I would be very thankful if somebody helps me with some practical advices about producing twin photons?
I know that twins are produced after a laser ray is directed on a BBO crystal. After the BBO there are two rings of photons and the twins are in the intersection of the rings.
1. I wonder in order to get the two rings must I have the laser ray falling strictly at 90 degree to the BBO? My crystal is very small 3x3 mm so I intend to focus the laser on it which means I would have different angles of them on the BBO so I fear I would not get the rings but a smeared spot. Is this right and what to do? I would like the beam to fall on a point but to be perpendicular. Is there a way to do this?
2. I am not at all sure how to capture the light from the intersection of the rings where the entangled twins are. I will filter the incident beam 405 nm by a filter at 405 but how to collect only the intersection spots to proceed and to direct them to a beam splitter b.e.? To make a mask with two openings? Or fibers?
Thanks in advance.
Producing and capturing entangled photons using a BBO (beta barium borate) crystal involves careful alignment and precise setup. Here are some practical steps and considerations to help you in your process:
Producing Entangled Photons
1. Laser Alignment:
- Ideally, the laser beam should fall perpendicular (at 90 degrees) to the BBO crystal to optimize the production of entangled photon pairs.
- To achieve this with a small crystal (3x3 mm), you can use a collimated laser beam with a narrow focus. The focal point of the laser should be at the center of the crystal.
2. Focusing the Laser:
- If you focus the laser beam on the crystal, ensure that the divergence is minimal to avoid a wide range of incidence angles. Using a lens to focus the beam precisely at the center of the crystal can help.
- Consider using an adjustable lens system to fine-tune the focus and angle of incidence.
3. Crystal Orientation:
- The orientation of the BBO crystal is crucial. Ensure it is aligned correctly along its optical axis to maximize the efficiency of the parametric down-conversion process.
- Use a goniometer or a precise rotational stage to adjust the angle of the crystal if needed.
Capturing Entangled Photons
1. Detection of Photon Rings:
- After passing through the BBO crystal, the entangled photon pairs form two rings. The intersection of these rings contains the entangled photons.
- Place a screen or a phosphor-coated detector behind the crystal to visualize the rings and ensure proper alignment.
2. Filtering
- Use a bandpass filter at 405 nm to block the incident laser beam. Ensure the filter is placed before the detection system to prevent the laser's residual light from reaching the detectors.
3. Collecting Entangled Photons
- To capture photons from the intersection of the rings, you can use spatial filtering techniques. Here are two options:
- Mask with Openings:
- Create a mask with two small openings positioned at the intersections of the rings. Place this mask in front of your detectors to allow only the entangled photons to pass through.
- Optical Fibers:
- Use single-mode optical fibers to collect the photons. Position the fibers precisely at the intersections of the rings. Mount the fibers on a precision translation stage to adjust their position accurately.
4. Directing Photons to Beam Splitter:
- Once you have collected the photons using fibers or through the mask, direct them to a beam splitter or other optical elements as required for your experiments.
- Ensure that the optical path lengths are matched to maintain the coherence of the entangled photons.
Additional Considerations
- **Alignment Tools:**
- Use alignment lasers, mirrors, and beam-splitters to fine-tune the setup. Laser alignment tools like beam profilers and alignment targets can help achieve precise alignment.
- **Stability:**
- Ensure that the entire setup is stable and free from vibrations, as small movements can misalign the system and affect the production of entangled photons.
- **Polarization:**
- Consider the polarization properties of the entangled photons. Using polarizers or waveplates may be necessary to control the polarization states.
By following these steps and carefully adjusting the alignment and focus, you should be able to produce and capture entangled photons effectively. Experimenting with the setup and making incremental adjustments will help you optimize the process for your specific crystal and laser system.
Thank you Harri
I supposed it would go as you describe. Good to hear it by experimentalists. I am a little afraid that when using a mask (fibers even more) diffraction can spread my rays but I suppose it would be negligible as the holes would not be to narrow but at 1-2 mm.
Regards: Ilian
PS. By the way what is the easiest practical way to rotate polarization from V to H. Can you recommend a rotator of polarization? Of course sugar water solutions can do but I look for something more compact.
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