Exploring Macroscopic Quantum Mechanics with Gravitational-wave Detectors
[Haixing Miao is the recipient of the 2010 GWIC (Gravitational Wave International Committee) Thesis Prize for his PhD thesis “Exploring Macroscopic Quantum Mechanics in Optomechanical Devices" (PDF). -- 2Physics.com]
Author: Haixing Miao
Australian International Gravitational Research Centre (AIGRC), University of Western Australia, Perth, Australia;
Theoretical AstroPhysics Including Relativity (TAPIR), California Institute of Technology, Pasadena, USA.
Do macroscopic objects have wavy behaviors predicted by quantum mechanics, the same as microscopic atoms? Is there any boundary or transition between the quantum world and the classical world which we experience daily? Interestingly, advanced laser interferometer gravitational-wave (GW) detectors may give answers to these fundamental questions.
Fig. 1 LIGO detector at Livingston (left). It consists of a Michelson interferometer which uses a highpower laser to measure differential motions of input test masses (ITM) and end test masses (ETM) caused by gravitational waves.
It might seem unlikely, at least from the first sight, that a GW detector (e.g., LIGO detector  shown in Fig. 1) can study something quantum, given its apparent classical features: (i) using a high-power laser and kilogram-scale mirrors as test masses, (ii) having these test masses widely separated by kilometers, and (iii) operating at the room temperature. How can we probe delicate quantum mechanics with such a giant? The answer lies in the fact that, to detect weak GWs from the distant universe, the GW detector has to be extremely sensitive to the tiny displacement of the kilogram test mass, even sensitive enough to probe the quantum zero-point motion of macroscopic test masses.
2Physics articles by past winners of the GWIC Thesis Prize:
Holger J. Pletsch (2009): "Deepest All-Sky Surveys for Continuous Gravitational Waves"
Henning Vahlbruch (2008): "Squeezed Light – the first real application starts now"
Keisuke Goda (2007): "Beating the Quantum Limit in Gravitational Wave Detectors"
Yoichi Aso (2006): "Novel Low-Frequency Vibration Isolation Technique for Interferometric Gravitational Wave Detectors"
Rana Adhikari (2003-5)*: "Interferometric Detection of Gravitational Waves : 5 Needed Breakthroughs"
*Note, the gravitational wave thesis prize was started initially by LIGO as a biannual prize, limited to students of the LIGO Scientific Collaboration (LSC). The first award covered the period from 1 July 2003 to 30 June 2005. In 2006, the thesis prize was adopted by GWIC, renamed, converted to an annual prize, and opened to the broader international community.
Fig. 2 Plot showing the sensitivity of LIGO Hanford detector compared with the standard quantum limit (SQL) -- a benchmark for quantumness. This figure is adopted from Ref., which reports cooling of kilogram test masses down to an effective temperature of 1.4μK (Experiment is led by Nergis Mavalvala and Thomas Corbitt from MIT).
Indeed, with state-of-the-art technology, initial LIGO detector is only a factor of 10 away from the Standard Quantum Limit (SQL) that is imposed by the Heisenberg Uncertainty Principle  (illustrated in Fig. 2). Currently, in the GW community, significant efforts have been put into improving the detector sensitivity by reducing the classical thermal noises that cause random jittering of test masses. The future AdvLIGO  and other advanced GW detectors  under construction are anticipated to be operating at or beyond the SQL, with their sensitivities limited by noises that have purely quantum origin. To further increase the detector sensitivity, we need to manipulate the light at the quantum level, e.g. the use of quantum squeezed light [6-7]. Advanced GW detectors can be viewed as quantum devices, regardless of their bulky appearance.
Fig. 3 Figure showing schematically the creation of quantum superposition of macroscopic test masses by coherently amplifying the momentum of a single photon with advanced GW detectors. Please refer to Ref.  for more details of the experimental protocol.
With a sequence of studies [8-11], it is shown that, by using appropriate protocols, advanced GW detectors allow us to prepare kilogram test masses in different quantum states, and to study their quantum dynamics. For example, by superimposing a single photon―the light quantum―onto a strong light field in the GW detector, the momentum of the photon can be coherently amplified, and can even place the macroscopic test masses into a quantum superposition, as depicted schematically in Fig. 3. The GW detector, in some sense, acts as a “quantum amplifier”, and brings the quantumness of the microscopic photon into the macroscopic world.
Besides, if we simultaneously measure the common and differential motions of test masses, we can create Einstein-Podolsky-Rosen type quantum entanglement among widely separated test masses . Furthermore, we can study the dynamics of such a macroscopic quantum entanglement, which allows us to explore some interesting decoherence effects that could be unique to macroscopic objects .
Future advanced GW detectors can, therefore, not only detect tiny ripples in the spacetime and open up a new window into observing our universe, but also help us to gain deeper understanding of quantum behaviors of macroscopic objects, which might reveal exciting new phenomena.
 LIGO website: ligo.caltech.edu and a recent review article by the LIGO Scientific Collaboration (LSC), “LIGO: the Laser Interferometer Gravitational-wave Observatory”, Rep. Prog. Phys. 72, 076901 (2009). Abstract.
 LIGO Scientific Collaboration (LSC), “Observation of a kilogram-scale oscillator near its quantum ground state”, New J. Phys. 11 073032 (2009). Abstract.
 V. B. Braginsky and F. Y. Khalili, "Quantum Measurement", publisher: Cambridge
University Press (1992).
 Advanced LIGO website: advancedligo.mit.edu
 Advanced VIRGO website: cascina.virgo.infn.it/advirgo ; Large-scale Cryogenic Gravitational wave Telescope (LCGT) website: gw.icrr.u-tokyo.ac.jp/lcgt/
 H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R. Schnabel, “Observation of squeezed light with 10dB quantum noise reduction”, Phys. Rev. Lett. 100, 033602 (2008). Abstract.
 K. Goda, O. Miyakawa, E. E. Mikhailov, S. Saraf, R. Adhikari, K. McKenzie, R. Ward, S. Vass, A. J. Weinstein, and N. Mavalvala, Nature Physics 4, 472 (2008). Abstract.
 Helge Müller-Ebhardt, Henning Rehbein, Chao Li, Yasushi Mino, Kentaro Somiya, Roman Schnabel, Karsten Danzmann, and Yanbei Chen, “Quantum-state preparation and macroscopic entanglement in gravitational-wave detectors”, Phys. Rev. A 80, 043802 (2009). Abstract.
 Helge Müller-Ebhardt, Henning Rehbein, Roman Schnabel, Karsten Danzmann, and Yanbei Chen, “Entanglement of Macroscopic Test Masses and the Standard Quantum Limit in Laser Interferometry”, Phys. Rev. Lett. 100, 013601 (2008). Abstract.
 Farid Ya. Khalili, Stefan Danilishin, Haixing Miao, Helge Müller-Ebhardt, Huan Yang, and Yanbei Chen, “Preparing a Mechanical Oscillator in Non- Gaussian Quantum States”, Phys. Rev. Lett. 105, 070403 (2010). Abstract.
 Haixing Miao, Stefan Danilishin, Helge Müller-Ebhardt, Henning Rehbein, Kentaro Somiya, and Yanbei Chen, “Probing macroscopic quantum states with a sub-Heisenberg accuracy”, Phys. Rev. A 81, 012114 (2010). Abstract.
 Lajos Diósi, “A universal master equation for the gravitational violation of the quantum mechanics”, Phys. Lett. A 120, 377 (1987). Abstract ; Roger Penrose, The Road to Reality: A Complete Guide to the Laws of the Universe, Publisher: Alfred A. Knopf (2005).