Homodyne Measurements of Shot Noise

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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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