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Measuring entanglement

I cannot believe it myself, and it is hardly thustworthy to write about it on April 1, but please take it for a fact. I have attempted to experimentally quantify entanglement of photon pairs today. I did not do any experiments for more than thirty years.

Val Zwiller is to blame. When asked to provide an experimental setup for undegraduate lab practice, he could not come with any fresh idea. So he’s just slightly modified a setup of pioneering experiment of Alian Aspect who has quantified the violation of Bell inequality in 1981. He has invited me and Leo Kouwenhoven to "inagurate" – that’s how he put it – the experimental setup. 

At 16:00 I was at his office. We brisky got to the measurement room. On the way Val asked me with soft heartfelt tone: " Do you believe in entanglement? Do you believe in non-locality?" This gave me a thrilling impression of being a part of important rite: he sounded like a priest asking faithful about their readiness. "I do. I do believe." I responded, trying to match the tone. I wanted to add that I believe in quantum mechanics, but, given the nature of the rite (where a participant should demonstrate a ritual doubt and sucsessfully overcome it by direct measurement) this would not sound polite. 

Me and Leo have been supervised by Julia Cramer and another young lady. The setup was placed in two large plastic boxes alike my children use to store old toys. One box housed a blue laser, non-linear crystal to chop a blue photon into an entangled pair of red ones. The two go by two fibers to the second box where their polarizations have been rotated by changeable angles and finally get to photon counters. The computer gave coincidence counts, those depending on the rotation angles. The math to be made afterwards, and its significance is explained in

There were about twenty curious people in the room, all irradiating excess heat. Val has attibuted the chaotic work of the setup to this mere circumstance. He kept re-tuning the fibers in the production box. He did it quickly and efficiently. I was turning polarizers. Leo read data and wrote it up. One needs to measure at four different positions of the polarizers. So we got 16 readings. After the experiment everybody was cheering while I had to do the math.

We did not manage to prove (our faithful devotion to ) quantum mechanics this time. This would happen if the final answer would exceed classical boundary of 2. Yet we got 1.91: Almost. Not bad for the first time.

Thanks very much, Val, Julia and all others involved: this was a wonderful and entertaining experience for me, and it will be fun for many generations of students.


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