Updates

3/16/2012

· LabVIEW programs updated. Upgraded to LabVIEW 2011. Support for Newport NewStep rotation stages (NSR1). New vi to measure the polarization state of a single photon.

1/29/2012

· Parts list updated.

3/10/2011

· Inexpensive detectors are here! See this information.

5/26/10

· Update on lasers

Thanks to Blu-Ray DVD technology, lasers are finally cheap! I bought some 405 nm laser diodes for about $17 each (!!) from hightechdealz.com. The lasers I bought were model PHR803T, which were surplus from an HD DVD player. I managed to get 40mW of power by driving them with 100mA of current from an ILX Lightwave laser diode current source. I had them in a temperature controlled mount, but I wasn’t using the active temperature control—as long as they’re well heat-sinked they should work fine. I could easily perform all of the experiments described on this site with this laser.

Only thing about these lasers were that the leads had been cut very short. To get them to fit into my mount I had to solder longer leads on, but then they worked fine.

I see that now there are even higher power lasers available from the same source. They’ll probably work as well.

5/26/10

· New Simulations!

We’ve developed some virtual laboratories. Check them out here.

6/25/09

· Improving Entanglement with Dispersion Precompensation:

When trying to do experiments with polarization entangled photon pairs it is necessary that the two entangled states be as indistinguishable as possible. For the two crystal geometry that we use in our experiments this means that if one can determine, even in principle, which of the two crystals the photons were produced in, then there is some distinguishing information, and the degree of entanglement is reduced. This leads to a lesser violation of inequalities testing local realism. One in principle means for determining which crystal the photons are produced in is via their arrival time. For example, if photons produced in one crystal take longer to reach the detector than photons produced in the second, it is in principle possible to determine the photon polarization. As long as the time delay between the production times is much less than the temporal duration of the photons, entanglement will be preserved. The effective temporal duration of the downconverted photons is given by their coherence length, which is the inverse of their bandwidth.

One way that photons could be produced at different times in the two crystals would be if the blue pump took different times to reach the two crystals. Even though the pump is continuous, it has a finite bandwidth, and hence it also has a finite coherence length. In some sense this coherence length behaves like a pulse width for the pump. When the pump enters the first crystal it has polarization components in both the horizontal and vertical directions. Since this crystal is birefringent and dispersive, these polarization components will propagate at different speeds through the first crystal. The important velocity here is the group velocity, and what we are most interested in is the difference in group delay times between the two orthogonal polarizations of the pump.  As long as the difference in group times is less than the coherence length of the downconverted photons then entanglement is preserved. 

The finite bandwidth of the pump means that there is some mismatch in the group delays, but it's not large.  It's not difficult to calculate, but I haven't bothered to do this calculation. However, I've found experimentally that while the entanglement in our experiments is degraded somewhat because of this effect, it's not enough to destroy the entanglement.  The question is, how pure do you need?  We are certainly able to easily violate local realism, even though the entanglement is somewhat degraded.

If you really need a very high purity source, then the best way to achieve this is to use a single-mode pump laser.

It's also possible to precompensate for the temporal walkoff by inserting a birefringent material in front of the downconversion crystal pair.  I've tried this by using a piece of BBO that's the same thickness as each of the other crystals.  It's oriented perpendicular to the first crystal of the pair. Thus, in the precompensating crystal the pump polarizations temporally walkoff.  This crystal is well in front of the pair, so any photons produced here are not collected.  In the first crystal of the pair the two pump polarizations come back together in time, so that they overlap at the interface between the two crystals of the pair, giving maximal entanglement.  By precompensating in this way the entanglement is definitely improved.  You can use this precompensation crystal in place of the quartz plate that adjusts the relative phase-- it can play both roles. 

So, if you want better entanglement and have extra money you can buy a single BBO crystal that's as thick as one of your other crystals.

Lastly, I should note that I borrowed the idea for precompensation from Morgan Mitchell and Paul Kwiat.

2/5/09

· I’ve had several people ask me about lasers. If I were buying a laser now I'd probably go with one of the blue laser diodes from Sanyo:

http://www.photonic-products.com/products/sanyo_violet_laser_diodes/sanyo_violetblue.html

I know someone who has used the 60mW laser with success.  The cost is about $2.2k for a bare laser.  You'd need to get a separate current source (and I'd think about a temp controlled head and temp controller), probably from Thorlabs or ILX Lightwave (make sure you get a current source with a large enough compliance voltage for blue lasers).  Despite this I think you'd save a thousand dollars or more over the Power Technology lasers I use, and you'd get a slightly more versatile system.  Just be careful with static!

The circularized beam on my Power Technology laser is very nice, and it helps with the collection efficiency, but the key is getting a single spatial mode.  As long as your laser is a single spatial mode I think you'll be able to get plenty of counts and good entanglement if you've got a 60+ mW laser.

6/9/08

· Updated LabView vi’s to use our new FPGA-based CCU.

6/7/08

· Updated Coincidence Counting Unit page to reflect our new FPGA-based CCU.

12/20/07

· Added course lecture notes and updated lab manual.

8/21/07

· Updated Labview vi’s.

8/1/07

· The coincidence circuit is ready: email beckmk at whitman.edu (replace "at" with @) to get the details.

7/9/07

· Updated the LabView vi’s. Now there are versions for LV 7 and LV 8.2.

· Added an updated parts list to the main page.

· The coincidence circuit should be ready for general release by the end of the month.

9/9/06

· We’ve tried the Power Technology 185mw laser. WOW! LOTS of coincidences! However, the jury is still out on the achievable level of entanglement you can generate with this laser. In tests of Bell’s Inequality we are able to get larger S values with the 50mW laser than the 185mW laser (although the error is less with the 185mW laser, because we get more counts and thus better statistics). We’re not sure if this is just because it’s a new laser to us, and we haven’t spent much time working on the alignment, or if it’s something more fundamental with the laser itself. This 185mW laser does have multiple spatial modes, and the beam is larger and more elliptical than our circularized 50mw laser.

· Our coincidence circuit is coming along. We’ve done two prototypes which work quite well. We’re currently working on a version with improved time resolution.

· I’ve added some new presentations on the entry page.

· I’ve updated the LabView vi’s.

· I’ve made available a copy of the Lab Manual for the lab portion of our QM course. This lab is being taught for the first time in the fall of 2006.

2/15/06

· We’re working on a circuit that makes coincidence counting cheaper, and hope to have more to say about this in the next few months.

1/23/06

· Power Technology http://www.powertechnology.com/ now has 185mw, 405 nm laser diode modules available!

· The Bell inequality and Hardy test experiments require making measurements at several different waveplate settings. While it’s not necessary to use a motorized rotation stage for this, it does make things nicer. Newport has recently released a relatively low-cost motorized rotation stage, the NSR1. It uses the same Newstep controller that we use for the linear stepper motor in the single-photon interference experiment. Total cost to automate the waveplate movement for these experiments would be about $2,600. I should note that I have never used these stages, so I can’t personally vouch for them. They only have a resolution of 1^o, but that’s probably sufficient for these experiments.

· I’ve just released updated LabView vi’s for these experiments. Included is a vi that allows one to mimic the behavior of a multichannel analyzer using an A/D board.

1/21/05

• We are currently using one of the 4-channel SPCM's mentioned in our AJP article (Perkin-Elmer APCM-AQ4C, $9k, available from Pacer).  It needs 3 external power supplies (2, 5, and 30 V), and the user manual is a bit short on details about how to wire it up. Putting it together is much easier if you purchase the adapter card [SPCM-AQ4C-IO, $265].   However, you’ll still need to know how to connect the power supplies to the board (not in the user manual), and a circuit diagram to help you do that is here. Another important thing to note is that the unit is gated ON when the gates are either grounded or floating. To gate the unit OFF you need to apply 5V to the gate. When we built a box to hold ours, we added a 5V pullup to the gate so that on powerup the counters are gated OFF (circuit diagram here). Some pictures of the unit in its box can be found on our pictures page.

• In the spring of 2004 we purchased a 50mW blue laser diode module from Power Technology http://www.powertechnology.com/ .  We got the circularized output option [Model IQ1C (LD1539) G2X23, $7k]  We’re using it in the Bell inequality and Quantum Eraser experiments and it works quite well.  They probably have modules with even higher powers now.

• In the experiments involving polarization entangled photons using two crystals mounted back-to-back (as in the article by Dehlinger and Mitchell), the company who sells the crystals has changed its name to Photop Technologies http://www.photoptech.com/.  We are using two 0.5mm thick crystals rather than the 0.1 mm thick crystals used by Dehlinger and Mitchell.  We get higher count rates with the thicker crystals, which makes things easier. The thicker crystals don't appear to hurt the quality of the Bell Measurement (I'd hesitate to go much thicker though.)

• We have found green LED lightbulbs that are great for illuminating the lab because their wavelength is well blocked by the RG780 filters in front of our detectors. The background singles count rates increase slightly with these LED’s on, but they contribute nothing to the coincidence rates. These bulbs screw directly into a standard incandescent lightbulb socket. Model PAR20-66-0AG-120AN , $108, from http://www.ledtronics.com/. It’s great to be able to do photon counting with the lights on!

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webpage updated 3/16/2012

beckmk at whitman.edu (replace "at" with @)