## Question Regarding Homodyne detector

In the notebook MichToLigo, a homodyne detector was used to measure the quantum noise limited sensitivity. And that’s where squeezing was used to reduce the quantum noise. However, in the notebook MichelsonSensitivity, the command qnoised was used to measure the quantum noise limited sensitivity. Which one of these methods should we use to find the QNLS and how are they different?

## 2 comments on “Question Regarding Homodyne detector”

1. Anna Green says:

Hi Agnibha,

To measure a gravitational wave signal, we need to extract the signal sidebands by beating them with a reference field. Homodyne Readout and DC readout are two different methods that can be used to do this.

In MichelsonSensitivity, you might recall that the end mirrors M1 and M2 were both slightly detuned so that a small amout of light reached the dark port. This is ‘DC readout’, and that small amount of light is used as the reference. You then measured the quantum-noise-limited sensitivity using (qnoised output)/Hs.

In MichToLigo (and Modelling_Advanced_LIGO) we instead injected a local oscillator (LO) to use as the reference field, and measured the sensitivity using QN/TF. This is homodyne readout.

LIGO uses DC readout currently, and the file you have is configured to be used with DC readout (i.e. the arms are slightly detuned, and no LO is included). I’d start with this. You are welcome to try implementing homodyne readout later on (but remember to adjust the tunings accordingly).

One handy thing you might not have seen is the qnoisedS detector, which gives an output equivalent to (qnoised output)/Hs all in one go, so might save you some time.

Hope that helps!

2. Agnibha Banerjee says:

Okay thanks a lot for the clarification.