Tuesday, September 30, 2008

Experiments supporting the concept of a g(2)-camera

Last weekend, as like an astronaut in a Mercury capsule I sat strapped in a small seat in a metal tube being flung across the Atlantic and Canada's Northern Territories, I was reading the day's press from both sides of the Atlantic to catch up with the last two weeks of news and get an appreciation of the reality field's distortions.

On both sides of the Atlantic, physicists made first page news, but for very different reasons, as you would expect in a Riemannian reality field. In the US newspaper, a journalist had been chasing so-called financial geniuses in New York and London to get the rap on $700 billion of toxic financial papers. In the European newspapers the story was on page four, with the question of why the US Government was talking about $700 billion when the actual amount of toxic paper was $3,500 billion, or $3,500,000,000,000.00.

Anyway, that is what you get with reality distortion, but it was not what caught my attention. The journalists were not able to get any financial genius to speak on the record, so they reported remarks from both sides of the Atlantic stating that the financial instruments were so complex that there was no way they (the geniuses) could understand them, that is why they hired quantum mechanics physicists to cook up risk models.

So, there it was written black on white: the quantum mechanics physicists are to blame for the $3,500 billion toxic papers. Hmm, and I thought the only toxic paper quantum physicists handle is that in the litter box of Schrödinger's cat. And they can even not known if the cat is dead or alive.

The story about the quantum physicists would have been more believable, if they had written the $3,500 billion disappeared in a black hole when the Large Hadron Collider (LHC) was turned on in Geneva (see this article on page 1291 of Science magazine of 5 September 2008).

Science 5 September 2008: Vol. 321. no. 5894, p. 1291

That is what I read in the US newspapers. In the European newspapers physicists made the first page for completely different reasons. The first reason was the LHC. There had been some apprehension about black holes, but the operation start on 10 September was a full success. Unfortunately, over a week later, a possible faulty electrical connection between two of the accelerator’s magnets caused a large helium leak into sector 3-4, moving the start of the experiments to March 2009.

What the newspapers explained in some detail, was how beneficial the $8 billions spent on the LHC was for European industry, because it spurred a large amount of new technology in fields like superconductors and low-temperature materials. While I was reading this, I thought, wow, $8 << $3,500 billion. We could have had our own supercollider in Texas for only the bonuses of one bank in one year!?

The second front page news related to physics in European newspapers was Zhai Zhigang's space walk and the impact the development of the Shenzhou 7 capsule and its launching technology had on Chinese industry, leading it to develop more advanced technologies.

As a whole, from a European perspective, quantum physics and rocket science are not as bad as it is believed to be on this side of the Atlantic. From an international point of view, that had already been decided in the Nüremberg trials, which lets me continue with the meat of this post without shame.

It did not make the newspapers, but last week our paper on experiments supporting the concept of a g(2)-camera was published. If your institution does not subscribe to SPIE's Digital Library, you can buy it for only $18.00 (those are plain dollars, not billions).

Recent experiments have reported the Bose-Einstein condensation (BEC) phase transition for exciton-polariton systems in a semiconductor microcavity. The macroscopic quantum degeneracy is typically detected by probing the statistical properties of light emitted from a microcavity, under the presumption that the statistics of the exciton polaritons are faithfully transferred to the emanating photons.

The macroscopic quantum degeneracy can be established by measuring the correlations viz., first-order in the electric fields:


and seconds-order in the electric fields:


Moreover, it has been assumed that observation of the interference fringes similar to those in Michelson or Young interferometers is sufficient to establish the fact of macroscopic coherence in exciton-polariton systems. Two points on the wave front separated by a distance x12 produce an intensity pattern

intensity pattern

such that the fringe visibility measures the magnitude of the first-order correlation function g(1)(x12, τ). But simply measuring this quantity alone is ambiguous because a coherent light source (e.g., a photon laser or decaying polariton BEC) can exhibit the same first-order correlations as a chaotic (or thermal) light source (e.g. Hg-Ar discharge lamp). The table below shows that proper disambiguation of a coherent state also requires measurement of the second-order correlation function

second-order correlation function

associated with intensity noise correlations. Here, I1,2(t) is the light intensity at a point ±½ x12 and time t.

Maximal values of respective correlation functions for incoherent, coherent and thermal light states

correlation function

photon states




g(1)(x, 0)




g(2)(x, 0)




∆g(2)(x, 0)




The minimal condition to confirm the BEC phase transition in a polariton system then becomes

minimal condition to confirm BEC

Our imager detects the spatial correlation excess shown as ∆g(2) ≡ g(2)(x, 0) – 1 in the third row of the table above.

In our paper, we present a novel g(2)-imager built with conventional CMOS technology, which is capable of measuring second-order spatio-temporal correlated photons and thereby offers an important means for verifying the existence of a BEC state of cavity exciton polaritons.

Exploded micrograph of the 4x4 SPAD array

One potential limitation when imaging BECs with our device is the requirement that ∆g(2) = 0, which corresponds to a null measurement. For BEC detection, however, we anticipate that a more practical device could combine conventional g(1)-imaging with g(2)-imaging, either as the same camera operated in two distinct modes or as two distinct cameras working together.

Future work will include the development of larger arrays of SPADs, the integration of on-chip data processing based on equation

and the extension to other g(2)-imaging applications.

A surprising feature of the g(2)-camera is that the parallelism of the sensor stemming from using N detectors does not scale linearly but binomially. For example with a 4 x 4 SPAD array all 16 detectors have separate parallel outputs so that (162) = 120 simultaneous pairwise measurements are possible.

You can get the full paper from this link: http://spie.org/x648.xml?product_id=795166.

Wednesday, September 17, 2008

That branch of Lake Como, which turns south

The airport in Frankfurt is an enclosed space where everybody comes from at least a thousand kilometers from the next person and speaks a radically different language. The planners made it all work by abstracting the semiotic common ground of humanity. The operators make it all work with a perfectly executed clockwork.

What a different experience getting off the airplane in Malpensa. Everybody looks the same and speaks the same language with the same accent. And while an hour earlier in Frankfurt it was silent like in a mausoleum, in Malpensa it is busy and loud like in a bazar.

Everybody seems to have two cell phones, one ringing and the other being used to exchange trivialities with remote people. And people keep talking to everybody around them. Sure, unlike the people in Frankfurt, these people can. But strangers keep asking me when their suitcase will come out, how long it will take, where they will have to go next…

I place myself under the carousel's display, where I can just rise my finger and point to the display. What do I know? Later, driving to the hotel, my host Prof. Rizzi explains me that I witnessed the Italian version of the GPS: you just constantly ask the people around you for directions. I guess that works in a society where most people walk.

The autostrada takes some getting used to, when one normally cruises on 101 or 280. But then, everybody is watching — no spaced out people juggling a coffee mug in one hand and a cell phone in the other. And there is not the heavy metal protection of an SUV.

We arrive at the hotel for the invited keynote speakers. I am surprised. The Comaschi, the people from Como, especially the business people, are known for having short arms (meaning they cannot easily reach in their pockets to take out their bill-fold). Yet, here they put us up in the best hotel in town.

Not only, they put us in the best rooms in the best best hotel in town. What a view! As you see in the pictures, in front of us is that branch of Como Lake, which turns south, between two uninterrupted mountain chains, all peninsulas and bays, depending on their sticking out or caving in, becomes, all the sudden, constricted, and becoming like a river…

Quel ramo del lago di Como, che volge a mezzogiorno, tra due catene non interrotte di monti, tutto a seni e a golfi, a seconda dello sporgere e del rientrare di quelli, vien, quasi a un tratto, a ristringersi, e a prender corso e figura di fiume, tra un promontorio a destra, e un'ampia costiera dall'altra parte; e il ponte, che ivi congiunge le due rive, par che renda ancor più sensibile all'occhio questa trasformazione, e segni il punto in cui il lago cessa, e l'Adda rincomincia, per ripigliar poi nome di lago dove le rive, allontanandosi di nuovo, lascian l'acqua distendersi e rallentarsi in nuovi golfi e in nuovi seni.

What a suffering it was in middle school, and what a gorgeous sight it is now.

When we arrive at the conference venue, we find a mixture of preoccupation, panic, and emergency. The proceedings did not arrive, and nobody knows who is in charge. Only the badges for people at the beginning and end of the alphabet are here, the others are missing. There is no wireless, where is IT? The janitor does not know. Short rapid conversations.

Then everybody pulls out their two cell phones. Is it high noon? No, every person in the hallways is communicating simultaneously on each of ten channels: the first cell phone in one hand talking to one person, the other cell phone in the other hand talking to a second person, both phones announcing the arrival of an SMS after another, talking to several people in the room, and orchestrating the hole gesticulating with the hands. What a cacophony!

Suddenly people start showing up. "Good afternoon, I am the one for the coffee." "Good afternoon, I am the IT manager." And so on. Everybody is proposing a contribution. What is amazing to somebody accustomed to the American way of doing business is that there are no negotiations, no power games, no delays to talk things over with the boss.

Everybody only makes positive proposals. Everybody has personal authority to make decisions. Numbering systems and spreadsheets are set up so there is full accountability on everything. After less than two hours the whole conference has been organized on the spot and everybody is satisfied that it is better than they had anticipated. It is time to part because it will be a long evening and night to get everything done to perfection.

It comes to my mind that everybody here in middle school has learned Nicolò Macchiavelli's Prince and knows how to manage well. In fact, here the whole Prince is summarized in the few lines of a vernacular haiku:

fin che ga né…

viva el re…

Quand ghe né pü…

crepa l'asan e quel che gh'é sù.

Mr. Bill Hewlett would have added "and we do not need any stinking Voice of the Workforce surveys."

Monday, September 15, 2008

Visit to the sausage factory

Scientific research is a struggle with many wrong turns and lots of meticulous detail work. Then, after the Eureka moment, we reorder our ideas and write it all up in a nice clean and easy-to-follow paper.

If you just read papers, scientific research is a glorious activity, where the researcher is like a Zen master waiting for the right moment and then letting the arrow reach the target. Even in the case of Zen, reality is very different from what you see in the movies.

Therefore, scientists gather in conferences. Papers are given in formal presentations, to quickly communicate the final result of the research. However, the real action takes place in the hallways during the breaks and during meals. That is when the presenters are approached and war stories are exchanged.

The trick is to learn from other researcher's experience. It is like visiting the sausage factory so you know what to eat and most of all, what not to eat.

So here is the handout to our next conference presentation: HPL-2008-109.

For those not attending the conference, here is a trip report from the sausage factory.

HP used to have a literature distribution center in Campbell. It warehoused the brochures and other marketing collaterals for all currently shipping products. When sales representatives prepared an offer, they would order the pieces from Campbell and assemble the material in a booklet to present the customer.

Then HP bought Indigo. Now all those printed materials could be replaced by a document repository. The sales reps could "shop" for the required collaterals on a web site and the resulting document would be printed on demand on the Indigo press and finished, resulting in a professional booklet.

Our project built the prototype of this system called DPOD (I cannot recall what the acronym stands for). In alphabetic order, team included Hui Chao, Scott Clearwater, Anna Durante, Parag Joshi, Xiaofan Lin, Greg Nelson, Henry Sang, and Nick Saw.

We were not happy to build just a shopping cart solution as a front-end to a document management system. To maximize the document's impact, we implemented a custom printing system, also known as variable data printing system.

In the middle of the implementation, HP changed the colors in the corporate palette. To have a uniform look for all pieces in a marketing document, we had to automatically change the old corporate colors to the perceptually closest new colors.

Things could go particularly wrong with colored text on colored background. If in the old palette the text was readable, in the new palette it could become illegible. We had to check each foreground-background color pair for readability and then nudge one of the colors.

In the first prototype we did not yet have the code to parse the PDF and nudge the colors there. Instead, we ripped each page and fixed it pixel by pixel. To keep up with the Indigo's print speed, the implementation had to be very efficient. Yet, it also had to be very precise, so the nudged colors were consistent across the entire document.

My first implementation of the small color piece was colorimetric, i.e., I calculated ∆E94 differences and nudged the colors in cylindrical CIELAB coordinates before converting them back to sRGB.

In the second implementation, instead of enforcing a minimal ∆E distance, I enforced a lexical distance. I calculated the color name of the foreground and background colors and ensured there was at least one color name in-between.

Years before, at PARC, Maureen Stone had used the ISCC-NBS color naming system, so I looked for something different to implement.

The basic scheme for ISCC-NBS color names

I found that most of the naming research in the literature was focused on the basic color terms, so I settled with the Coloroid color naming system. Here is how the leaf for hue A = 20 looks:

Coloroid color names

There are many white areas in the plot. In addition, to make things run fast, there is no time for gamut mapping, so the entire addressable color range must be tiled, for a resulting naming algorithm as follows:

modified Coloroid names

The system worked so well, we made a follow-up for a product called MoD, for marketing on demand. It was a web service that would allow small and medium businesses (SMB) to create their own professional marketing collaterals. By "their own" we mean that instead of hiring an expensive graphic designer, they can get done a comparable collateral by an administrative assistant.

To avoid complexities like spec sheets for logos, we extended the color discrimination algorithms to work for any vector graphics element on a page. For example, the colored logo works on any background color from HP's corporate palette, as shown in these examples:

on black on blue 2004

on white on ocher

on ocher dark on ocher liht

on orange on orange light

on purple dark purple light

If we compare the web service to a minivan, then with our system we provided it with a racing car engine. Of course, from car manufacturers we learned that the buyer of a minivan hardly ever opens the hood, so there is no point in outfitting it with a racing engine, a lawn mover engine will do it. As my friend Irwin Sobel keeps telling me, the computer industry is learning fast from the US car manufacturer's cost-cutting strategies.

Now that you know what happened in the sausage factory you can read HPL-2008-109. Even better, come to the conference, where you can ask all your questions and learn from our experience.

Sunday, September 14, 2008

Name a color

For some years, Nathan Moroney has been running a multilingual color naming experiment. In the experiment, participants are shown seven color patches picked at random from the sRGB cube. Next to each patch there is a field for entering a name for the color.

The random distribution is uniform, so one would expect to obtain a uniform distribution of color names. However, this is not so. There are eleven basic color terms that come to mind very fast, so one can expect to hear them more often.

Naming colors is an acquired skill, so it will depend on the participant's socioeconomic status (SES). Participants in the color naming experiment tend to be from a high SES, so one would still expect a to get a fair amount of non-basic color terms.

Well, here is the data:

distribution of color names

We are plotting the 101 most frequent color names collected so far in the experiment. For navigation, the abscissa marks each fifth color name from the ordered list above.

For a number of reasons, in such a crowd-sourcing experiment there is low precision and there is a certain amount of bogus data. This is handled by weighting and averaging the contributions.

The distribution's long tail entails that a large amount of data needs to be collected to obtain a reasonably rich color name dictionary. A rich dictionary is important, because for us color scientists it is not the dictionary itself that is of most interest (linguists deal with that). What we are interested in, is to invert the data and build a tool that given a color name shows a color patch of that name. This allows us to help you improve your color naming skills.

English being the internet's lingua franca, we have sufficient data in English. However, in the other languages we have insufficient data to be able to average the non-basic color terms.

At this time we call for readers speaking Italian. If you do, please help us by naming colors in Italian at this link. Your contribution will make the color thesaurus more useful.

Grazie mille!