Hong Ou Mandel Lab
HOM Resources:
Fiber Optics:
Fiber Optics Essentials, K. Thyagarajan and Ajoy Ghatak, 2007: Chapters 4, 5, and 12
Can be found at the library, both as a physical copy and an online version
HOM:
Yang, Y., Xu, L. & Giovannetti, V. Two-parameter Hong-Ou-Mandel dip. Sci Rep 9, 10821 (2019).
Equipment List
Single Photon Setup
HOM Setup
Necessary Parts
You will need various fiber and free space optics equipment to complete these experiments. Some pieces of equipment, like mirrors are general equipment, others, such as the fiber couplers are highly specialized and the exact part number listed should be used
Free Space Optics Parts:
Four identical collimators with xy angle adjustment: Make sure the mounts are identical, Thorlabs KM100T mounts work great. All four collimators should have Thorlabs F280FC-780 lenses. Check this before setting up! The correct lenses are vital to the success of the HOM effect. For best results the three collimators that are fixed should also use the exact same posts and post holders, the height of these is up to you.
One collimator with xy angle and translation adjustment: This will be used to couple the HeNe laser into fiber which will be important for alignment. This collimator should have a 633 nm collimating lens.
At least two mirrors: Any mirrors with xy angle adjustment will do.
Near infrared non-polarizing beamsplitter: All it has to do is split the 810 nm beam 50:50
Thorlabs BS017 works great.
At least two irises.
Translational Stage: Should have a range of at least 10 mm and a resolution less than 50 microns. Thorlabs PT1 translation stage works great.
Fiber Optics Parts:
Pump laser and biphoton source: Both from the Qubitekk kit, operation instructions are in the Qubbitekk manual.
1x2 polarization maintaining fiber coupler: Thorlabs PFC780F. This coupler splits two combined orthogonally polarized beams and splits them into separate linearly polarized beams.
2x2 polarization maintaining fiber coupler: Thorlabs PN780R5F2. This coupler takes two linearly polarized signals and splits each of them evenly between the two outputs.
Multimode fiber patch cables: There are many of these in the lab. The orange cables and black cables are multimode.
Single mode fiber patch cables: These cables are wavelength specific. The yellow ones are rated for 780 nm. When used at lower wavelengths they may operate in the multimode regime. The blue cables are also rated for 780 nm and are polarization maintaining. These cables are extremely thin and break easily. HANDLE GENTLY. NEVER BEND SHARPLY.
Fiber to Fiber connectors: All of the fibers we use are terminated with male FC/PC connectors. The fiber to fiber connectors have two female FC/PC connectors.
Tools:
Level
Square
Allen Wrench Set
Adjustable Spanning Wrenches: One is the Thorlabs spw801 with pointed tips, the other should have flathead tips
Single Photon Experiment
In this manual. MM stands for multimode, SM for single mode, and PM for polarization maintaining.
Step 1. Familiarize yourself with pump and biphoton source.
The Qubitekk lab manual has operating instructions for both of these. Plug in but do not change the temperature of the biphoton source. Plug in the pump laser and get used to changing the power by feeding the output of the biphoton source directly into one of the cages and turning on the labview program. You should see that changing the pump power drastically changes the output of the biphoton source.
You can use the biphoton source software as shown in the Qubitekk manual to get used to changing the temperature. If you don’t have working software, a serial emulator can be used to communicate with the biphoton source. Instructions for that are in the section below.
Step 2. Couple HeNe into fiber.
Place the collimator with angular and translational adjustment in the beam of the HeNe laser by adjusting the height of the collimator mount. This collimator should have one of the 633 nm collimating lenses. Adjust the angles and translation until the beam is most efficiently coupled. You can maximize coupling efficiency by finding the angles where the beam can be seen brightest inside of the fiber. Do not send the HeNe laser directly into the SPCM.
Step 3. Align one in-air beam.
Connect the input of the 1x2 fiber coupler to the output of the biphoton source using a fiber to fiber connector. This will produce two 810 nm polarized beams, one from each output of the 1x2 coupler.
Put together two collimators with 780 nm lenses and screw their post holders directly into the optics table. The posts should be allowed to sit as low as possible in the post holders to make sure the two collimators are the same height.
Align this in-air section as shown in the thorlabs video. It is best to go in this order:
Connect the HeNe cable to the input collimator and maximize counts with an orange MM fiber running from the output collimator to the SPCM.
Connect one output of the 1x2 coupler and maximize counts again for the 810 nm signal with the orange MM output.
Swap the orange MM output with a yellow SM cable running to the SPCM. Once again maximize coupling efficiency using the technique in the Thorlabs video. At this point you are making incredibly fine adjustments to the collimator mounts and you have to be very careful not to bump anything when connecting and disconnecting cables from the collimators. A good way to test the coupling efficiency is to measure counts directly at the output of the 1x2 coupler and compare to the counts measured
Step 4. Measure α2d.
α2d stands for 2 detector anticorrelation coefficient. It is a measure of the correlation of the two photons and details on calculation of α2d are found in the Pearson and Jackson paper.
With the first in-air section aligned, run one 810 nm signal through it, and attach a yellow SM fiber patch cable to the other 810 nm signal. The output of the in-air section and the yellow patch cable should both be connected to cages, at which point you can take measurements of A, B, and AB coincidences. It would be best at this point to use an orange MM patch cable at the output of the in-air section in order to maximize transmission. Take measurements for varying values of pump laser power. With the laser at 25 mW, you should find alpha to be upwards of 600.
Step 5. Align Detector B’ and Beamsplitter.
Set up two irises along the in-air beam and position them by measuring the counts as you adjust the height, position, and aperture.
Once the irises are locked in place, use mirror walking to direct the HeNe beam (straight from the laser, not the fiber) through both irises. You will also be able to see that it is in the right place by maximizing counts on B from the HeNe signal.
Place the beamsplitter directly in the path of the HeNe laser by screwing the post holder directly into the optics table along the B arm and adjusting the height and angle of the beamsplitter so that the reflected HeNe laser comes out at roughly 90 degrees top the B arm.
Place the third identical height collimator where it will catch the reflected beam by also screwing the post holder directly into the optics table and letting the post sit as low as possible.
Connect an orange MM patch cable to the B’ collimator and run it to cage C. Maximize counts at both B and B’ by adjusting the B’ collimator and the angles of the beamsplitter
Remove the mirror that directs the HeNe laser and realign collimator B’ and the beamsplitter with the 810 nm signal
Step 6. Measure α3d.
Once again details on α3d can be found in the pearson and jackson paper.
Keep the yellow patch cable running from the second 810 nm output to cage A. Once again you will be recording counts, but this time for A, B, B’, AB, AB’, and ABB’. These values can be used to calculate the three detector alpha. It should be many standard deviations less than 1.
Congrats! You can move on to HOM interference now. The beamsplitter and B’ collimator can be removed, but the other two collimators form one of the two in-air sections of the HOM interferometer.
HOM Experiment
In the first part of this project you can afford to be a little imprecise. Now, it gets a little bit harder. Make sure to make very small adjustments when performing alignment, and never change multiple things at once. In order to get interference, the alignment has to be very very precise.
Step 1. Orient Collimation lenses.
Before putting together the interferometer, the keys on the FC/PC connectors on the back of the collimation lenses must be oriented. It is necessary that the keys point upwards on all the collimators. This is because the key orientation determines the polarization of the light inside of any PM cables, including the 1x2 and 2x2 couplers. Matching the polarization of the two inputs of the 2x2 coupler is vital to see interference.
FC/PC connector key
You can adjust the orientation of the lenses by loosening the lockrings holding the lenses in, rotating the lens to the desired orientation, and then tightening the lockrings again. Keep in mind that the collimators in the translation stage arm should be mirror images of each other when deciding which direction is up for the lenses.
Thorlabs Lockrings
Step 2. Set up translation collimator.
The fourth collimator that was not used for the single photon experiment will be mounted on the translation stage. Mount it on the front, opposite from the knob, so that the knob is able to hang off the optics table for easy access. Do not attach the stage to the table yet though. Use a post or threaded stainless steel cylinder to screw the collimator into the translation stage at approximately the same height as the other collimators. It doesn’t need to be exact yet, as the final collimator placed will be adjusted to the same height as the translation collimator.
Step 3. Place final stationary collimator.
Screw the post holder of the final collimator into the table so that the second in-air section will be parallel and the same length as the first in-air section. Insert the final collimator and post into the post holder. Place the translation collimator directly facing the stationary one, roughly two inches away. Place a level on the tops of the two collimators and adjust the height of the stationary collimator until the level is level, implying they are the same height.
Step 4. Attach translation stage to table.
Find the correct holes on the table to screw the translation stage in such that it is opposite the stationary collimator and the new in-air section is the same number of table holes long as the other one. Once again it is convenient for the translation stage to be placed such that the knob hangs off the table. Screw two short screws in to attach the stage but do not tighten them yet. Screw two long screws into the table in front of the stage to use as a guide. Use a square to square the stage as you tighten the short screws attaching it to the table as shown in the picture. This ensures that the translation collimator stays aligned as the translation stage moves.
Step 5. Align second in-air section.
Before beginning alignment, zero the translation stage by moving it until the translation collimator is roughly lined up with the equivalent collimator in the other in-air section. Now repeat the same alignment done in step 3 of the single photon experiment. That is, use the techniques of the Thorlabs video and align from HeNe to MM, then from 810 nm to MM, and finally from 810 nm to yellow SM. You can test how the alignment holds up as the translational stage moves by monitoring the counts and turning the knob. If there is a steep drop in counts, then most likely your translation stage is not square or the collimators are not the same height. The best way to fix this would be to repeat the leveling and squaring from steps 3 and 4. It is vital for steady counts that the collimators are exactly the same height and the translation stage is exactly square with the table.
Step 5.5: recheck the alignment of the other in-air section as well to make sure you are getting maximum counts.
Step 6. Attach the 2x2 coupler and find the dip.
Attach the 2 inputs of the 2x2 coupler to the translation collimator and its equivalent collimator in the other in-air section. Run the two outputs to cages A and B. Before you start looking for the dip here is a checklist of necessary conditions:
All keys in the collimator are oriented exactly vertically to match the polarization of the two photons interfering.
Both in-air sections have maximized counts for SM fiber.
The translation stage can move at least a couple mm in either direction from the estimated midpoint without affecting counts.
Loose fibers are taped down or otherwise secured so that jostling doesn’t affect the data and the fibers are protected against sharp turns or being accidentally pulled on (these fibers are extremely delicate and must be treated with care)
When the whole setup is connected, A and B counts are close to equal. Within 20% should be enough.
To start looking for the dip, set the pump laser to a fairly high power, something that brings the AB coincidence rates up to around 2000 or more. Move the translation stage to a position at least 3 mm away from your estimated center point. While running the LabVIEW program, slowly rotate the knob of the translation stage to move it back through the estimated middle point and past the middle another 3 mm or so. Moving it only a couple of tick marks each turn will get you the precision to find the dip. (each tick mark is a thousandth of an inch on the Thorlabs PT1) As you move through the center point you will see at some point the coincidence counts begin to decrease while the A and B counts stay relatively constant. The minimum coincidence rate should be roughly half of the initial coincidence rate. As you move past the minimum, the coincidence rate will increase until it returns to the initial value. Take note of the location of the translational stage at the minimum. It will most likely be a bit off from the estimated center point as the 1x2 coupler’s polarization maintaining qualities slightly affect the path length difference of the two photons.
Step 7. Record the dip!!!
The final step is to actually record the dip. You can go about this whatever way you feel will bring you the best data. Once you have your data you can plot AB coincidence rates against translation stage position and recreate the HOM dip! From this data you can calculate the depth/visibility of the dip as well as the FWHM. Matlab’s curve fitting software can allow you to fit a gaussian curve to the data set to find more exact values for these as well as error analysis.
Step 8. Record more dips.
Repeat step 7 at various values for the biphoton source temperature setpoint. (make sure to record the default degenerate setting first!) Degeneracy should correlate directly with the dip width and this is something that you should investigate.
Bi-Photon Temperature Control
If you don’t have a working copy of the QES2.2 temp control software, a serial emulator can be used to control the temperature in the biphoton source. I recommend Termite, it is easy to use and you can start the application straight from a .zip file without installation.
When You have Termite running you will get this window. Click settings and copy these exact settings:
Everything in Port configuration should be as shown above, but make sure you have chosen the COM port that the QES2.2 is plugged into, in my case that is COM5. The baud rate, data bits, stop bits, and parity are all properties of the port on the QES2.2 and should be set correctly so that Termite can send and receive data. It is also important that Transmitted text is set to just Append CR. This ensures that the text that is sent and received displays correctly on your screen. The other options are left as default.
Once the settings are correct, and the QES2.2 is powered on and connected to your computer via USB you are ready to go. Go back to main interface by pressing okay and you can now send commands and queries to the temperature control board by typing into the bottom text bar as shown below. It is very important that you check and record the default setpoint before changing anything as the default setpoint produces degenerate photons. The default setpoint for the MXP QES2.2 is 38100 but it is still good practice to check and make sure that that is what it is set to.
An example of using the commands
Here is a list of commands and queries for the board:
:SETT N
Set the temperature setpoint for the QES crystal (Min = 10000 | Max = 50000)
SETP?
Returns the temperature setpoint for the QES crystal (Min = 10000 | Max = 50000)
CURR?
Returns the current reading from the I/O board (Min = 0A | Max = 5A)
FAUL?
Returns whether there is a fault in the temperature control system (0 = No fault | 1 = Fault)
HORC?
Returns whether the system is heating (H) or cooling (C) (0 = Heating | 1 = Cooling)
TEMP?
Returns the temperature reading (in deg C) of the crystal. (Min = -40C | Max = 150C)
VOLT?
Returns the voltage reading from the I/O board (Min = 0V | Max = 5V)
FIRM?
Returns the firmware version of the temperature control system
Written by Ziggy Bjurlin, 2023
-Updated September 2023 - Tracy Chmiel