than vertically, and I label the topmost qubit 0. Hence in my papers, each row represents a different qubit. A 4 qubit circuit would have qubits 0,1,2,3 in this order

0
1
2
3

To remember how these two circuit representations are related, I have a simple mnemonic that I would like to share with my readers, so that they won’t get confused when reading my picture files. The mnemonic is this: I use a 12 o’clock/3 o’clock convention (The other possibility, which I do not use, would have been to use a 12 o’clock/9 o’clock convention).

You see, I like to use terminology that impresses the girls, like this 12 o’clock/3 o’clock term, which sounds like jet-fighter-pilot language. If you say “You’ve got a pair of bogeys at 11 o’clock. Roger, Wilco”, you’ll get the girls swooning.

P.S.- As I discussed in a previous blog post, when drawing circuits with time flowing horizontally, I use the Dirac Convention rather than the Dan Quayle Convention.

P.S.2- In case you were wondering what I look like, here are some pictures of me when I’m not doing quantum computing.

GOODDAY SIR, I AM A CAPTAIN AND JET PILOT IN THE NIGERIAN AIR FORCE. I HAVE BEEN AUTHORIZED TO GIVE YOU $1,000,000

December 21, 2010

The BlackBerry Quantum Computer

Filed under: Uncategorized — rrtucci @ 8:51 am

Mike Lazaridis is the founder of RIM (Research in Motion), makers of the BlackBerry handheld device. He has made extremely generous donations to the following scientific institutions located in the city of Waterloo, Canada

Pi (Perimeter Institute)

In the Univ. of Waterloo:

According to Wikipedia, Lazaridis has donated about $150 million to PI and about $100 million to IQC/WIN. The government of Canada contributed another $50 million to IQC/WIN in 2009.

These are still pretty young institutions. PI opened in 2001. The IQC/WIN buildings are slated for completion in 2011.

PI currently employs over 100 resident theoretical physicists and many other visiting ones. Even though PI employs only theorists, this certainly will not be the case with IQC/WIN.

As I mentioned in my CHIMPS blog post, so far Perimeter (and IQC/WIN) have not excelled in their experimental QC work. A quick look at the IQC website reveals that they currently specialize in turkey technology like quantum cryptography and NMR quantum computers. But this is likely to change soon, and it’s likely that they will soon begin to make a serious attempt to build a scalable QC. Like I said before, WIN/IQC is still a very young institution; it hasn’t even opened its new building yet. All that money is bound to produce some positive results, even if some of it is wasted in politics and pomp & pageantry. So the future looks very promising for a BlackBerry quantum computer.

It’s certainly refreshing to find rich people who believe in QCs enough to invest in them. At present, trying to get an American venture capitalist or philanthropist to invest in QCs is nearly impossible. I know. I’ve tried.

Let’s hope that the BlackBerry doesn’t continue to lose market share to Android and iPhone, so that Lazaridis can continue to donate to QC research.

China is very rich right now and could easily outspend the Canadian QC program. A Chinese/Canadian QC race would certainly be exciting to watch. America is in a shambles economically right now, but, in my opinion, we are at present winning the QC race, because the QC American Riviera phenomenon started earlier than its Canadian and Chinese counterparts, and it is already bearing fruit. Will the US continue to lead the QC race? …I for one can’t wait to find out.

P.S.: Canada also has D-Wave Systems, which is doing great work, but I haven’t mentioned them because, at least for now, they are the only ones building an adiabatic QC, so they are in that sense sui generis.

December 19, 2010

Quantum Computing’s American Riviera

Filed under: Uncategorized — rrtucci @ 8:48 am

Aerial photo of Santa Barbara, CA


Will UCSB (Univ. of California at Santa Barbara) become the Silicon Valley of the quantum computing era? God only knows, but the initial signs are very promising.

Just this month, the journal “Science”, published by the AAAS (American Association for the Advancement of Science), awarded the title of “2010 Breakthrough of the Year” to the following paper in the journal “Nature”. The paper was written by a UCSB group under the tutelage of professors John M. Martinis and Andrew Cleland.

Quantum ground state and single-phonon control of a mechanical resonator by A. D. O’Connell, M. Hofheinz, M. Ansmann, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, John M. Martinis & A. N. Cleland

link to abstract in Nature here
No arxiv version available (sigh!)

For a more detailed report, Prof. Martinis has put online the Ph.D. theses of 3 of his students

  • A Macroscopic Mechanical Resonator Operated in the Quantum Limit, by Aaron O’Connell (2010).

  • Generation of Three-Qubit Entanglement Using Josephson Phase Qubits, by
    Matthew Neeley (2010).

  • Benchmarking the Superconducting Josephson Phase Qubit – The Violation of Bell’s Inequality, by Markus Ansmann (2009)

In a previous blog post, I mentioned that Neeley now works at Lincoln lab. I don’t know what the future plans of O’Connell and Ansmann are. Hopefully they will scatter far and wide, and be given the opportunity to continue to work developing a QC.

UCSB has become one of the premier hot beds of QC R&D, both experimental and theoretical. The history of super-successful laboratories like Bell Labs seems to indicate that theorists perform best if they are in close contact with experimentalists, and vice versa. UCSB has managed to achieve this explosive mixture, unlike Perimeter Institute and the other CHIMPS.

In the experimental physics side, UCSB is host to

In the theoretical physics side, UCSB is host to

The roster of the UCSB Physics Department Faculty is quite impressive, including not only quantum computer/solid state specialists like (David Awschalom, John Martinis, Andrew Cleland and Wim Van Dam(computer science theorist) ) but also some very famous String Theorists like Joe Polchinski and Nobel prize winner David Gross.

The only thing UCSB is missing are some QC startups. I hope those will start sprouting soon.

Last but not least, the city of Santa Barbara is a seashore paradise. It lies on the Pacific coast of California, pinched between the Pacific Ocean and the steeply-rising Santa Ynez Mountains.

Santa Barbara’s climate is often described as Mediterranean, and the city is sometimes referred to as the “American Riviera.” Temperatures range from
45-65 degs F in January
to
60-77 degs F in August.
No snow year round.

The city became notorious for an oil spill in 1969, but that was a long time ago, and almost no trace remains of that event. There are some oil-drilling derricks offshore, in the Santa Barbara Channel, but you can also see humpback whales there, as the channel is part of the route of their yearly migration. Santa Barbara was a favorite of Julia Child. She spent the last years of her life there.

December 16, 2010

The CHIMPS of Quantum Computing

Filed under: Uncategorized — rrtucci @ 1:27 pm

We’ve all heard people refer to the European countries that are in financial trouble as the PIGS (Portugal, Ireland or Italy, Greece, and Spain). In the race to build a scalable quantum computer, there are some places that are buzzing with activity, like UCSB (Martinis/Awschalom), Yale (Schoelkopf/Girvin), U. of Wisconsin (Saffman), NIST (Wineland), USTC in China, etc., etc., while there are other places that are a sleepy dullsville. In the dullsville category, the CHIMPS come to mind

C- Caltech
H- Harvard
I- Indian Institute of Technology
M- Massachusetts Institute of Technology
P- Perimeter Institute, and its identical twin, IQC. Oh Canada!  
S- Stanford

The amazing thing is that each of these institutions is super-prestigious and has enough scientific intellectual firepower to sink an aircraft carrier, and yet, as far as the race to build a QC is concerned, they are performing at chimp level. Some of them are still working on NMR quantum computers, which are not scalable, and are therefore a dead-end street. Others are working on quantum cryptography and a quantum internet (what a turkey idea! totally pointless and impractical). Others are doing very little in the area of QC experiments. Please correct me if I’m wrong and I’ve missed some recent (in the last 2 years) experimental work done by one of the CHIMPS.

December 9, 2010

Agile, Micron-Wide Spotlight

Filed under: Uncategorized — rrtucci @ 10:10 pm

Normally, I don’t say much about experimental stuff, because I’m not an experimental physicist, and I feel uncomfortable teaching outside my area of expertise, but this experiment is easy to explain and really cool. Check out:

“Independent individual addressing of multiple neutral atom qubits
with a micromirror-based beam steering system”
by C. Knoernschild, X. L. Zhang, L. Isenhower, A. T. Gill, F. P. Lu, M. Saffman, and J. Kim (ArXiv version available here)

Some scientists from Saffman’s group at the Univ. of Wisconsin (Madison, Wisc.) and from Duke Univ. (N.Carolina) have built a device that traps 5 NEUTRAL Rubidium-87 atoms and then shines a spotlight on one atom at a time. (so far, the NIST people have used ions)

The five Rb atoms are trapped in a straight line; adjacent ones are 8.7 micrometers apart. The diameter of the spotlight is about 5 micrometers. (bacteria are typically 1 micrometer in size, so this is a spotlight for bacteria) The spotlight can be moved from one atom to another in 10 to 20 microseconds.

The spotlight is so powerful 🙂 that it causes a Rb atom to transition between the |0\rangle and |1\rangle quantum states which define a qubit. In this case, these two states are defined as two hyperfine energy levels of the atom; more specifically, |0\rangle := | F=1, m_F=0\rangle and |1\rangle := |F=2, m_F=0 \rangle. It takes about 0.1 microsecs for an atom to go from |0\rangle to |1\rangle. This so called Rabi flopping can be induced in a single target atom, with minimal disturbance on the quantum state of adjacent atoms.

In the near future, these researchers plan to produce 2-qubit interactions and 2-dimensional arrays of qubits.

The last sentence of the paper is:

While we have focused on its use in neutral atom QIP, it is applicable to other qubit systems in a periodic lattice including atomic ions, diamond nitrogen-vacancy color centers, and quantum dots.

I find it awesome how good the experimentalists are getting at controlling single qubits. True, qubits floating in the middle of a vacuum cavity, held in place by ephemeral “tractor beams” of light, manipulated by a spotlight which is steered by mechanical means, is probably not the most sturdy and convenient way of implementing a scalable quantum computer. But we can certainly learn a lot of physics from such experiments, and this will illuminate the way towards more convenient devices.

Press release for paper here

December 6, 2010

Wikileaks and the Quantum Space Race

Filed under: Uncategorized — rrtucci @ 10:18 am

The cable communications recently released by Wikileaks can be searched at cablesearch.org. I searched for the keyword “quantum” and got just one hit, for a communiqué sent from the American embassy in Beijing, on Feb. 2010. I saved it here. It alludes to the quantum teleportation work being done at USTC (University of Science and Technology of China) in Hefei, China. By coincidence, the USTC people recently published a paper in PNAS (Proceedings of the National Academy of Sciences of the USA) (ArXiv version available here) about their teleportation work. An excellent report of their paper appeared in ArsTechnica:

Quantum teleportation finds a place in quantum computers
By Casey Johnston (ArsTechnica, December 1, 2010)

It looks like USTC is emerging as a serious contender in the race to build a QC. In my opinion, China is still substantially behind the USA in that race, but it appears that they are quickly catching up with us.

December 5, 2010

Lincoln Laboratory Joins the Race To Build a Quantum Computer

Filed under: Uncategorized — rrtucci @ 8:23 am

Lincoln Lab (LL) is a laboratory funded by the DOD (Department of Defense) and partly run by MIT university. Since it is funded by DOD and is a favorite son of theirs, it has very deep pockets and its opinion holds much sway in US defense circles. Since it is partly run by MIT and is only about a 20 mile car ride away from the MIT campus, MIT engineers often collaborate fruitfully with LL staff. Like most defense labs, LL is often wasteful, bureaucratic and political. Nevertheless, it has done in the past some excellent, world-class work in the fields of electronics, optics, communications, radars, etc. For example, the gifted electrical engineer Ken Olsen got his start at LL. Olsen left his job at LL circa 1957 to start DEC, the company that made VAX computers.

Recently (Nov.30, 2010), a radio station in Santa Barbara, California (KCSB 91.9FM) interviewed a Lincoln Lab “Project Scientist” called Matthew Neeley. Let me quote from a web page of KCSB’s website:

Project Scientist at Lincoln Laboratory at MIT and former UC Santa Barbara Graduate Student in Physics, Matthew Neeley, Ph.D. talks about his recent paper in Nature on his team’s breakthrough in quantum computing. Dr. Neeley is an alumni of the Martinis Group at UC Santa Barbara. The breakthrough work has shown for the first time the entanglement of three quantum bits of information, or qubits. The long term goal in this is to create a high functioning quantum computer.

What type of project is Dr. Neeley leading at Lincoln Lab? Is it one that will try to extend the work of Martinis’ group? I sure hope so. (It’s not a forgone conclusion that he is, though. LL might have him there for a completely different reason. Much of the work done at LL is classified secret. I can’t just call LL and ask them what Neeley is doing.)

If LL is truly trying to build a scalable quantum computer, that could have important repercussions for the QC race. It might act as an incentive for some American defense companies to join the QC action, and try to get a piece of the QC pie. Cash rich China may see this as a challenge and put more effort into building their own QC.

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