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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 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. Homodyne Detection 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. 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) 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. 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) Pointing • Experiments by McKenzie et al.1 demonstrated 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) 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. 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 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. Non-linear electronic Noise Low-pass filtered DC voltage with huge (~80 dB) common mode rejection showed voltage dependent noise 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 Shot Noise (I) Shot Noise (II) 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. Squeezing Proof of concept experiments have shown sensitivity improvements (ANU, MIT, AEI) GEO is also investigating the introduction of squeezed states currently 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) The LIGO Injection Test OPO PZT Actuator Squeezing Out Pump light In Crystal Oven/ Temperature Sensor Squeezing June 2009 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 Improvements Homodyne isolation chamber: Squeezing January 2010 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.
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