Squeezed Light – the first real application starts now

Fig. 1: Roman Schnabel at the squeezed light experiment.

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Authors: Roman Schnabel and Henning Vahlbruch

Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover and Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)

Not all light is the same. When looking at a light bulb, the light particles (photons) arrive at our eyes not in a well-organized stream, but in a chaotic fashion. Even in ultra-stable, single-color coherent laser beams, the photons are randomly distributed. This is demanded by the statistical nature of quantum physics. Similar to a rain shower, where many or just a few rain drops may hit the ground, sometimes a bunch of photons arrive, and sometimes just one. This fluctuation of the intensity, known as photon shot-noise, perturbs especially sensitive measurements.

However, quantum physics also allows for light with less (squeezed) photon noise. The very special property of such squeezed light is that the photons are not independent from each other but show quantum correlations. One way of interpreting squeezed light is the following. The occurrence of a photon, say on a photo-electric detector, is still unpredictable, because it obeys the statistical nature of quantum physics, but whenever a first photon has shown up, a second photon is guaranteed to follow a given time later. No wonder that the peculiar properties of squeezed light have been used to demonstrate quantum entanglement and teleportation [1].

A new world record in the strength of squeezing has been achieved this year by the team made up of Henning Vahlbruch, Roman Schnabel and co-workers at the Albert-Einstein-Institute (AEI) in Hanover/Germany. In their experiment an ultra-stable, infra-red laser beam was further refined by squeezing its photon shot-noise by a factor of ten (or 10 dB in the Decibel scale) [2]. The first squeezed light sources were demonstrated in 1985. However, after 20 years of intensive research, doubts arose whether strong squeezing could ever be realized as required for imminent applications. The new experiment at the AEI showed that strong squeezing of light’s quantum noise is possible. If a 10 dB squeezed laser beam replaces a standard coherent laser beam in an optical measurement device, its noise variance would decrease by a factor of 10. Similarly, the new world-record technology can be used to improve entanglement and teleportation experiments.

At the heart of the AEI squeezing experiment was an artificially grown birefringent crystal, that was precisely cut and polished. Two laser beams were sent through the crystal. A green laser beam of wavelength 532 nm caused the electron clouds of the crystal's atoms to oscillate with the frequency of the green light. In this state, the crystal could redistribute photons from a second, infrared beam that had exactly twice the wavelength (1064 nm). When the infrared beam sent a large bunch of photons through the crystal, the crystal stored those photons, and only returned them to the infrared beam when the photon flux became less. In this way, a more regular photon distribution for the infrared laser beam was achieved, and the photon noise of the infra-red beam became squeezed.

Figure 2 shows a photograph of the original optical components of the squeezed light source: a magnesium oxide doped lithium niobate crystal pumped with green laser light. The crystal had a length of 6.5 mm. The additional mirror was used to form a resonator together with one dielectrically coated crystal surface. In the real experiment the crystal was inside a temperature controlled housing and not visible.







Figure 3 shows the result of the successful squeezing experiment. When the green pump light was switched on, the photon noise of the infrared laser dropped by 10 dB to the lower noise level (red). The experimental techniques used for this result are independent of the absolute laser power. In principle any laser beam of arbitrary power could be realized with the same quantum noise reduction of a factor of 10.

Squeezed light offers a lot of fantastic applications in quantum communication and optical quantum computation. However in gravitational wave detection, squeezed light will find its first real application. Gravitational wave detectors are sophisticated laser interferometers which use cutting-edge technology in order to observe tiny changes in space-time that originate from distant black hole binaries or neutron star mergers. The circulating laser powers in these interferometers reach up to several kilowatts in order to reach high measurement sensitivities. A further increase of laser power can in principle further improve the detectors. But there may be a better way.

Imagine that you had a fixed amount of money, say a billion Euros, to build the most sensitive gravitational wave detector. Squeezed light would certainly be one of your key technologies. In order to reduce thermal motions inside your detector, you would use cryogenic techniques to cool it to liquid Helium temperature (-269°C). At the same time you would want to use high laser powers to boost its sensitivity to gravitational waves. You would find that the high laser power inside the detector would heat it up, such that any operation at low temperatures would become technologically extremely challenging, if not impossible. The 10 dB squeezed light technology can solve this problem. It ensures the same high sensitivity to gravitational waves for only one tenth the laser power, and your ultra-sensitive gravitational wave detector could be cost-effectively realized.

All prototype experiments for the application of squeezed light in gravitational wave detectors have been rather successful [3], and the implementation in the gravitational wave detector GEO600 [4] is currently in preparation. The routine use of squeezed light in GEO600 is envisaged for 2009.

References
[1] "Experimental investigation of continuous variable quantum transportation"
A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble, and E. S. Polzik, Science 282, 706 (1998); W. P. Bowen, N. Treps, B. C. Buchler, R. Schnabel, T. C. Ralph, H.-A. Bachor, T. Symul, and P. K. Lam,
Phys. Rev. A 67, 032302 (2003), Abstract Link, arXiv:quant-ph/0207179.
[2] "Observation of squeezed light with 10dB quantum noise reduction"
H. Vahlbruch, M. Mehmet, N. Lastzka, B. Hage, S. Chelkowski, A. Franzen, S. Gossler, K. Danzmann, and R. Schnabel,
Phys. Rev. Lett. 100, 033602 (2008), Abstract Link, arXiv:0706.1431.
[3] "Coherent control of vacuum squeezing in the gravitational-wave detection band"
H. Vahlbruch, S. Chelkowski, B. Hage, A. Franzen, K. Danzmann, and R. Schnabel,
Phys. Rev. Lett. 97, 011101 (2006), Abstract Link, arXiv:0707.0164.
[4] http://geo600.aei.mpg.de/

Milestones in a Continuing Tale of Big and Small : Large Magnitude Squeezed Light at 100 Hz, and a Squeezed 4km Gravitational-wave Detector

Sheon Chua

[Sheon Chua is the recipient of the 2013 GWIC (Gravitational Wave International Committee) Thesis Prize for his PhD thesis “Quantum Enhancement of a 4km Laser Interferometer Gravitational-Wave Detector” (PDF). -- 2Physics.com]


Author: Sheon Chua

Affiliation:

Currently at: Laboratoire Kastler Brossel, University of Pierre and Marie Curie (UPMC), Paris, France.

PhD research performed at: Centre for Gravitational Physics, Australian National University (ANU), Canberra, Australia.

Gravitational-wave sources of astronomical size from our Universe. Gravitational-wave displacement signals expected at one thousandth of the diameter of a single proton. Interferometric instruments with kilometre-long arms. Light ‘squeezed’ on the quantum scale.

The construction and implementation of second-generation laser-interferometric gravitational-wave detectors [1] are rapidly progressing [2], forming a detector network expected to be online over the next few years. These amazing instruments will use state-of-the-art isolation systems, optics, and hundred watt input lasers, and have kilometre-scale arms. For these detectors, the effect of a passing gravitational wave causes a relative displacement change between the interferometer arm end mirrors, which is encoded in the relative phase of the light beams propagating in the arms [3]. The relative displacement sensitivities will be of order of 10^-19 m in the 10 Hz to 10 kHz Fourier frequency band, achieved after monumental efforts in research and development across many fields of physics and engineering.

2Physics articles by past winners of the GWIC Thesis Prize:

Paul Fulda (2012): "Precision Interferometry in a New Shape: Higher-order Laguerre-Gauss Modes for Gravitational Wave Detection"
Rutger van Haasteren (2011): "Pulsar Timing Arrays: Gravitational-wave detectors as big as the Galaxy"
Haixing Miao (2010): "Exploring Macroscopic Quantum Mechanics with Gravitational-wave Detectors"
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.

However, even after these impressive technological efforts, there remain fundamental noise sources that limit measurement sensitivity that arise from the underlying physics of the instrument itself. One such noise source is the quantum nature of light [4], that comes from a non-zero commutation relationship between a light beam’s phase (ϕ) and amplitude (A) [5]. The Heisenberg Uncertainty Principle for this specific pair of quantities is given by ∆ϕ ∆A ≥ 1. Figure 1(a) shows this relation diagrammatically as a noise ‘ball’. As the gravitational-wave signal is encoded in the relative phase, with quantum phase noise Δϕ present we reach a signal-to-noise level where we can no longer distinguish a passing gravitational wave in the measurement. Therefore, the quantum noise of light is a limitation to achievable sensitivity.

However, the Heisenberg Uncertainty Principle relation is multiplicative. This means that one of the uncertainties can be below the quantum level, or ‘squeezed’, if the other uncertainty is above the level, or ‘antisqueezed’. This is illustrated in Figure 1(b), where the overall uncertainty is the same, but the individual uncertainties have been ‘rearranged’. The amount of squeezing has units of decibels [dB], referenced to the unsqueezed quantum noise level amplitude, given by [dB]=20 log₁₀[(anti)squeezed noise / unsqueezed noise].

Figure 1 (a) Quantum noise ‘ball’, showing the even distribution of uncertainty between the two quantities amplitude (A) and phase (ϕ). (b) Squeezed noise, where the uncertainty in one quantity is less than quantum noise, while the other uncertainty is greater than quantum noise.

As an example, if we have 6 dB of squeezing, we mean that the noise is squeezed to about half the value of the quantum noise level, or that the noise is reduced by a factor of 2. It follows that if we inject squeezed light into an interferometer so that it results in the phase uncertainty being reduced, the measurement sensitivity limited by quantum phase noise will be improved.

The tale of squeezed light for enhancing gravitational-wave detectors is now three decades young, with theoretical proposals for injecting squeezed light into interferometers published in the early 1980s [6], a few years before first experimental measurement of squeezing [7] took place. Since then, there has been a steady advancement in techniques and technologies to generate squeezed light within the 10 Hz to 10 kHz detection band [8-10], as well as to implement squeezed light with interferometers [11-14]. The GEO600 detector is now routinely using squeezed light, with ever-increasing timescales and duty cycles [15].

Figure 2: First measurement of greater than 10 dB squeezing across the audio gravitational-wave detection band, with 11.6 dB from 200 Hz and above. The degradation of squeezing level below 100 Hz is due to remaining residual classical noise entering the squeezing detector. Adapted from [16], and includes resolution bandwidth and window information.

The first milestone recently added to this story is the measurement of greater than 10 dB squeezing across the 10 Hz – 10 kHz frequency band [16]. This measurement was achieved by a team at the Australian National University, with valuable input from the Albert Einstein Institute. Figure 2 shows the result, with a maximum of 11.6 dB measured at 100 Hz and above. This was achieved after a detailed study characterizing and minimizing classical noise sources that impacted the squeezing measurement. This result represents the current record for squeezing in the 10 Hz – 10 kHz band, and further demonstrates the availability of large squeezing magnitude applicable for gravitational-wave detector enhancement.

The second milestone recently achieved is realising a squeezed 4 km interferometric gravitational-wave detector [17]. This was an experiment completed on the Enhanced LIGO 4 km interferometer in Washington State USA, performed by scientists from across the LIGO Scientific Collaboration, with LIGO Hanford Observatory, LIGO Massachusetts Institute of Technology, Australian National University and the Albert Einstein Institute being the lead institutions.

Figure 3: Enhanced LIGO interferometer with squeezing. (a) The Reference trace shows the displacement sensitivity of the interferometer without squeezing being injected, while the Squeezing trace shows the interferometer with squeezing injected. (b) Squeezing enhancement in LIGO’s most sensitive frequency band, at a lesser level due to significant contributions from noise sources other than quantum noise. Adapted from [17].

Figure 3(a) shows the interferometer displacement sensitivity curve with and without squeezed light. Up to 2.15 dB of squeezing enhancement is measured in the quantum noise limited regime (above 150 Hz). This is in line with the expected experiment parameters. Furthermore, as shown in Figure 3(b), in the most sensitive band between 150 Hz and 300 Hz, there is enhancement gained by squeezing. This result confirmed the compatibility of squeezing at lower detection frequencies where future gravitational-wave detectors will have their best sensitivity.

Squeezed light is a tool that is now available for, and being used for enhancing interferometric gravitational-wave detectors [15]. Third generation detector designs, such as the Einstein Telescope [18], have squeezed light injection as part of baseline technology. To realise maximum benefit from squeezed light injection, further improvements and refinements are needed, such as for improved parameters for squeezing injection and for minimizing adverse impacts on future detectors with more stringent requirements. This development work continues on as I write. It is safe to say that there are many more milestones to come in this continuing tale of big and small.

This article is a ‘synopsis’ of the squeezed light story and the two milestone results. For an in-depth review of squeezed light, squeezed light technologies and injection experiments up to 2013 (including both of these recent milestones), a Topical Review article is to be published soon [19]. I also recommend the LIGO Magazine, Issue 3 [20], which is focussed on squeezed light.

References:
[1] Advanced LIGO website: www.advancedligo.mit.edu ; Advanced Virgo website: wwwcascina.virgo.infn.it/advirgo ; KAGRA website: gwcenter.icrr.u-tokyo.ac.jp/en/; GEO600 website: www.geo600.org
[2] For example: www.advancedligo.mit.edu/adligo_news.html .
[3] For an expanded introduction to interferometric gravitational-wave detector measurement, I recommend this short video: www.youtube.com/watch?v=RzZgFKoIfQI .
[4] P.R. Saulson, "Fundamentals of interferometric gravitational wave detectors". World Scientific, Singapore (1994).
[5] D.F. Walls and G. Milburn, "Quantum Optics". Springer-Verlag, 2nd edition, Berlin (2008).
[6] Carlton M. Caves, "Quantum-mechanical noise in an interferometer". Physical Review D, 23, 1693 (1981) . Abstract.
[7] R.E. Slusher, L.W. Hollberg, B. Yurke, J.C. Mertz, J.F. Valley, "Observation of Squeezed States Generated by Four-Wave Mixing in an Optical Cavity", Physical Review Letters, 55, 2409 (1985). Abstract.
[8] Kirk McKenzie, Nicolai Grosse, Warwick P. Bowen, Stanley E. Whitcomb, Malcolm B. Gray, David E. McClelland, Ping Koy Lam, "Squeezing in the Audio Gravitational-Wave Detection Band". Physical Review Letters, 93, 161105 (2004). Abstract.
[9] Roman Schnabel and Henning Vahlbruch, "Squeezed Light – the first real application starts now". 2Physics : April 03, 2008.
[10] Tobias Eberle, Sebastian Steinlechner, Jöran Bauchrowitz, Vitus Händchen, Henning Vahlbruch, Moritz Mehmet, Helge Müller-Ebhardt, Roman Schnabel, "Quantum Enhancement of the Zero-Area Sagnac Interferometer Topology for Gravitational Wave Detection", Physical Review Letters, 104, 251102 (2010). Abstract.
[11] Kirk McKenzie, Daniel A. Shaddock, David E. McClelland, Ben C. Buchler, and Ping Koy Lam, "Experimental Demonstration of a Squeezing-Enhanced Power-Recycled Michelson Interferometer for Gravitational Wave Detection", Physical Review Letters, 88, 231102 (2002). Abstract.
[12] Henning Vahlbruch, Simon Chelkowski, Boris Hage, Alexander Franzen, Karsten Danzmann, Roman Schnabel, "Demonstration of a Squeezed-Light-Enhanced Power- and Signal-Recycled Michelson Interferometer", Physical Review Letters, 95 211102 (2005). Abstract.
[13] Keisuke Goda, Alan Weinstein, Nergis Mavalvala, "Beating the Quantum Limit in Gravitational Wave Detectors". 2Physics : May 10, 2008.
[14] Hartmut Grote, Roman Schnabel, Henning Vahlbruch, "A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit". 2Physics : September 25, 2011.
[15] H. Grote, K. Danzmann, K. L. Dooley, R. Schnabel, J. Slutsky, H. Vahlbruch, "First Long-term Application of Squeezed States of Light in a Gravitational-Wave Observatory". Physical Review Letters, 110, 181101 (2013). Abstract.
[16] M S Stefszky, C M Mow-Lowry, S S Y Chua, D A Shaddock, B C Buchler, H Vahlbruch, A Khalaidovski, R Schnabel, P K Lam, D E McClelland, "Balanced Homodyne Detection of Optical Quantum States at Audio-Band Frequencies and Below". Classical and Quantum Gravity, 29 145015 (2012). Abstract.
[17] The LIGO Scientific Collaboration, "Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light". Nature Photonics 7,  613 – 619 (2013). Abstract.
[18] Einstein Telescope: www.et-gw.eu .
[19] S. Chua et al, "Quantum Squeezed Light for Advanced Gravitational-wave Detectors". Classical and  Quantum Gravity Topical Review, accepted for publication (2014).
[20] LIGO Magazine, Issue 3: www.ligo.org/magazine/LIGO-magazine-issue-3.pdf .

A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit

[L to R] Hartmut Grote, Roman Schnabel, Henning Vahlbruch













Authors: Hartmut Grote, Roman Schnabel, Henning Vahlbruch

Affiliation: Institute for Gravitational Physics, Leibniz Universität Hannover and Max-Planck-Institute for Gravitational Physics (Albert-Einstein-Institute, AEI), Hannover, Germany

Quantum-squeezed light has now gone beyond the development-phase in the confinement of laboratory, and is now improving the sensitivity of the German/British gravitational-wave (GW) observatory GEO600 located close to Hannover (Germany). This is what our recent publication in Nature Physics reports [1]. Arguably -- for the first time -- a technology, which exploits the special findings of quantum mechanics, is put into a real application in metrology. The idea that squeezed states of light might be valuable in the field of GW detection is already 30 years old [2], but only now realized.

Past 2Physics article by this group:
April 03, 2008: "Squeezed Light – the first real application starts now"
by Roman Schnabel and Henning Vahlbruch

Gravitational waves are predicted by Einstein’s general theory of relativity, and are generated, for example, by black-hole binary systems. In principle, they can even be observed on earth by kilometer-scale Michelson-type laser interferometers measuring the changes in distance between mirrors suspended in vacuum. However, so far they have not been observed directly. Fig. 1 shows one of the suspended mirrors of the GW observatory GEO600 (mirror at bottom right). More details about GEO600 can be found in Ref. [3].

Fig. 1: One of four suspended 5 kg test-masses of the GEO600 gravitational wave detector (mirror at bottom right) together with reaction and suspension point masses. Changes in the 600 m distance between the test-masses are now measured with new quantum-squeezed laser light (image by courtesy of Harald Lück, AEI).

In the past, its measurement sensitivity at frequencies above several hundred hertz has been limited by the vacuum (zero-point) fluctuations of the electromagnetic field. Now, GEO600 incorporates an additional laser – a squeezed light laser – which has previously been presented in Ref. [4]. This new laser operates below its laser threshold and is based on parametric down-conversion of 532nm light thereby producing quantum correlated quasi-monochromatic photon-pairs at 1064 nm.































Fig. 2: View into the GEO600 central building. The squeezed-light laser is shown in the front. Its optical table is surrounded by several vacuum chambers containing suspended interferometer optics such as the mirror shown in Fig.1.

This rather dim laser mode is matched into the GEO600 laser interferometer where it interferes with the observatory’s ordinary 3 kW laser beam. As a result, the vacuum fluctuations at the photo-diode in GEO600’s output port are reduced (“squeezed”). GEO600 now operates with its best ever sensitivity being 50% higher than before at signal frequencies above 1 kHz, as shown in Fig. 3. Our success has finally proven the qualification of squeezed light as a key technology for future GW astronomy.





















 
Fig. 3: Nonclassical reduction of the GEO600 instrumental noise (calibrated to a space strain) using squeezed vacuum states of light. The black trace shows the observatory noise spectral density without the injection of squeezed light. An injection of squeezed vacuum states into the interferometer leads to a broadband noise reduction of up to 50% (3.5 dB in power, red trace). The peaks are not due to gravitational waves. They appear at well-known frequencies and are mainly due to violin modes of the mirror’s pendulum suspensions.

During the past years GEO600 has been made one of the most sensitive measuring devices ever built. Up to one hundred scientists from Germany, UK and other countries have contributed [5]. All of them are also members of the LIGO Scientific Collaboration (LSC) [6]. A number of new technologies have been developed, some of which are by now also implemented in the other gravitational wave observatories, namely the US LIGO and the Italian/French Virgo project. In this course of steady improvements, “classical” techniques have been driven to its extremes and GEO600 eventually became so sensitive that the squeezing technology became worth the effort.

Squeezed light has been generated in several research laboratories in the world before, however, it is a rather involved technique and leaving the conditions of a laboratory to an environment of continuous operation is difficult. All these aspects explain why only now the squeezed light is used for the first time in a GW observatory. In the past 5 years about a dozen of physicists have been working on the squeezed laser development in Hannover to enable this leap [7]. A recent review article summarizes the progress on squeezed light generation over the past years [8].

We are convinced that squeezed light will be used in all GW observatories around the globe in near future. The squeezing technology is certainly not exhausted yet. We believe that improvements of up to 200% are feasible with current technology.

References
[1] The LIGO Scientific Collaboration, "A gravitational wave observatory operating beyond the quantum shot-noise limit", Nature Physics, doi:10.1038/nphys2083 (Published online September 11, 2011). Abstract. Free Download.
[2] C. M. Caves, "Quantum-mechanical noise in an interferometer". Phys. Rev. D 23, 1693 (1981). Abstract.
[3] Willke, B. et al., "The GEO 600 gravitational wave detector", Class. Quantum Grav. 19, 1377 (2002). Abstract; Grote, H. et al., "The GEO 600 status", Class. Quantum Grav. 27, 084003 (2010). Abstract.
[4] H. Vahlbruch, A. Khalaidovski, N. Lastzka, C. Gräf, K. Danzmann, R. Schnabel, "The GEO600 squeezed light source", Class. Quantum Grav. 27, 084027 (2010). Article.
[5] The GEO600 team: www.geo600.org
[6] The LIGO scientific collaboration: www.ligo.org
[7] The Quantum Interferometry Group at the AEI: www.qi.aei-hannover.de
[8] R. Schnabel, N. Mavalvala, D. E. McClelland, P. K. Lam, "Quantum metrology for gravitational wave astronomy", Nature Communications, 1:121, doi: 10.1038/ncomms1122 (2010). Abstract.

Quantum Up-Conversion of Squeezed Vacuum States

From Left to Right: Christina E. Vollmer, Christoph Baune, and Aiko Samblowski

Authors: Christina E. Vollmer, Christoph Baune, Aiko Samblowski, Tobias Eberle, Vitus Händchen, Jaromír Fiurášek, Roman Schnabel

Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Hannover, Germany

Squeezed vacuum states of light belong to the special class of ‘nonclassical states’ that can not be fully described by either a classical or a semi-classical model. Overlapped with a (bright) coherent laser beam, they are able to reduce (to ‘squeeze’) the photon counting statistics, i.e. the light’s shot noise, thereby enhancing the sensitivity of optical measurement devices. There are applications in spectroscopy [1] and imaging [2] and in particular in interferometric length measurements in gravitational wave detectors [3-6], once the kick-off for research in squeezed states. With squeezed vacuum states it is also possible to teleport quantum states [7], to generate so-called Schrödinger kitten states [8], and to improve quantum cryptography [9].

Past 2Physics articles by this group:
September 25, 2011: "A Gravitational Wave Observatory Operating Beyond the Quantum Shot-Noise Limit" by Hartmut Grote, Roman Schnabel, Henning Vahlbruch.
April 03, 2008: "Squeezed Light – the first real application starts now" by Roman Schnabel and Henning Vahlbruch

Fig.1: Schematic of the experiment. A squeezed vacuum state at 1550 nm is overlapped with a bright pump field at 810 nm inside a periodically poled KTP crystal inside an optical resonator for sum-frequency generation. The output is a squeezed vacuum state at 532 nm.

In our recent work [10] we demonstrated for the first time the frequency up-conversion of squeezed vacuum states of light in an external setup, i.e. ‘on the fly’. Our scheme can be applied to quantum networks that first use a squeezing wavelength of 1550 nm for transmission through optical fibres and then use a shorter wavelength to meet the requirements of a quantum memory for storing the squeezed state. In our experiment we converted a 4dB squeezed state at 1550nm to a 1.5dB squeezed state at 532nm. The degradation was due to optical loss and in full agreement with our model.

With our experiment we also demonstrated a scheme that provides access to short squeezing wavelengths. Today, squeezed states are most efficiently produced at near-infrared wavelengths. Due to the lack of appropriate nonlinear media it is difficult to produce them with conventional techniques at visible or even ultra-violet wavelengths. In future work we plan to reduce the optical loss of our setup to be able to demonstrate strong squeezing at visible wavelengths.

Fig. 2: Photograph of parts of the experiment. In total, the experiment required five frequency conversion steps. First, a 1064 nm continuous-wave laser beam was frequency doubled. The produced 532 nm beam was used to generate two beams at 1550 nm and 810 nm via optical parametric oscillation. The 1550 nm light was frequency doubled and the generated 775 nm light used to pump a parametric down converter to produce squeezed vacuum states at 1550nm. The final step was the up-conversion as shown in Fig. 1.

References:
[1] E. Polzik, J. Carri, H. Kimble, “Spectroscopy with squeezed light”. Physical Review Letters, 68, 3020 (1992). Abstract.
[2] G. Brida, M. Genovese, I. Ruo Berchera, “Experimental realization of sub-shot-noise quantum imaging”. Nature Photonics 4, 227 (2010). Abstract.
[3] Carlton M. Caves, “Quantum-mechanical noise in an interferometer”. Physical Review D,  23, 1693 (1981). Abstract.
[4] Roman Schnabel, Nergis Mavalvala, David E. McClelland, Ping K. Lam, “Quantum metrology for gravitational wave astronomy”. Nature Communications, 1:121 (2010). Abstract.
[5] The LIGO Scientific Collaboration, “A gravitational wave observatory operating beyond the quantum shot-noise limit”. Nature Physics, 7, 962 (2011). Abstract. 2Physics Article.
[6] The LIGO Scientific Collaboration, “Enhanced sensitivity of the LIGO gravitational wave detector by using squeezed states of light”. Nature Photonics, 7, 613 (2013). Abstract.
[7] A. Furusawa, J. L. Sørensen, S. L. Braunstein, C. A. Fuchs, H. J. Kimble, E. S. Polzik, “Unconditional Quantum Teleportation,” Science 282, 706 (1998). Abstract.
[8] Alexei Ourjoumtsev, Rosa Tualle-Brouri, Julien Laurat, Philippe Grangier, “Generating Optical Schrödinger Kittens for Quantum Information Processing,” Science 312, 83 (2006). Abstract.
[9] Christian Weedbrook, Stefano Pirandola, Raúl García-Patrón, Nicolas J. Cerf, Timothy C. Ralph, Jeffrey H. Shapiro, Seth Lloyd, “Gaussian quantum information,” Review of Modern Physics, 84, 621 (2012). Abstract.
[10] C. E. Vollmer, C. Baune, A. Samblowski, T. Eberle, V. Händchen, J. Fiurášek, and R. Schnabel, “Quantum Up-Conversion of Squeezed Vacuum States from 1550 to 532 nm”, Physical Review Letters, 112, 073602 (2014). Abstract.

Beating the Quantum Limit in Gravitational Wave Detectors

- Manipulating quantum noise can significantly improve the sensitivity to small displacements in gravitational wave detectors -

Authors: Keisuke Goda¹, Alan Weinstein², Nergis Mavalvala¹
Affiliation:
¹LIGO Laboratory, Massachusetts Institute of Technology
²LIGO Laboratory, California Institute of Technology

Nergis Mavalvala and Keisuke Goda at MIT

[This is an invited article based on recent works of the authors. -- 2Physics.com]

Gravitational waves are ripples in the fabric of spacetime. Predicted by Albert Einstein in his general theory of relativity in 1916, gravitational waves are emitted by accelerating masses analogous to electromagnetic waves emitted by accelerating charges [1]. Although gravitational waves have not yet been detected, they have been indirectly shown to exist by Russell A. Hulse and Joseph H. Taylor Jr., who received 1993 Nobel Prize in physics. What makes direct detection difficult is that these waves are extremely faint due to their weak coupling with matter – even for violent astronomical events such as collisions of black holes and supernova explosions. Direct detection, therefore, requires instruments with unprecedented precision.

Laser-interferometric gravitational-wave detectors, such as those of the Laser Interferometer Gravitational-Wave Observatory (LIGO) [2], are designed to measure displacements of the order of 10^-18 m, or one-thousandth of the diameter of the proton, caused by passing gravitational waves that propagate from the distant universe. The LIGO detectors employ Michelson interferometry in which the difference in light travel time between the two arms of the interferometer is measured with high precision using controlled laser light.

But the story is not that simple. The quantum nature of photons prohibits us from increasing the detector sensitivity infinitely – Heisenberg’s uncertainty principle imposes a fundamental limit on the precision with which displacements caused by gravitational waves can be measured. The so-called quantum limit is set by the zero-point fluctuations of the light in the interferometer. Laser-interferometric gravitational-wave detectors such as LIGO are so sensitive that they have already confronted the quantum limit. In next generation detectors such as Advanced LIGO, optimization of classical parameters will reach the limits of conventional technology.

So, is this the end of the story? Fortunately, the answer is no. We haven’t used all the resources yet. The quantum limit can be overcome by use of non-classical or squeezed states of light [3] – states in which fluctuations are reduced below the symmetric quantum limit in one quadrature (like x) at the expense of increased fluctuations in the orthogonal quadrature (like p), while preserving the uncertainty principle (Δx Δp ≥ ћ/2). In the 2Physics article dated April 3, 2008, Roman Schnabel and Henning Vahlbruch reported their achievement of a world record in the strength of squeezing – more than 10 dB of squeezing for the first time.

In a recent paper entitled “A quantum-enhanced prototype gravitational-wave detector” [4], a team of LIGO scientists demonstrated improved sensitivity in a prototype gravitational wave detector at frequencies where the detector sensitivity was limited by photon shot noise, by injecting a squeezed state of light into the output port of the detector. The prototype detector consists of suspended quasi-free optics with a readout and control scheme similar to those used in the currently operational LIGO interferometers. In the demonstration, the team prepared a squeezed state with about 9 dB of sub-shot noise using a below-threshold optical parametric oscillator pumped by a powerful second-harmonic field [5], and injected it into the interferometer.

Figure 1 shows the 40m prototype interferometer at Caltech (left) and the squeezed light generator (right)








Figure 2 shows the noise floor of the prototype gravitational wave with a simulated gravitational wave signal at 50 kHz detector without (red) and with (blue) the injection of squeezed light. The sensitivity of the interferometer is limited by photon shot noise at frequencies above 42 kHz. The broadband shot noise floor was reduced by the injected squeezing while the strength of the simulated gravitational wave signal was intact, thereby improving the signal-to-noise ratio.

The result of the squeezing-enhancement in the prototype detector is shown in Figure 2. The comparison between the two spectra shows that the noise floor of the interferometer was reduced by the squeezed light injection at frequencies where the detector sensitivity was limited by shot noise. Figure 2 also shows the noise floor with a simulated gravitational wave signal at 50 kHz, with and without the injected squeezing. The broadband quantum noise floor was reduced by about 3 dB while the strength of the simulated gravitational wave signal was intact. This corresponds to a 44% increase in signal-to-noise ratio or detector sensitivity. In kilometer-scale gravitational wave detectors, this would correspond to a factor of 1.44³ ≈ 3.0 increase in detection rate for uniformly distributed gravitational wave sources such as coalescing neutron star binaries. This increase by a factor of 3 is significant because the occurrence of detectable gravitational waves is extremely rare – once every few years for binary neutron star coalescences at the present LIGO sensitivity.

This demonstration is an important step towards implementation of squeezing injection to improve the sensitivity of existing gravitational wave detectors worldwide. In fact, the installation of a squeezed source in a LIGO interferometer in the near future is under consideration, making gravitational wave detectors an important practical application of squeezed states of light. It is expected that within the next decade all interferometric gravitational wave detectors in the world will routinely use squeezed light.

References
[1] K.S. Thorne in “300 Years of Gravitation,” Cambridge Univ. Press, Cambridge (1987)
[2] “LIGO: The Laser Interferometer Gravitational-Wave Observatory,”
Alex Abramovici, William E. Althouse, Ronald W. P. Drever, Yekta Gürsel, Seiji Kawamura, Frederick J. Raab, David Shoemaker, Lisa Sievers, Robert E. Spero, Kip S. Thorne, Rochus E. Vogt, Rainer Weiss, Stanley E. Whitcomb, and Michael E. Zucker
Science 256, 325 (1992), Abstract Link.
[3] “Squeezed states of light,” D. F. Walls, Nature 306, 141 (1983), Abstract Link.
[4] “A quantum-enhanced prototype gravitational-wave detector,”
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, advance online publication, doi:10.1038/nphys920 (2008), Abstract Link.
[5] “Generation of a stable low-frequency squeezed vacuum field with periodically poled KTiOPO₄ at 1064 nm,” K. Goda, E. E. Mikhailov, O. Miyakawa, S. Saraf, S. Vass, A. J. Weinstein, and N. Mavalvala,
Optics Letters 33, 92 (2008), Abstract Link.

Tiniest Semiconductor Laser

Xiang Zhang [Photo courtesy: Roy Kaltschmidt/ Lawrence Berkeley National Laboratory]

In an advanced online publication of the journal Nature dated Aug. 30, a team of researchers from USA and China -- led by Xiang Zhang of University of California, Berkeley -- has reported the creation of the world's smallest semiconductor laser, capable of generating visible light in a space of only 5 nanometer -- smaller than a single protein molecule. The team not only successfully squeezed light into such a tight space, but found a novel way to keep that light energy from dissipating as it moved along, thereby achieving laser action. The research was performed at the NSF Nanoscale Science and Engineering Centre of University of California -- Berkeley, the Materials Sciences Division of Lawrence Berkeley National Laboratory, and the State Key Lab for Mesoscopic Physics and School of Physics of the Peking University -- China.

The achievement helps enable the development of such innovations as nanolasers that can probe, manipulate and characterize DNA molecules; optics-based telecommunications many times faster than current technology; and optical computing in which light replaces electronic circuitry with a corresponding leap in speed and processing power.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

While it is traditionally accepted that an electromagnetic wave - including laser light - cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals. This interaction between light and oscillating electrons is known as surface plasmons.

Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.

Recently, another team of researchers from Norfolk State University, Purdue University and Cornell University reported the creation of "spaser-based nanolasers" which were spheres 44 nanometers in diameter - more than 1 million could fit inside a red blood cell [Read 2Physics article dated August 22, 2009]. Those nanolasers are based on lasing action of gold spheres in a dye-filled, glasslike shell immersed in a solution. The dye coupled to the gold spheres could generate surface plasmons when exposed to light.

The UC Berkeley researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.

The Berkeley team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire - 1,000 times thinner than a human hair - with a silver surface separated by an insulating gap of only 5 nanometers, the size of a single protein molecule. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished. With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.

[Image courtesy of Xiang Zhang Lab/UC Berkeley] Left: Light being compressed and sustained in the 5 nanometer gap — smaller than a protein molecule — between a nanowire and underlying silver surface. Right: Electron microscope image of the hybrid design shown in the schematic.

"When you are working at such small scales, you do not have much space to play around with," said Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."

Trapping and sustaining light in radically tight quarters creates such extreme conditions that the very interaction of light and matter is strongly altered, the study authors explained. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; in this study, the researchers measured a six-fold increase in the spontaneous emission rate of light in a gap size of 5 nanometers.

"Plasmon lasers represent an exciting class of coherent light sources capable of extremely small confinement," said Zhang. "This work can bridge the worlds of electronics and optics at truly molecular length scales."

"What is particularly exciting about the plasmonic lasers we demonstrated here is that they are solid state and fully compatible with semiconductor manufacturing, so they can be electrically pumped and fully integrated at chip-scale," said Volker Sorger, a Ph.D. student in Zhang's lab and a co-lead author of the paper.

Scientists hope to eventually shrink light down to the size of an electron's wavelength, which is about a nanometer, or one-billionth of a meter, so that the two can work together on equal footing.

"The advantages of optics over electronics are multifold," added Thomas Zentgraf, a post-doctoral fellow in Zhang's lab and another co-lead author of the Nature paper. "For example, devices will be more power efficient at the same time they offer increased speed or bandwidth."

Reference
"Plasmon lasers at deep subwavelength scale"
Rupert F. Oulton, Volker J. Sorger, Thomas Zentgraf, Ren-Min Ma, Christopher Gladden, Lun Dai, Guy Bartal & Xiang Zhang,
Nature advance online publication 30 August 2009 doi:10.1038/nature08364; Abstract

[Our presentation of this work is based on a write-up by Sarah Yang of University of California, Berkeley]

'Quantum Cats' Made of Photons

NIST research associate Thomas Gerrits at the laser table used to create "quantum cats" made of photons [Photo courtesy: NIST, Boulder, CO, USA]

Researchers at the National Institute of Standards and Technology (NIST) have created "quantum cats" made of photons (particles of light), boosting prospects for manipulating light in new ways to enhance precision measurements as well as computing and communications based on quantum physics.

The NIST experiments, described in a forthcoming paper in Physical Review A, repeatedly produced light pulses that each possessed two exactly opposite properties—specifically, opposite phases, as if the peaks of the light waves were superimposed on the troughs. Physicists call this an optical Schrödinger's cat. NIST's quantum cat is the first to be made by detecting three photons at once and is one of the largest and most well-defined cat states ever made from light. (Larger cat states have been created in different systems by other research groups, including one at NIST.)

A "cat state" is a curiosity of the quantum world, where particles can exist in "superpositions" of two opposite properties simultaneously. Cat state is a reference to German physicist Erwin Schrödinger's famed 1935 theoretical notion of a cat that is both alive and dead simultaneously.

"This is a new state of light, predicted in quantum optics for a long time," says NIST research associate Thomas Gerrits, lead author of the paper. "The technologies that enable us to get these really good results are ultrafast lasers, knowledge of the type of light needed to create the cat state, and photon detectors that can actually count individual photons."

These colorized plots of electric field values indicate how closely the NIST "quantum cats" (left) compare with theoretical predictions for a cat state (right). The purple spots and alternating blue contrast regions in the center of the images indicate the light is in the appropriate quantum state [Image credit: Thomas Gerrits/NIST]

The NIST team created their optical cat state by using an ultrafast laser pulse to excite special crystals to create a form of light known as a squeezed vacuum, which contains only even numbers of photons. A specific number of photons were subtracted from the squeezed vacuum using a beam splitter. The photons were identified with a NIST sensor that efficiently detects and counts individual photons [2]. Depending on the number of subtracted photons, the remaining light is in a state that is a good approximation of a quantum cat says Gerrits—the best that can be achieved because nobody has been able to create a "real" one, by, for instance, the quantum equivalent to superimposing two weak laser beams with opposite phases.

NIST conducts research on novel states of light because they may enhance measurement techniques such as interferometry, used to measure distance based on the interference of two light beams. The research also may contribute to quantum computing—which may someday solve some problems that are intractable today—and quantum communications, the most secure method known for protecting the privacy of a communications channel. Larger quantum cats of light are needed for accurate information processing.

References
[1] T. Gerrits, S. Glancy, T. Clement , B. Calkins, A. Lita, A. Miller, A. Migdall, S.W. Nam, R. Mirin and E. Knill, "Generation of optical coherent state superpositions by number-resolved photon subtraction from squeezed vacuum", Physical Review A. Accepted for publication.
Abstract: We have created heralded coherent state superpositions (CSS), by subtracting up to three photons from a pulse of squeezed vacuum light. To produce such CSSs at a sufficient rate, we used our high-efficiency photon-number-resolving transition edge sensor to detect the subtracted photons. This is the first experiment enabled by and utilizing the full photon-number-resolving capabilities of this detector. The CSS produced by three-photon subtraction had a mean photon number of 2.75 (errorbar: -0.24+0.06) and a fidelity of 0.59 (errorbar: -0.14+0.04) with an ideal CSS. This confirms that subtracting more photons results in higher-amplitude CSSs.
[2] A. E. Lita, B. Calkins, L. A. Pellouchoud, A. J. Miller, S. Nam, "Superconducting transition-edge sensors optimized for high-efficiency photon-number resolving detectors", Proc. SPIE, Vol. 7681, 76810D (2010); doi:10.1117/12.852221. Abstract.

[We thank National Institute of Standards and Technology, Boulder, CO, USA for materials used in this posting]

Quantum Entangled Images

Paul Lett [photo courtesy: Joint Quantum Institute]

Conventional photographic films or digital camera sensors only record the color and intensity of a light wave striking their surfaces. A hologram additionally records a light wave’s “phase”—the precise locations of the crests and valleys in the wave. However, much more happens in a light wave. Even the most stable laser beam brightens and dims randomly over time because, as quantum mechanics has shown, light has inherent “uncertainties” in its features, manifested as moment-to-moment fluctuations in its properties. Controlling these fluctuations—which represent a sort of “noise”—can improve detection of faint objects, produce better amplified images, and allow workers to more accurately position laser beams.

Quantum mechanics has revealed light’s unavoidable noise, but it also provides subtle ways of reducing it to values lower than physicists once imagined possible. Researchers can’t completely eliminate the noise, but they can rearrange it to improve desired features in images. A quantum-mechanical technique called “squeezing” (read our past postings on squeezed light) lets physicists reduce noise in one property—such as intensity—at the expense of increasing the noise in a complementary property, such as phase. Modern physics not only enables useful noise reduction, but also opens new applications for images—such as transferring heaps of encrypted data protected by the laws of quantum mechanics and performing parallel processing of information for quantum computers.

In a recent communication published in Science Express, a team of researchers led by Paul Lett of the Joint Quantum Institute (JQI) of the Commerce Department’s National Institute of Standards and Technology (NIST) and the University of Maryland reported a convenient and flexible method for creating twin light beams to produce “quantum images,” pairs of information-rich visual patterns whose features are “entangled,” or inextricably linked by the laws of quantum physics. In addition to promising better detection of faint objects and improved amplification and positioning of light beams, the researchers’ technique for producing quantum images—unprecedented in its simplicity, versatility, and efficiency—may someday be useful for storing patterns of data in quantum computers and transmitting large amounts of highly secure encrypted information.

“Images have always been a preferred method of communication because they carry so much information in their details,” says Vincent Boyer, lead author of the new paper. “Up to now, however, cameras and other optical detectors have largely ignored a lot of useful information in images. By taking advantage of the quantum-mechanical aspects of images, we can improve applications ranging from taking pictures of hard-to-see objects to storing data in futuristic quantum computers.”

Perhaps most strikingly, the quantum images produced by these researchers are born in pairs. Transmitted by two light beams originating from the same point, the two images are like twins separated at birth. Look at one quantum image, and it displays random and unpredictable changes over time. Look at the other image, and it exhibits very similar random fluctuations at the same time, even if the two images are far apart and unable to transmit information to one another. They are “entangled”—their properties are linked in such a way that they exist as a unit rather than individually. Together, they are squeezed: Matching up both quantum images and subtracting their fluctuations, their noise is lower—and their information content potentially higher—than it is from any two classical images.

A laser beam (marked as “probe”) first passes through a mask that imprints a visual pattern. Along with a second laser beam (marked “pump”), it enters a cell containing a gas of rubidium atoms. Interactions between the rubidium gas and the beams produce an amplified version of the imprinted image as well as a second version of the image, rotated 180 degrees around the pump. The bottom panel shows, from left to right, an incoming probe beam imprinted with the letters “N” and “T,” an outgoing probe beam with an amplified image, and an upside-down version of the letters. The middle image is “entangled” with the rightmost image; the images’ changes over time are highly related to one another [Credit: Vincent Boyer et al., JQI]

To create quantum images, the researchers use a simple yet powerful method known as “four-wave mixing,” a technique in which incoming light waves enter a gas and interact to produce outgoing light waves. In the setup, a faint “probe” beam passes through a stencil-like “mask” with a visual pattern. Imprinted with an image, the probe beam joins an intense “pump” beam inside a cell of rubidium gas. The atoms of the gas interact with the light, absorbing energy and re-emitting an amplified version of the original image. In addition, a complementary second image is created by the light emitted by the atoms. To satisfy nature’s requirement for the set of outgoing light beams to have the same energy and momentum as the set of incoming light beams, the second image comes out as an inverted, upside-down copy of the first image, rotated by 180 degrees with respect to the pump beam and at a slightly different color.

In this photo montage of actual quantum images, two laser beams coming from the bright glare in the distance transmit images of a cat-like face at two slightly different frequencies (represented by the orange and the purple colors). The twisted lines indicate that the seemingly random changes or fluctuations that occur over time in any part of the orange image are strongly interconnected or “entangled” with the fluctuations of the corresponding part in the purple image. Though false color has been added to the cats’ faces, they are otherwise actual images obtained in the experiment. [Credit: Vincent Boyer/JQI]





Reference
"Entangled Images from Four-Wave Mixing" by V. Boyer, A. Marino, R. Pooser, and P. Lett,
Science Express, 12 June 2008, Abstract Link.

[We thank Media Relations, National Institute of Standards and Technology (NIST) for materials used in this posting. -- 2Physics.com]

Use of Squeezed Light to perform Distance Measurement below the Standard Quantum Limit

(from Left to Right) Nicolas Treps, Brahim Lamine and Claude Fabre

A team of researchers (B. Lamine, N. Treps and C. Fabre) from the Laboratoire Kastler Brossel (LKB) at the University Pierre and Marie Curie (Paris, France) have shown how to use squeezed light to perform distance measurement below the standard quantum limit imposed by the quantum nature of light [1].

Any distance measurement involves the propagation of light between two observers and the measurement of its phase (interferometric measurement, which gives distance within a wavelength) or its amplitude (time of flight measurement, giving absolute measurement). The quantum nature of light introduces fluctuations in the phase and the amplitude of the light used for ranging, therefore leading to a noisy measurement. The scientists have shown how to combine both a time of flight and a phase measurement, using frequency combs and homodyne detection, to minimize the effects of this quantum noise.

When classical light is used, then the sensitivity cannot go below what is called a standard quantum limit, which is smaller than previously existing standard quantum limits based either on interferometric or phase measurement. More interestingly, when squeezed frequency combs are used to perform the measurement, the sensitivity can significantly dive below the previous standard quantum limit. Squeezing light consists in tailoring its quantum fluctuations.

Ranging using frequency combs have already been proposed in the past [2] while it has long ago been realized that quantum resources is a way of improving ranging [3] (in particular entanglement and squeezing). Nevertheless the combination of both technology in an adapted optimal scheme is a major first.

Potential applications could be for future space-based experiments such as DARWIN (to detect Earth-like exoplanets) or LISA (to detect gravitational waves), and even for precise dispersion measurement. Indeed, when dispersion occurs, it does not affect in the same way the phase and the envelope --an effect which can be seen in the detection scheme proposed by the scientists.

References
[1] "Quantum Improvement of Time Transfer between Remote Clocks"
B.Lamine, C.Fabre and N. Treps,
Physical Review Letter 101, 123601 (2008). Abstract. [arXiv:0804.1203].
[2] "Absolute measurement of a long, arbitrary distance to less than an optical fringe",
J. Ye, Optics Letters 29, 1153 (2004). Abstract.
[3] "Quantum-enhanced positioning and clock synchronization",
V. Giovannetti, S. Lloyd, and L. Maccone, Nature 412, 417 (2001). Abstract.

Exploring Macroscopic Quantum Mechanics with Gravitational-wave Detectors

Haixing Miao

[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

Affiliation:
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 [1] 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.[2], 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 [3] (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 [4] and other advanced GW detectors [5] 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. [10] 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 [9]. 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 [12].

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.

References:
[1] 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.
[2] LIGO Scientific Collaboration (LSC), “Observation of a kilogram-scale oscillator near its quantum ground state”, New J. Phys. 11 073032 (2009). Abstract.
[3] V. B. Braginsky and F. Y. Khalili, "Quantum Measurement", publisher: Cambridge
University Press (1992).
[4] Advanced LIGO website: advancedligo.mit.edu
[5] Advanced VIRGO website: cascina.virgo.infn.it/advirgo ; Large-scale Cryogenic Gravitational wave Telescope (LCGT) website: gw.icrr.u-tokyo.ac.jp/lcgt/
[6] 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.
[7] 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.
[8] 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.
[9] 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.
[10] 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.
[11] 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.
[12] 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).