[Stefan, Lisa, Kiwamu, Yuta, Evan]

We are trying to center the transmitted X-arm IR on the transmon QPDs, so that we can have an IR alignment reference. The expected power on each QPD is determined as follows:

• 8.8 W into the IMC
• 80% transmission through the IMC
• 3% tranmsission through the PRM (misaligned)
• 50% transmission through the BS
• Power buildup of 2 x 445 / pi in the X-arm
• 5 ppm transmission through ETMX
• 5% transmission through mirror M12 on the transmon table
• 50% power splitting on the beamsplitter on the IR QPD sled

These numbers give an expected power of 3.7 µW on each QPD.

What we measured was about 600 cts for the sum output on each QPD. We can back out the power on the QPDs as follows:

• 2¹⁶ = 65536 counts over a range of 40 V on the ADC
• 1 kΩ transimpedance on the QPDs
• 0.8 A/W for the QPDs

This gives a power of 4.6 µW on each QPD.