1. Quantum noise reduction techniques for the Einstein telescope
Helge Müller-Ebhardt
on behalf of ET WG3
Max-Planck-Institut für Gravitationsphysik (AEI) and Leibniz Universität Hannover
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2. Requirements on the sensitivity for ET
a hundred times better sensitivity than initial detectors
significantly increase the detection band towards frequencies as low as a few Hz
highly broadband
extremely sensitive
1st generation
2nd generation
3rd generation 1 10
100
1000
10000
-25
-24
-23
-22
-21
-20
-19
f [Hz]
Noise Spectral Density
[Punturo, 2008]
Seismic
shot noise
thermal noise

3. ET’s classical noise budget
detector with 10 km long arms of few hundred kg test-mass mirrors
underground facility with long tunnel system and high caverns
cryogenic environment for test-masses and suspension system
up scaled super attenuator
Newtonian noise subtraction
[Hild, 2010]

4. Simple position meter: Michelson interferometer
detector with 10 km long arms of few hundred kg test-mass mirrors
power-recycling and arm cavities increase circulating optical power
finite arm-cavity bandwidth → shot noise rises at high frequencies
tuned signal-recycling → effective bandwidth
quantum noise touches SQL (depending on optical power)

5. Overview: quantum noise reduction options
optical-spring interferometer
speed-meter interferometer
optical inertia interferometer
optical transducer with local readout
(frequency-dependent) input-squeezing
interferometer
variational-output interferometer
single interferometer detector ↔ xylophone detector

6. QNR technique: optical-spring interferometer
Michelson interferometer with detuned signal-recycling cavity
optomechanical coupling induces a restoring force acting on test-masses: optical spring
mechanical resonance up-shifted into detection band
sensitivity enhanced around resonances
well-investigated in prototypes and table-top experiments

7. QNR technique: speed-meter interferometer
different optical realizations proposed: sloshing cavity, polarizing optics, Sagnac topology, long signal-recycling cavity, …
frequency-independent back-action noise at low frequencies is cancelled in output → shot noise limited
SQL beating depends on optical power / cavity bandwidth
combinable with frequency-dependent input squeezing
speed meter effect not yet experimentally observed
technical challenges: ring cavities,...
[Chen, 2003]
[Purdue, 2002]
[Danilishin, 2004]

8. QNR technique: input-squeezing
squeezed vacuum input at dark port increases sensitivity
optimal frequency-dependent squeezing ellipse
→ overall sensitivity enhancement
squeezed field reflected at filter cavities realizes frequency dependency
2 filter cavities required for e.g. optical-spring interferometer
optical loss in filter cavities degenerates squeezing
→ sets requirements on filter cavities
well-investigated technique

9. QNR technique: variational output
at every frequency there exists optimal readout quadrature
output field reflected at filter cavities → frequency-dependent readout quadrature
2 filter cavities required for e.g. optical-spring interferometer
quadrature without signal content in output
→ optical loss increases noise and decreases signal
highly susceptible to optical loss
technique less investigated

10. Single interferometer detector: Michelson
frequency-dependent input-squeezing
300 ppm loss in two 10 km (1 km) long filter cavities
circulating optical power 3 MW
arm length 10 km
test-mass mirror weight 120 kg
variational output
20 ppm loss in two 10 km (1 km) long filter cavities

11. Single interferometer detector: Sagnac
frequency-dependent input-squeezing
300 ppm loss in two 10 km (1 km) long filter cavities
circulating optical power 3 MW
arm length 10 km
test-mass mirror weight 120 kg
variational output
20 ppm loss in two 10 km (1 km) long filter cavities

12. Two-band xylophone detector
hard to realize broadband single interferometer detector
- in terms of quantum noise, especially optical loss
- in terms of technical noise even more
split detection band into LF interferometer and HF interferometer
LF interferometer:
- up scaled advanced detector
- cold
- optical-spring
HF interferometer:
- up scaled
initial detector
- room-temperate
- high-power
[Hild, 2010]

13. LF interferometer: filter cavities vs. Sagnac
Sagnac interferometer with frequency-independent input squeezing
- no filter cavities
- need ring cavities in the arms
- moderate test-mass weight possible
- moderate circulating optical power (180 kW)
optical-spring interferometer
with frequency-dependent
input squeezing
- need filter cavities
- heavy test-mass mirrors
- low circulating
optical power (18 kW)
ET-LF option