The Sub-SQL Interferometer

A view along the south arm. Image by S. Köhlenbeck

What is the Sub-SQL Interferometer?

The sub-SQL interferometer is the interferometer which is housed inside the vacuum system of the AEI 10 m prototype facility. It will be set up to mimic current gravitational wave detectors to provide an interferometer for prototyping gravitational wave detector techniques and technologies. Unlike a gravitational wave detector, which must maintain and maximise observation time, a dedicated prototype facility allows for rapid and regular testing of different technologies and techniques.

Although we are not aiming to measure gravitational waves, we will still measure the position of our mirrors with extremely high precision. A standard Michelson interferometer has a sensitivity limit governed by the Standard Quantum Limit (SQL). We are building our interferometer to be initially limited by the SQL. As the name implies, the sub-SQL interferometer will test techniques to overcome this limit.

Optical layout of the AEI sub-SQL interferometer. The laser beam enters the vacuum system at the central AEI-SAS via an optical fiber. Its spatial mode is cleaned by the pre-mode cleaner (PMC) (1). A part of the PMC output is split and sent to the intensity stabilization system (ISS) (2) to stabilize the laser intensity. The residual beam is split again, with one part entering the frequency reference cavity (3) to stabilize the laser frequency and the second part being steered to the south AEI-SAS, where it propagates through the input mode cleaner (IMC) (4) to clean the spatial mode of the laser beam further. Afterwards, the laser beam is steered into the main interferometer (5). The anti-symmetric port of the interferometer is propagated to the west AEI-SAS and coupled into the output mode cleaner (OMC) (6) to clean the spatial mode of the signal of interest. The OMC output is detected by a photodetector (7).

What is the SQL?

Unlike a photon or an electron, a macroscopic object is regarded as a classical object. The macroscopic object has been exposed in the messy environment and has lost the quantum property. However, if we can do the measurement with the precision at the level of the SQL, it will recover the quantum property. We will then be able to see the quantum behavior of the macroscopic object and may find out a fundamental difference of the quantum world and the classical world.

Another motivation is the development of so-called quantum non-demolition techniques (QND). The SQL is indeed a limit for a “standard” measurement, however, this is not a fundamental limit. There have been a number of ideas to overcome the limit. Once the sensitivity of the interfereomter reaches the SQL, we can experimentally demonstrate these ideas (hence the name ‘sub-SQL interferometer‘). Successful demonstration of QND techniques will pave the way for them be implemented in full scale gravitational wave detectors to improve their sensitivity. Our facility will therefore play an important role in research and development.

Is it possible to see the SQL?

Plot made with GWINC [https://git.ligo.org/gwinc/pygwinc]

Yes, indeed. A design noise budget is shown in the figure below which shows the dominant noise sources for the sub-SQL interferometer. The dashed red line is the sum of the classical and technical noise sources accounted for in the design of the sub-SQL interferometer. The solid black line shows the SQL with the quantum noise for an example input power of 5 W shown in dark blue.

The shaded region shows the difference between the SQL and the total classical and technical noise in a frequency region between 20 and 500 Hz. We aim to demonstrate QND techniques which can surpass the SQL in this region.

We expect various sources of technical noise to reduce this sub-SQL band which we will not be able to mitigate, for example: imperfect suspensions and stray light in the experiment. We initially target the more feasible goal of being limited by the SQL in the band between 100 and 500 Hz.

Current Status

The full interferometer is not yet installed in the vacuum system. We currently have a single arm cavity (the single arm test) installed with pilot optics to test and refine the method of controlling our 100 g suspended optics. The following sub-systems are installed and operational:

  • Three seismically isolated platforms (AEI-SAS), controlled with with both local inertial sensors and the suspension platform interferometer
  • The pre-stabilised laser system, including power and frequency stabilisation
  • The control and data system (CDS)

The path towards the sub-SQL interferometer includes:

  • Installing the interferometer beam splitter (suspension prepared, awaiting optic)
  • Preparing the quasi-monolithic suspensions for the low thermal noise 100 g mirrors
  • Installing the low thermal noise 100 g optics into the vacuum system
  • Building, testing and installation of an output mode cleaner and readout photodiodes

IFOCAD Model of the Interferometer

The video below gives a virtual tour of the inside of the vacuum system showing the envisioned core optical components and laser beam. The model was created by Philip Koch using the IFOCAD software developed by our Interferometery in Space colleagues at the AEI Hannover.