Quantum Noise Measurements at the ANU

1. Quantum Noise Measurements at the ANU Sheon Chua, Michael Stefszky,Conor Mow-Lowry, Sheila Dwyer, Ben Buchler, Ping Koy Lam, Daniel Shaddock, and David McClelland Centre for Gravitational Physics Australian National University

2. Homodyne Detection • Homodyne detectors work by comparing a weak signal beam with a strong local oscillator • The two beams are interfered on a beamsplitter and detected on two photodiodes • The subtraction of the diodes can give either the amplitude or the phase projection of the noise on the signal beam • The subtraction gives enormous common mode rejection • Uncorrelated technical noise masks the signal.

3. Homodyne Detection

4. Scatter • Small angle scatter which propagates in the (0,0) mode interferometrically couples in phase fluctuations from mirror motion and air currents • Depending on the location of the principal scattering sources, this can create uncorrelated intensity noise.

5. Scatter • By sweeping the phase of a parasitic interferometer with a PZT, the phase noise can be moved out of band. • This technique can be used to diagnose the presence of scattered light, and to shift it out of the measurement band.1 1 de Vine et. al., Phys. Rev. Lett., Accepted for publication (2010)

6. Scatter • A PZT was used to modulate the path length at two separate points of the apparatus at a variety of modulation frequencies and amplitudes. • In an effort to increase the effect, a scatter source was introduced. • In all cases, there was no evidence that a parasitic interferometer was present, neither in reduction of low frequency noise nor in the broadening of the modulation peak.

7. Dust • Dust moving through the beam after the beamsplitter causes non-stationary uncorrelated intensity fluctuations 1 • For the figure below, each diode had an equivalent of 6 Volts incident, with measured subtraction to 1 part in 1000 • The largest dust excursions result in worse than 1 part in 100 subtraction 1 Chua et al., J. Phys.: Conf. Ser. 122 012023 (2008)

8. Pointing • Experiments by McKenzie et al.1demonstrated coupling of pointing to homodyne readout • Confirmed in our apparatus by driving PZTs • Pointing noise generates uncorrelated noise on the two diodes due to detector inhomogeneities. • Even after sealing the chamber, the homodyne readout was very susceptible to anthropogenic noise. 1 McKenzie et al. Applied Optics 46 3389 (2007)

9. Pointing • After the homodyne chamber was sealed, noise slowly improved with time • Monday morning anthropogenic noise caused further large disturbances, exciting the spectrum (not shown) • No modecleaner installed, using AEI detector.

10. Modecleaner • One of the key improvements was placing a small, moderate finesse (~300) modecleaner inside our chamber. • The modecleaner converts uncorrelated pointing noise and mode shape disturbances into common intensity noise • This truly common noise is rejected by more than 60 dB, finally rendering the homodyne output resistant to anthropogenic noise

11. Electronic Noise • We investigated two couplings of electronic noise: • Additive dark noise, and • Non-linear electronic noise • One potential mechanism for non-linear noise is uncorrelated ‘gain noise’ which couples due to the large dynamic range required to see shot noise.

12. Non-linear electronic Noise Low-pass filtered DC voltage with huge (~80 dB) common mode rejection showed voltage dependent noise

13. Current Subtraction • It is possible to avoid gain noise by directly subtracting the diode photocurrent. • Both homodyne diodes are placed on the same circuit-board and subtracted before the transimpedance amplifier1: 1 Designed by the squeezing team at AEI Hannover

14. Shot Noise (I)

15. Shot Noise (II)

16. Conclusions • Isolation from the general lab environment was required to prevent dust and air current disturbances • Scatter and stray light did not cause an issue despite stock optics and imperfect cleanliness • Beam jitter was a strong source of noise mitigated by the introduction of a modecleaner inside a common chamber • Non-linear electronic noise was limiting performance in prior experiments, but is no longer an issue when using a current-subtraction detector.

17. Squeezing Proof of concept experiments have shown sensitivity improvements (ANU, MIT, AEI) GEO is also investigating the introduction of squeezed states currently

18. Squeezed Hanford 4km Project • Squeezing to be injected into Hanford 4km detector asymmetric port Faraday Isolator • Investigation into: • The Impact of the squeezer on LIGO operation • injection losses • The effect of scattered light from LIGO on the OPO • The effect on LIGO sensitivity (!) Coherent control of vacuum squeezing in the gravitational-wave detection band Vahlbruch et al. Phys. Rev. Lett. 97, 011101 (2006)

19. The LIGO Injection Test OPO PZT Actuator Squeezing Out Pump light In Crystal Oven/ Temperature Sensor

20. Squeezing June 2009

21. Improvements • New OPO constructed (Mk II) including new crystal • Further optimised locking loops • New homodyne detector installed (courtesy H. Vahlbruch, AEI) • Chamber used to isolate homodyne detector and modecleaner • Mitigation of scattered/ stray light with dichroics, dumps, and cleaning of optics

22. Improvements Homodyne isolation chamber:

23. Squeezing January 2010

24. Future Directions • Installation of new, high quality optics, including new crystals • ANU OPO delivered to MIT, awaiting installation and testing • Investigation of long term squeezing stability • Delivery of complete squeezing table from MIT to Hanford • Injection of squeezing into an operational gravitational wave detector.