Comparison of LISA and Atom Interferometry for Gravitational Wave Astronomy in Space

1
Comparison of LISA and Atom Interferometry for
Gravitational Wave Astronomy in Space

• Peter L. Bender
• JILA, University of Colorado and NIST

46th RENCONTRES DE MORIOND Gravitational Waves
and Experimental Gravity March 20 27, 2011, La
Thuile, Valle dAosta, Italy
2
REFERENCES
Atomic gravitational wave interferometric
sensor Savas Dimopoulos,1, Peter W. Graham,2,
Jason M. Hogan,1, Mark A. Kasevich,1, and
Surjeet Rajendran PHYSICAL REVIEW D 78, 122002
(2008) Comment on Atomic gravitational wave
interferometric sensor Peter L. Bender PHYSICAL
REVIEW D, accepted for publication An Atomic
Gravitational Wave Interferometric Sensor in Low
Earth Orbit (AGIS-LEO) Jason M. Hogan, David M.
S. Johnson, Susannah Dickerson, et
al. arXiv1009.2702v1, 14 Sep 2010
3
Continuous Laser Beam
Pulsed Laser Beam
FIG. 11. The proposed setup for the AGIS-Sat. 3
experiment. Two satellites S1 and S2 house the
lasers and atom sources. The atoms are brought a
distance d 30 m or 130 m from the satellites at
the start of the interferometer sequence. The
dashed lines represent the 100 m paths traveled
by the atoms during the interferometer sequence.
4
(No Transcript)
5

• Parameters for Proposed AGIS- Satellite 2 GW
  Sensor
•  
• S. Dimopoulos et al., Phys. Rev. D 78, 122002
  (2008)
•  
• Parameters Satellite separation L 1 103
  km
• Atom cloud path length IL 200 m
• Atom cloud temperature ? 100 pK
• Atom thermal velocity V 1 10-4 m/s
• Separation of Raman pulses T 100 s
• Repetition rate 1/s
• Sensitivity, 0.004 to 0.5 Hz h 2
  10-20/vHz
•  
• Est. Parameters Atom cloud radius b 5 mm
• Laser beam radius a 500 mm

6
Assumed AGIS-Sat. 2 Mission Design

• The mission characteristics given in Table III of
  Dimopoulos et al. (2008) are assumed, where they
  differ from those given in the text.
• Confocal laser beams are transmitted between the
  telescopes, as recommended in the AGIS-LEO paper.
  The beams have to be apodized to reduce Fresnel
  ripples.
• Successive stimulated Raman transitions based on
  many added ? pulses are used to produce 400
  photon momentum splittings. This approach is
  discussed by McGuirk, Snadden and Kasevich in PRL
  85, 4498 (2000)

7
Overlooked Error Sources in the AGIS-Sat. 2 and 3
Proposals

• The laser beam from one end will be on
  continuously and serve as a phase reference.
  However, there will be some fluctuations in its
  wavefront aberrations over the 100 s intervals
  between the ?/2, ?, and ?/2 pulses from the other
  laser.
• These aberration fluctuations will have more
  effect on the laser phase seen by the near atom
  clouds than those for the far atom clouds because
  of the small atom cloud radius, the long laser
  path, and diffraction.
• The laser wavefront aberration fluctuations would
  have to be attenuated to a level of 210-8
  wavelengths in order to achieve the quoted
  gravitational wave sensitivity for the Sat. 2
  proposal, and 10 times better for Sat. 3.
• Because of the 100 s time between pulses for the
  Sat. 2 and 3 proposals, the atom cloud
  temperature would have to be stable to less than
  2 pK from cloud to cloud.

8
Specific Problems with the AGIS-Sat. 2 Proposal

• The authors say the paper gives The details of
  our proposal for an atomic gravitational wave
  interferometric sensor (AGIS). However, no
  sketch or description of what the satellites
  might look like has been given.
• There is no mention in the 2008 paper of needing
  mode-cleaner cavities after the lasers, even
  though large mode-cleaner cavities are included
  in discussions of ground-based laser
  gravitational wave detectors. A possible rough
  design for the mode-cleaner cavities is needed in
  order to permit consideration of the impact on
  the satellite design.
• A requirement on the temperature fluctuation
  between atom clouds is not given.
• It is stated that telescopes with about 1 meter
  diameter and 1 Watt of laser power would permit
  operation over 1,000 km baselines. However, such
  operation with 200 atom clouds in the
  interferometer at the same time, as specified,
  does not seem possible.

9
COMPLEXITY

• Even without much tighter requirements on
  wavefront aberration noise mitigation and cloud
  temperature fluctuations
• The proposed AGIS-Sat. 2 mission is far more
  complex than LISA!
• The statement that, compared with LISA, the
  proposed AGIS missions would be able to reach the
  suggested sensitivities with reduced engineering
  requirements is not supported by anything in the
  published paper.
• With the much tighter requirements
• It seems unlikely that the suggested sensitivity
  could be achieved in an affordable mission.

10

• Parameters for Proposed AGIS-LEO GW Sensor 
• J. Hogan et al., arXiv1009.2702v1, 14 Sept.
  2010
• Parameters Satellite separation L 30 km
• Atom cloud path length IL 15 m
• Separation of Raman pulses T 4 s
• Repetition rate 20/s
• Satellite altitude 1000 km
• Sensitivity, 0.07 to 10 Hz h 3
  10-19/vHz
•  
• Est. Parameters Atom cloud radius b 5 mm
• Laser beam radius a 150 mm

11
(No Transcript)
12
Issues Concerning the AGIS-LEO Proposal

• The laser wavefront aberration fluctuations would
  have to be attenuated to a level of 210-8
  wavelengths in order to achieve the quoted
  gravitational wave sensitivity.
• None of the gravitational wave sources shown in
  the sensitivity figure for AGIS-LEO have a
  reasonable probability of being observable during
  a mission lifetime.
• Operation at 1000 km altitude in Earth orbit
  appears to complicate the mission operations
  seriously.
• At 1000 km altitude, it does not appear possible
  to obtain any new information about
  time-variations in the Earths mass distribution.
• Possible operation only when in Earth shadow is
  suggested to avoid the need for large sunshields
  over the atom interferometers, but would
  substantially interfere with the scientific
  objectives.

13
Does the AGIS-LEO Proposal Discussion of
Wavefront Aberration Noise Mitigation Help the
AGIS-Sat. 2 Proposal?

• The need for A high-finesse mode-scrubbing
  optical cavity after the laser to reduce the
  laser wavefront aberration noise is recognized.
• However, no estimate of the amplitude of this
  noise is given, and the required performance
  level of the mode-cleaner cavities is not
  discussed.
• Use of an extra propagation segment to reduce
  wavefront aberration noise does not seem
  practical.
• Effects due to atom spatial distribution
  variations and atom velocity variations also are
  discussed. They are much less for AGIS-LEO than
  for AGIS-Sat. 2 because the pulse separation time
  T is 4 s rather than 100 s. But the suggested use
  of spatially resolved detection of the atoms in
  the cloud does not appear to help.

14
Summary

• The following two issues need to be addressed
• The wavefront aberration noise level from lasers
  with adequate output power levels
• The design and performance level of mode-cleaner
  cavities that can handle the required laser power
  levels.
• A modified version of the AGIS-Sat. 2 proposal
  with a reduced time T between pulses seems more
  realistic to pursue. However, it still would be
  much more complex than LISA.
• Studies of an AGIS-LEO mission appear to be
  considerably less attractive than studies of a
  modified AGIS-Sat. 2 mission. This is partly
  because the design problems for operation in
  Earth orbit are more severe, and partly because
  the science justification given so far appear to
  be very weak.