Hardy's Test

4-Detector Hardy Test Alignment Procedure

This alignment procedure assumes that no optical components have been set up. If you have already done the other SPQI experiments, you should be able to reuse parts of your setup.

Considering the level of precision required for Hardy's test, the summer group of 2014 decided to start from a nearly empty table. This was done so that every level of alignment could be checked.

Figure1: Here is a block diagram of the final setup. Note that the elements indicated with dashed lines are for alignment purposes only and will need to be removed before data can be taken for Hardy's Test. Specifically, the HeNe laser should be turned off when collecting data and the detector in front of the laser will be needed to collect data(We only have four detectors). The pickoff mirror needs to be removed since it is blocking the path of the down-converted light.

Initial Alignment

Figure 2: Initial Setup: THIS IMAGE IS NOT ENTIRELY CORRECT: Iris #1 (I1) should be placed AFTER Mirror #2 (M2)

We refer you to the Pump Steering page for initial alignment. You will find it useful to place the steering mirrors, M1 and M2, as close to the edge of the table as you can. This experiment samples down converted photons from the Beta Barium Borate crystal( BBO). Refer to Figure 2.

The down converted photons exit in a small angle cone. We want to only select pairs of entangled photons. The photons which are entangled will occur at opposite sides of the cone. So, a larger separation between the BBO and detectors will allow a more specific selection.

We found an iris immediately after the pump laser and an iris after the second steering mirror allows for collimating the beam as well as controlling the intensity. When aligning, it is often useful to select only the center of the bright portion of the beam. This allows you to very precisely align components in the best position without the width of the beam obscuring the best position.

It is recommended that you height align all four fiber optic detectors(Detectors A, B , B'/C, and A'/D) with the walked pump beam. It is also advised that you center the fine adjustment knobs before doing any alignment. These steps are non-essential, but will likely save you effort later on.

Pump Beam and Down Conversion Components

Whenever we place components in a beam arm, it is easiest to start from the last component in the beam and work toward the source. Start by placing the components that will be in the pump beam path, i.e., (1) the BBO's, (2) the 405nm half wave plate (HWP or λ/2), and (3) the 405nm Quarter Wave Plate (QWP or λ/4).

1. For this experiment we will be using two BBO crystals. Verify that both crystals are in the holder and are aligned perpendicularly to each other. One should be aligned vertically and the other horizontally relative to the table. Each BBO has an arrow on its edge. For both BBO's, this arrow should point towards the detectors. Place the assembly in the path of the beam and retro-reflect. (For details on how to retro-reflect, read Step 1 of Down Conversion Alignment) When placing the BBO, keep in mind how much room you will need for the other components. The 405nm HWP and the QWP must go in between the mirror and the BBO. It is also advisable to leave room for an iris if not already placed. I also recommend leaving an open slot in case you want to insert another component such as a polarizing beamsplitter or linear polarizer. The summer 2014 group found 12 inches to be sufficient, but restrictive. (See Figure 4 below for a close-up view of these components.)
2. Next place the QWP immediately before the BBO. This wave plate should be mounted on a rotational stage. Set the rotation of the stage to zero and lock it in place so it won't move during adjustment. Retro-reflect the crystal using the fine adjustment knobs.
3. Place the 405nm HWP in front of the QWP and retro-reflect it.

Figure 3: Definition of Angles:

Figure 3 depicts an optical component and is meant to help define the terms "tilt" and "turn" as they are used in this experiment. "Tilt" refers to the rotation about the vertical axis of the optical component and is denoted by the angle φ. If it helps, when the component is attached to a rod, rotating the rod will change the component's tilt. The Rotational Stage of the QWP also adjusts φ. "Turn" refers to the rotation of the component about the axis of the beam (so long as the beam is orthogonal to the optical component, which is the case after you have retro reflected the component). The angle of this turn is denoted by θ. All of the wave plates are secured in mounts that allow you to adjust θ.

Figure 4: Close-up view of the HWP (λ/2), QWP (λ/4) and the 2 BBOs. (Note: the BBOs are obstructed from view by they their mount.)

Detector Alignment

Refer to the Down Conversion Alignment page for aligning Detectors A and B.

After placing detectors A and B, refer back to the instructions here.

Position the 810nm band-pass filter in front of each detector. The tilt of the filter affects the count rate in a nearly parabolic fashion. So the derivative of the count rate with respect to the angle is zero when it is perpendicular. As such it is not critical to perfectly align the filter for now. Roughly align the filter so that it is perpendicular to the down-converted beam.

Remember, when we refer to Detector A, B, etc, we are referring to the collimating lens that focuses the beam into the fiber optic cable. The units that actually detect and count photons are the SPCM's that are inside the Black Box.

Setting up the Polarizing Beam Splitters

This section has three parts: the setup of the alignment HeNe laser, the setup of B-arm components, and the setup of the A-arm components. Since this experiment measures invisible down-converted light, the use of a HeNe laser will facilitate the alignment of the detectors. The goal is to get the HeNe laser light to mimic the path of the down-converted light as closely as possible. To do this, we will be using pick-off mirrors placed directly after the BBO(on the side with the down-converted light.) These instructions will differ from those used in setting up the Anticorrelation Parameter of a HeNe Laser experiment to a small extent. The previously used method should work well, but requires a little more work and components. Feel free to refer back to the previous method if you would prefer. Remember that for Hardy's test we will have a symmetric setup and both arms require a PBS.

The justification for the alignment procedure is as follows. Detectors A and B have been "tuned" using the count rates in such way that they are directly facing the BBO crystals. You can verify that this is the case by shining HeNe laser light out of Detectors A and B. The beams should hit the center of the BBO crystals(if they don't, your initial alignment is not optimally aligned and you should adjust it.) If the alignment of Detectors A and B are optimal, the detectors are pointed at the BBO and intersect the path of the down converted light. Therefore, a beam coming out of the detectors should share the same path as the down converted light of interest. Now if we use mirrors to walk a beam coming out of the Detectors into the path of the HeNe laser, then when the laser is unobstructed, the light will travel back into the detectors, and hence along the path of the down converted light. To provide a means of making sure the mirror walked beam is aligned with the HeNe laser, we set up two irises in the path of the HeNe. Then, by mirror walking into the irises, we guarantee the alignment of the HeNe laser with the down converted path, which is very important when using the HeNe beam to align the polarizing beam splitters and Detectors C and D. Refer to Figures 5(a) and 5(b) below for clarification.

Note: In order to avoid confusion, when we place a detector in front of the HeNe for transmission purposes, we will refer to that detector as Detector E.

Alignment of HeNe Laser

First, we will setup the HeNe beam to mimic the down-converted light, that is, we will mirror walk the beam so that it strikes one of the detectors(we will start with the B-arm.) Once we have the beam aligned correctly, we can begin aligning the B-arm and A-arm.

To facilitate the mirror-walking, place the HeNe laser at a spot where the beam can most easily be made to mimic the down-converted light. We found that placing the HeNe laser parallel to the pump laser made mirror-walking simple. Refer to Figure 1.

Place the HeNe near Detector A and make the beam parallel to the holes on the table. The beam should be aimed away from Detectors A and B(refer to Figure 1.) Make sure that the HeNe laser beam does not coincide with where you plan to place Detector D. Remember, the location of Detector D will depend on the location of the A-arm PBS and the distance between the PBS and Detector A. We found that 13-15 inch/holes was a sufficient( a tad cramped) distance between Detector A and the HeNe laser. Place an iris directly in front of the HeNe. This will be used for intensity control and light collimation.

After the iris, the next component in the path of the HeNe will be a stand which will hold Detector E. Place a stand at approximately the correct position, but don't put the detector in just yet. Next in the path you will place and center two irises. The further apart you can place the irises, the easier it will be to achieve a high level of precision in your alignment. Make sure these irises are well aligned!

Since a detector is just a collimating lens, we can use a detector to focus the HeNe laser into a fiber-optic cable. By attaching the opposite end of the cable to another detector(say, Detector B), we can shine the HeNe laser through that detector and back onto the laser(if the pickoff mirror is in place) or the BBO(if the pickoff mirror is not in place.) This will ensure that the HeNe closely mimics the down-converted light. Since we will be aligning the B-arm first, we use Detector D as our transmission detector(Detector E.)

Figures 5(a) and 5(b): These figures are block diagrams showing the process of aligning the HeNe Laser. By shining the HeNe beam through Detector B, as shown in Figure 5(a), the HeNe beam can be mirror walked to shine back into the HeNe laser. Note that if the pickoff mirror was not in place, the beam from Detector B would strike the center of the BBO(so long as Detector B is aligned correctly). If the beam has been correctly walked, removing Detector E will allow the main HeNe beam to follow the path of the down converted light and strike Detector B, as shown in Figure 5(b).

Place and fasten down a mirror in the path of the HeNe beam such that it will reflect the beam near the BBO, where you will be placing a mirror on a pick-off mount. Place and fasten a mirror on a pickoff mount at the location of the reflected beam near the BBO. If you can get the mirror closer to the BBO, you will be able to more accurately mimic the down-converted light. Just remember that you will need use the adjustment knobs on the mirror mount, so make sure you have access to these, ie don't place the mirror too close to the BBO. Roughly mirror walk the beam toward the Detector B. Don't bother trying to get it exact at this point. Place Detector E in the mount in front of the HeNe laser. Attach an unused fiber optic cable to Detector E, and adjust the detector such that you get a strong beam coming out the unconnected end. Initial alignment is possibly the most difficult part of setting up this experiment. Don't be discouraged if you can't get it right away. When you are close to the right alignment, the cable will start to glow, although there might not be much light coming out the other end. Iteratively adjust the horizontal and vertical tilt to maximize first the glow of the cable and then the light coming out of the end. Once the emitted light is maximized, disconnect the cable. You might find it useful to add a collar to the post of Detector E as we will be removing and replacing it.

Figure 6(a):

6(b):

Figures 6(a) and 6(B): These figures demonstrate the difference between a weak transmission signal, 6(a), and a strong transmission signal, 6(b). The best way to get a strong signal is to point the unconnected end of the fiber optic cable at a piece of paper so you can clearly see the transmitted light. Then,using the adjustment knobs, adjust the height of the detector until you get a maximum brightness for the dot on the paper. Then,(using the knobs) adjust the horizontal position of detector until you again reach a maximum. Then, adjust the tilt of Detector E and the H/V knobs until you get a well defined center of the dot. Your transmitted signal should resemble Figure 6(b). If using the adjustment knobs doesn't seem to be working, you may have strayed the lens too far from the beam. Remove the fiber optic cable from Detector E and visually align it with the HeNe laser. This should get you near the sweet spot. Reconnect the fiber optic cable and use the adjustment knobs to get the maximum transmittance, ie the "sweet spot."

Figure 7: This figure depicts the alignment of the HeNe laser and the detector that will be used for alignment of the other detectors.

Aligning the B arm

Disconnect the B fiber from the cage assembly. (Be careful not to disturb the adjustments of the cage assemblies.) Then connect the disconnected end to the Detector E. Go ahead and disconnect the A fiber from its cage assembly as well. We want to ensure that there is no way a strong HeNe beam can enter a single photon counting module. Put a plastic cap on the disconnected end of fiber A. Remember, the 810nm band-pass filter will block the 632nm HeNe light, so go ahead and remove the filter from in front of Detector B. We will be changing the position of the filter, so remove both the filter and stand. Adjust Detector E until you get a strong signal projected through Detector B. You can now double check that the HeNe beam emitted from Detector B mimics the down-converted light by confirming that the beam strikes the center of the BBO. In an ideal setup, the beam will reflect off the BBO and enter into Detector A.

Now place the mirror on the pick-off mount, and mirror walk the beam into the irises in the HeNe laser path. The beam should strike the back end of Detector E. Once you are confident in the alignment, remove Detector E from its stand in front of the HeNe laser. If your alignment is good, the light should now pass through the irises and be directed by the mirrors into Detector B(Figure 5(b)). You should see a nice strong beam projected out of Detector E(If B and E are still connected.) Make sure that you NEVER have a detector connected to a single photon counting unit while the HeNe laser is on and aimed toward the detector.

Place one of the small polarizing beam splitters (PBS) in the B arm. The exact location doesn't matter, but what does matter is that both Detectors B and C are of equal distance from the beam splitter. Also, keep in mind that you will be placing the 810nm Band-pass filter and an 810nm HWP in between the PBS and the BBO. When placing the PBS, make sure that the adjustment knobs face the incident beam, ie away from Detector B. Use the knobs to retro-reflect the alignment laser. Try to position the PBS such that the alignment beam hits the center of the face. Each PBS is marked with a dot. The corner with dot should be in between the incoming down converted beam and the C detector.

Figure 8: This diagram is a close up view of the PBS in the B-arm. As shown, the reflected light(which is vertically polarized) travels in the direction of Detector C while the transmitted light(which is horizontally polarized) travels towards Detector B. A similar diagram can be made for the A-arm.

Place the 810nm HWP in front of the PBS and retro-reflect it. This HWP will not work as well for the 632nm HeNe laser, but it will still sufficiently shift the polarization such that you will be able to significantly modify the relative intensities of the outputs of the PBS.

Position a ruler on the table the same distance from the PBS as Detector B, but in the path of the split beam. Place and align Detector C with the split beam. The HeNe makes this a much easier task than it would have been otherwise. For alignment of the Detector C, disconnect its fiber optic cable and position the Detector C so that the beam passes through the lens. Disconnect the fiber optic cable from Detector C's cage assembly. Reattach that fiber optic cable to Detector C and adjust Detector C until you get a strong signal coming out the free end of the cable.Place the 810nm band-pass filter before the PBS. It does not matter if it comes before or after the HWP.

Figure 9: This shows the final HeNe alignment of the B_arm.

Turn off the HeNe and reconnect Detectors A, B, and C to their cage assemblies. You will need to remove the mirror from the pick-off mount. Turn on the pump laser, but make sure to turn off the room lights before turning on the SPCM's. If you adjusted the 405nm HWP, readjust it to maximize A counts. Now turn the B arm HWP to maximize B counts. Verify that you are still sampling entangled pairs. ie. make sure that AB alpha is still sufficiently high (in excess of 100). If it is really low you can try tilting one of the bandpass filters to maximize coincidences or adjusting the horizontal and vertical alignment of Detector B.

Now turn the B arm half-wave plate to minimize the B count rate. This should simultaneously maximize AC coincidences. You will now align the C detector to maximize AC coincidences like you did previously with the B detector.

Aligning the A arm

Since the experimental setup is symmetric, setting up the A arm will be very similar to setting up the B arm. The main difference is that, once Detector D is in place, we will no longer have a detector to transmit the HeNe Laser. Make sure the SPCM's are off before turning on the alignment laser!

You will be mirror walking the alignment beam along the A arm. You can either place another set of mirrors to accomplish this, or adjust the ones you used for the B arm. The summer 2014 group used two sets which came in useful for checking alignment later when the setup wasn't working correctly. However, as of August 2014 there are only two pick-off mounts. This means that in order to switch between the A and B arms you will need to either remove or replace a mirror. The block diagrams on this page use one mirror in the main HeNe beam path and two pick off mirrors(one for each arm), which can be placed on the same stand since you will only be aligning one arm at a time.

Place Detector E in the stand to collect the alignment laser. If necessary, readjust Detector E so that you get a strong signal coming from Detector A. Remove the bandpass filter in front of Detector A(if you haven't already.) Once again make sure that the beam coming out of A is narrow and strikes the center of the BBO. (In a nearly ideal setup, the reflection from the BBO should then follow the path of the B arm.) Now mirror walk the beam back through the irises in the HeNe Laser arm. Once again remove detector E to verify the beam travels through the irises, and is reflect by the mirrors into detector A. We are now (hopefully) done with Detector E, and it can be positioned to function as Detector D.

Follow the instructions outlined for the B arm to place and align the second PBS, the 810nm HWP, Detector D, and the bandpass filter. In this case, the PBS corner with the dot on it should be between the incident beam and the Detector D.

When maximizing the BD coincidences, you may want to switch which cage assemblies the detectors are connected to such that the program will give you a graph of the correct coincidence rate. For example you could connect the D detector to the A cage assembly, then maximize AB coincidences. Don't forget to switch the detectors back to their original cage assemblies.

Figure 8: This picture shows the final setup a the A-arm and B-arm.

Tuning The Alignment

Visually adjust both band-pass filters such that they are close to perpendicular to the down converted beams. Set both 810nm HWP to 0 degrees. Now turn the 405nm HWP to maximize AB coincidences. Adjust the vertical tilt of the BBO to maximize the AB coincidence rate. Now turn the 405nm HWP to maximize CD coincidences. Adjust the horizontal tilt of the BBO to maximize the CD coincidence rate. To get a graph of the CD coincidence rate, connect the Detector D to the A cage assembly.

The setup should now be ready to conduct the experiment. Quantum mysteries tested by Carlson, Olmstead, and Beck provides a good explanation on how to conduct the experiment. At this point you should be ready to follow their instructions in section C, "Tuning the state." After that is done all that remains is to take data at the analysis angles. Remember, the 810nm HWP's create the analysis angles.

If you get good results, you can switch the QWP with the Quartz Plate and see how that affects your results.

Results

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