Technology Transfer Case Studies

 

Photo-thermal common-path interferometry

Technology type: High performance optics and optical metrology

Institution: Stanford University
Contact: Alexander Alexandrovski
Stanford Photo-Thermal Solutions
15-1598 6th Avenue, PO Box 493039, Keaau, HI 96749
(408) 898-0446
www.stan-pts.com
Supporting agency: National Science Foundation
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

The commercial use of nonlinear optical materials requires that properties other than nonlinear coefficients and birefringence be measured. These properties include absorption, surface and bulk damage limits, grey tracking and green induced infrared absorption. One of the most important properties is simple light absorption. In the early years of the field the absorption was large but as the materials improved and the absorption became smaller, more sensitive measurement techniques were needed. When Advanced LIGO began development in the late 90s, [ptcpi]it was clear that a close comparison between the properties of silica and sapphire for test mass substrates would require measurements techniques with much higher sensitivity. The absorption of the ion beam sputtered coatings in aLIGO would be in the sub-ppm region. Several different pump probe techniques were developed, but the best approach was the Photo-Thermal Common Path Interferometry (PTCPI) developed at Stanford University by Alexander Alexandrovski.

In PTCPI a small high-power pump laser beam is used to illuminate a sample to be measured, and a second larger low-power probe is centered on the first beam. The thermo-optic distortions due to the heating of the sample by the pump beam produce a small lens-like distortion on the probe beam phase front and CTCPI measures this distortion with dual-beam interferometric sensitivity. In response to a broad interest in this approach, a company was incorporated, Stanford Photo-Thermal Solutions (SPTS), to meet the needs of the optics community for industrial and research equipment for the measurement of surface and bulk absorptions, and also to provide services for the measurement of samples. Since its founding SPTS has had sold 30 complete measurement systems and has over 100 service customers in the optics and homeland security communities and is currently incorporated and operating in Hawaii.

 

Adaptive laser beam shaping

Technology type: Optical components

Institution: University of Florida Gainesville
Contact: David Tanner tanner(at)phys.ufl.edu, (352) 392-4718
Funding agency: National Science Foundation
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

Thermally-induced laser-beam wavefront distortions are typically caused when a high-power laser beam passes through an nominally transparent optical element. A small amount of residual optical absorption causes the optical element to be slightly heated where the beam intensity is highest. This heating produces wavefront errors through either thermo-elastic distortion of the element figure or thermo-optical change in the refractive index. The high laser power and extreme sensitivity to beam distortions has required the LIGO team to develop adaptive laser beam shaping techniques. We have developed an approach to correcting this distortion for our application in gravitational wave detectors by using a second transparent element and placing heaters on the edge to apply a compensating heat induced distortion, thus flattening the wavefront. By using several heating elements, more complicated aberrations can be compensated.

This technology can benefit the larger industrial and homeland security base. High-power laser systems are now used in a wide variety of applications, including directed laser energy weapons, laser radars, scientific and research applications of high power laser systems, welding, material cutting, and hole drilling. As the laser power continues to increase and new applications emerge, the need to control the beam wavefront will become an increasingly difficult challenge.

 

High power electro-optical modulator

Technology type: Optical components

Institution: University of Florida Gainesville
Contact: Volker Quetschke volker.quetschke(at)utb.edu, (956) 882-6723
Funding agency: National Science Foundation
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

Laser metrology and many other scientific applications of high-power laser beams require the phase modulation of the laser field. Previous commercial phase modulators were incapable of handling laser powers in excess of a few watts without beam heating resulting in thermo optic distortion of the laser beam. Our newly developed electro-optical modulators (EOM) can handle up to 200W of continuous laser power with no beam degradation. These high-power EOMs attracted such strong interest that they were initially prototyped by the New Focus company (now a division of Newport Corporation), and inspired their new high-power KTP modulator standard product line. Our new design was also proven to reduce the spurious polarization and amplitude modulation of phase modulators by several orders of magnitude while eliminating the very strict alignment tolerances of earlier modulators. This enables new applications in high resolution metrology, sensing, or ranging in many different environments. A schematic of the newly developed Advanced LIGO modulator is shown below to the right. The wedges in the crystal separate the two polarization components, which reduces the spurious amplitude and polarization modulation by orders of magnitude. The three electrodes shown allow modulation of the crystal with multiple frequencies simultaneously reducing the number of crystals required in multi frequency modulation applications.

 

Pound-Drever-Hall locking: A novel approach to very high performance stabilization of laser frequency

Technology type: Lasers

Authors: R.W.P. Drever, J.L. Hall, F.V. Kowalski, J. Hough, G.M. Ford, A.J. Munley and H. Ward
Contact: Prof. James Hough james.hough@glasgow.ac.uk, 0141 330 4701
Funding agencies: National Science Foundation, National Bureau of Standards, the Office of Naval Research, Science and Engineering Research Council (UK), University of Glasgow and Caltech
Technology source: Enabling technology from the pre-LIGO Lab era

Prior to the development of RF Reflection Locking, the use of very high-finesse optical cavities as stable references to which lasers could be locked in frequency was limited by the control bandwidth achievable, and hence by the resulting stability possible. Then a key development transformed the field. It was demonstrated that high-frequency phase modulation of the light directed toward the optical cavity, together with coherent detection of the light reflected from the optical cavity, resulted in the generation of a useful error signal over a wide detuning bandwidth around the cavity resonance. This signal also gave a proportional response to relative phase fluctuations above the cavity linewidth. It was found that these features could be effectively used to implement very wideband and high-gain servo control.

The promise of this approach was announced in the "Pound-Drever-Hall" publication (1) from the Joint Institute for Laboratory Astrophysics in the US (where the interest was mainly for precision measurements), and the University of Glasgow in Scotland (where interest was driven by the requirements of interferometric gravitational-wave detectors). For a decade the technique remained of largely academic interest. Then, numerous practical applications stimulated its adoption in a wide range of technical areas and markets.

The use of RF Reflection Locking -- or Pound-Drever-Hall (PDH) locking, as it has become known -- is now widespread in many fields, including spectroscopy, chip and reticule inspection, precision standards definition, optical frequency standards, space borne metrology applications, development and testing of ultra-low-loss mirrors, frequency reference cavities, fiber optic sensing and nonlinear laser frequency conversion.

The benefits from this research have influenced the growth of companies such as Toptica and Sacher Lasertechnik in Europe, and Advanced Thin Films and Vescent Photonics in the US; the development of a range of fast photo detectors and modulators from New Focus; advances in communication and metrology systems provided by precise frequency control of infrared fiber lasers by NP Photonics, and the generation of EUV light from modelocked visible lasers. Other developments not yet near-market have also been aided. Indeed the technique underpins almost all advances in frequency metrology, fundamental measurement, and any field in which lasers must be controlled in frequency, such as ultra-stable optical clocks, and dissemination of laser frequency standards.

References

Laser Phase and Frequency Stabilization using an Optical Resonator R.W.P. Drever, J.L. Hall, F.V. Kowalski, J. Hough, G.M. Ford, A.J. Munley and H. Ward Appl. Phys. B 31, 97-105 (1983)

 

Initial LIGO diode-pumped solid state laser

Technology type: Lasers

Institutions: Lightwave Electronics and LIGO Laboratory
Contact: Tom Kane, FASORtronics, PO Box 50370 LLC Albuquerque, NM 87181
www.fasortronics.com
Funding agency: National Science Foundation
Technology source: Initial LIGO

Lightwave Electronics (LWE) Corporation, of Mountain View CA, built the 10-Watt Diode Laser Pumped Solid State Lasers used in the Initial LIGO (iLIGO) Prestabilized Laser System (PSL). This project benefited LWE in three ways. First, it allowed LWE to make improvements to their laser system components product line. Second, it allowed LWE to hire a talented individual who has since become a top performer as both an engineer and a product marketer. Third, it created a small but profitable business, as LWE sold LIGO-design lasers to other customers.

The design for the iLIGO laser used a Master Oscillator Power Amplifier (MOPA) approach using as the Master Oscillator the LWE Non Planar Ring Oscillator (NPRO) invented by Tom Kane (shown in photo). The Power amplifier was based on technology already working in the LWE labs when Kane wrote a proposal to LIGO to build iLIGO lasers. LWE had invented and was working to improve a new pumping laser geometry for coupling the pump light from broad-stripe semiconductor lasers into cylindrical Nd doped YAG laser amplifier rods. This geometry, called "side pumping," uses a "light trap" to make certain the pump light from the semiconductor lasers is efficiently absorbed in the laser amplifier rod. This innovation led to the development of a standard LWE component called a "side-pumped laser engine." Under contract to the LIGO project, LWE refined the basic laser engine technology and designed supporting components, such as the current- and temperature-control systems needed to make complete laser systems using the technology.

LWE sold about a dozen lasers of the exact iLIGO design but more importantly the firm went on to develop a new product line based on the improved "side-pumped laser engine," which sold several thousand lasers. Its greatest success is in materials processing using pulsed ultraviolet solid state lasers. LED chips, Intel microprocessors, and iPhone printed circuit boards are examples of devices manufactured using this technology. Our newly-hired engineer saw great commercial promise in the materials processing market and was able to turn this iLIGO technology into a long term successful commercial venture. He further emerged as both a technical and a marketing leader, pushing LWE forward in laser materials processing. In 2005, LWE was purchased by JDS Uniphase, primarily to obtain the successful LWE business in laser materials processing.

 

Edge pumped zig-zag slab laser

Technology type: Lasers

Institution: Stanford University
Contact: Robert L. Byer 650.723.0226, rlbyer(at)stanford.edu
Funding agency: National Science Foundation and DARPA
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

The September 1999 LIGO Scientific Collaboration (LSC) White Paper (1) on detector research and development produced a strawman design for an interferometer that used a 180 watt near-infrared laser. This lead to a competition among laser groups within the LSC to produce a diode laser pumped solid state laser that could meet the stringent Advanced LIGO requirements. In particular a competition began between three LSC groups investigating end pumped rod lasers, unstable optical resonator and zig-zag slab laser geometries. The invention of the edge-pumped zig-zag slab laser (U.S. Patent 6,134,258) resulted from this research.

Improvements in the brightness of laser-diode pump sources, both fiber-coupled and bare arrays, have provided the ability to push the limits of high-power solid-state laser design. Stress-induced birefringence, thermal lensing, and the stress-fracture limit make scaling of rod lasers to high average powers difficult when good beam quality is required. Zigzag slab geometry lasers have been scaled to high average power levels while maintaining good beam quality and polarization contrast. However, practical use of slab lasers has been limited by the low laser efficiency that is typically seen in side-pumped slab lasers and by the complexity of the pumping and cooling through the same surface in the slab laser-head design. This zigzag slab laser design is based on conduction cooling and a novel pumping geometry called transverse or edge pumping. The edge-pumping geometry decouples the cooling and optical pumping interfaces, simplifying the laser-head design. The advantages of this design include efficient pump light absorption, acceptance of high-numerical-aperture pump sources, uniform conductive cooling, and scalability to higher power.

References

LSC White Paper on Detector Research and Development, E.K. Gustafson, D. Shoemaker, K. Strain and R. Weiss, https://dcc.ligo.org/LIGO-T990080/public/main

 

Manufacturable vacuum cable clamp

Technology type: Ultra-high vacuum components and techniques

Institution: LIGO Laboratory
Contact: Richard Abbott abbott_r(at)ligo.caltech.edu, 626.395.3449
Funding agency: National Science Foundation
Technology source: Initial LIGO

The most important inventions are often the simplest. Interferometric gravitational wave detectors are themselves complex scientific instruments. The in-vacuum optical systems (extremely high performance mirrors and beam-splitters) must be held free from all contamination (organic and particulate) while simultaneously being isolated from seismic noise. However, it is in addition necessary to apply electrical signals to control the active Internal Seismic Isolation (ISI) and passive Suspension System (SUS) and in addition command the optical components positions and pointing to maintain the alignment of the optical system. This requires the use of a large number of ultra high vacuum (UHV) compatible electrical cables to route signals from the control electronics outside the vacuum system through the vacuum envelope to the ISI, the SUS and the controlled optics. All of this must occur without compromising the ISI performance or the SUS. Enabling a convenient method of fixing the cables in a way compatible with the UHV cleanliness standards is the purpose of these clamps. Since many cables must be used with the SUS and the ISI systems, many clamps must be used. Therefore it is important that the cable clamps be UHV compatible and mass produced. The best high volume manufacturing technique is injection molding using UHV compatible plastics. The cable routing problem is complex and the available mounting surface area small, thus the clamps must be able to handle different numbers of cables. This is accomplished using two identical nesting parts that can accommodate a wide range of cable bundles.

 

EUCLID displacement sensor

Technology type: Sensor technology

Institution: University of Birmingham
Contact: Clive Speake 0121 414 4679
Funding agency: PPARC Innovative Technology Fund
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

Interferometers are excellent instruments for measuring displacements. In fact interferometric gravitational-wave detectors provide the best displacement sensitivity of any scientific instruments. However, there are three challenges to using simple interferometers as displacement sensors. First, they require that the mirror whose displacement is to be sensed must be aligned in pitch and yaw with respect to the light beam. Second, interferometers are often limited in their dynamic range. And third, the sensor must have very good internal mechanical and thermal stability consistent with the required displacement sensitivity. The Advanced LIGO suspension alignment sensors use shadow sensors. These are simple and sensitive at the nm level. The interferometric displacement sensor was developed at Birmingham University as part of the United Kingdom contribution to Advanced LIGO risk reduction. This work resulted in a patented invention (US2010/0238456 A1) that uses homodyne interferometry to overcome the dynamic range problem, a compact and very stiff mechanical design using low CTE material to achieve stability, and a clever optical design to achieve a tolerance against mirror pitch and yaw misalignment of more than 1 degree. In addition to providing risk reduction for Advanced LIGO the EUCLID sensor also provides a possible upgrade path to an improved suspension system beyond Advanced LIGO.

References

An Interferometer-based Optical Readout Scheme for the LISA Proof-Mass, Stuart Aston and Clive Speake AIP Conference Proceeding Nov. 26, 2006 Vol. 873, pp. 326-333
Optical Readout Techniques for the Control of Test-Masses in Gravitational Wave Observatories, Stuart Aston, https://dcc.ligo.org/LIGO-G1100826/public
Mirror Tilt Immunity Interferometry with a Cat's Eye Retroreflector, Peña-Arellano et al. Applied Optics Vol. 50, Issue 7, pp. 981-991 (2011)

 

Oxide bonding techniques for jointing silicon carbide

Technology type: Materials engineering

Inventors: Sheila Rowan, James Hough, Eoin John Eliffe
Institutions: Glasgow University and Stanford University
Patent application: US2007/0221326
Contact: Sheila Rowan, Sheila.Rowan(at)glasgow.ac.uk, 0141 330 4701
Supporting agencies: NSF and PIPSS
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

Jointing techniques with high mechanical strength and stability are required for use in the fabrication of optical systems used in space-based applications such as telescope assemblies and optical benches. The original technique of hydroxy-catalysis, or "silicate" bonding was invented and patented at Stanford University by Gwo (1) for the purpose of jointing the fused silica pieces forming the star-tracking telescope assembly used in the Gravity Probe B space experiment. LIGO has used a variant of this technique to fabricate the quasi-monolithic fused silica suspensions now being installed for Advanced LIGO and previously used in the GEO 600 gravitational wave detector. This technique was selected for use in ground-based gravitational wave detectors because of its high strength with very thin bonding layers combined with low mechanical loss (2), thus maintaining low thermal noise.

The original technology has been transferred to a number of optical vendors. Development of Oxide-bonding techniques was supported by a PIPSS technology transfer award to SpanOptic Ltd (UK), with a patent granted for bonding silicon carbide (US2007/0221326 A1). In addition, further contracts and partnerships have been established in this area with Gooch and Housego (UK), HighYaG (Germany) and Calyxo (USA).

References

Gwo, D-H2001 Ultra precision and reliable bonding method United States Patent Number US6284085B1
Mechanical Losses Associated with the Technique of Hydroxide-catalysis Bonding of Fused Silica, S. Rowan et al. Phys. Lett A, 246 (1998) pp471-478

 

Fast chirp transform

Technology type: Computation and time-series data analysis

Institution: California Institute of Technology
Inventor/Contact: Professor Thomas A. Prince, prince(at)caltech.edu, Caltech Office of Technology Licensing 626.395.3822
Patent Number: 6,509,866
Supporting agency: National Science Foundation
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

When two neutron stars combine into a single neutron star this process is described by three phases - the inspiral, merger and ringdown. In the initial inspiral stage of a binary neutron star coalescence the two stars spiral toward each other at a greater and greater angular velocity. During this inspiral phase the generated gravitational waves are expected to result in changes of gravitational strain with increasing amplitude and frequency resulting in a "up-chirp" signal. During the merger phase the prediction is much more difficult and currently rather uncertain but the signal properties are expected to be much more complex than a simple chirp. The final phase is called the ringdown phase in which the merged star undergoes damped body oscillations and is expected to create a gravitational wave characterized by a damped chirp signal where both the amplitude and frequency of the signal decrease in time which is called a "down-chirp". In general chirped signals are characterized by signals whose frequency change in time in a monotonic fashion. Chirped signals are not only seen in exotic systems like binary neutron star coalescences but are common in nature and engineering including: modern radar systems, sonar systems, femtosecond laser pulses and many natural physical phenomena, for instance those with dissipation.

As part of the LSC data analysis effort an algorithm was invented called the Fast Chirp Transform (FTC) to improve the detection and production of quasi-periodic signals. The FTC is a generalization of a multidimensional Fast Fourier Transform (FFT). Phase coefficients for boundary intervals are calculated using phase functions describing the time dependent frequency characteristic of an input signal in the time domain. A multidimensional FFT is performed on the dot product of the phase coefficients and the input signal in the frequency domain. The FCT eliminates the need for generating individual matched filters for each possible chirp waveform and allows a single algorithm to perform an optimal search over a very broad class of waveforms.

 

A new blind search method for analyzing the Fermi-LAT data for gamma-ray pulsars

Technology type: Computation and data analysis

Contact: Professor Bruce Allen, Albert-Einstein-Institut Hannover: Max-Planck-Institut für Gravitationsphysik und Institut füaut;r Gravitationsphysik, Leibniz Universitäaut;t Hannover, Callinstr. 38 D-30167 Hannover, Germany, +49-511-762-17148; Adjunct Professor, Physics Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
Working group: LIGO Scientific Collaboration Continuous Wave Search Group
Funding agencies: Max Planck Society and the National Science Foundation
Technology source: LIGO Scientific Collaboration (LSC) members outside of LIGO Laboratory

While technology transfer is often thought to flow mainly from the scientific to the commercial or defense realms, there are many instances when technology is transferred between scientific disciplines. This is one such example. Moreover it is a case in which the technology is based not on pieces of metal or glass, but on algorithms.

The LIGO Scientific Collaboration (LSC) data analysis working groups have, for over 10 years, been developing techniques to search through the data streams from the LIGO, VIRGO, and GEO interferometric gravitational-wave detectors. They are on the hunt for gravitational-wave signals from compact binary coalescences, exploding stars, the Big Bang, and spinning neutron stars, to name a few. These signals are rare, weak, and buried under the instrumental noise of the detectors, requiring sophisticated data analysis algorithms and pipelines to identify them.

LSC's Continuous Wave Search Group searches for coherent, continuous quasi-sinusoidal gravitational-wave signals generated by rapidly spinning neutron stars with slight asymmetries in their shapes. They have developed data analysis pipelines with particularly sophisticated procedures for efficiently searching a large set of data for weak signals from anyplace on the sky and with an unknown and continuously changing frequency.

In 2009 members of the group began to investigate the possibility of using some of their algorithms, pipelines and infrastructure (including the ATLAS computing system and Einstein@Home), to analyze data from other non-gravitational-wave astrophysical detectors. One of the first sources investigated was gamma-ray pulsars. These are incredibly dense neutron stars that produce short periodic pulses of gamma rays associated with the spin of the star. Even though the pulsars spin many times per second, space-based gamma ray telescopes only detect a gamma ray photon perhaps once in every 100,000 rotations. This may seem very different from the sinusoidal signal detection problem for continuous gravitational waves, but the timing of both signals are similarly affected by the Earth's orbital motion and spin, and by the gradual deceleration of the neutron star over time. Also in both cases years of data must be included to accrue sufficient signal strength to detect a weak signal over the detector noise. This makes both kinds of searches very sensitive to the location of the source on the sky; and if the source location and pulse period are unknown, both types of searches must be repeated for each sky location and period in a similar way. Computationally, this is a very challenging problem, and one of the innovations made by the LSC members was to devise strategies to maximize the sensitivity of a search given a fixed amount of computational resources.

Members of the LSC Continuous Wave Search Group have collaborated with scientists working on the space-based Fermi gamma ray telescope and with radio pulsar astronomers to apply the methods developed for gravitational wave detection to the detection of gamma-ray pulsars. They applied a slightly modified version of the method to data from the Fermi Large Area Telescope (LAT) and found signals for nine pulsars previously undetected in either gamma-rays or radio in those data. This increased the number of pulsars without radio counterparts discovered in the Fermi LAT data by more than a third, and further discoveries are expected.

References

Discovery of Nine Gamma-Ray Pulsars in Fermi Large Area Telescope Data Using a New Blind Search Method, H.J. Pletsch et al. The Astrophysics Journal 744:105, 2012 January 10
Exploiting Large-Scale Correlations to Detect Continuous Gravitational Waves, H.J. Pletsch and B. Allen Physics Review Letters 103, 181102 (2009)

 

Federated identity management for large distributed science communities

Technology type: Distributed computing

Architect: Scott Koranda
Contact: Scott Koranda scott.koranda@ligo.org 414.229.5056
Supporting agency: National Science Foundation
Technology source: Advanced LIGO

How many IDs and passwords do you need to remember? To buy an ebook, stream a movie, or download that latest app often means logging in with different IDs. Your bank, health insurance company, and physician offer the convenience of online services but each requires a separate ID, and smart online users create a unique ID/password combination for every site to protect themselves. The more you do online the more combinations there are to jot down on that sticky note pasted to your computer screen. Managing so many IDs and keeping your online accounts safe slows you down and is no fun.

Today's scientists and engineers face a similar challenge. Increasingly their research involves widespread collaborations. As they work with colleagues across disciplines and projects they typically access online resources both inside and outside their collaborations. All these services required the use of yet another ID and password, and scientists soon realized they were doing more systems administration than science. Each group hosting an online scientific tool or service also found itself expending valuable time resetting passwords and providing other support for users managing so many IDs.

The tools of "federated identity management" and "single sign-on" software like Shibboleth from the Internet2 project remove this burden and enable efficient online collaboration by allowing users a single ID and password to access resources both within and outside an organization.

Developed initially for use within higher education and universities, LIGO is leading the effort to bring the benefits of federated identity management to large scientific research organizations and projects. LIGO has joined the InCommon identity federation in the U.S. and is working to integrate with identity federations in Europe, Japan, Australia, and Canada. Integrating the LIGO computing infrastructure with these identity federations enables scientists who wish to collaborate with LIGO to use their existing login and password to access LIGO resources.

Enabling access by using a federated identity solves part of the problem. A larger challenge is to efficiently manage authorization to those web services and applications scientists need for productive collaborations. Equally important is to provide scientists the power to quickly create their own virtual groups and to manage user access to web pages, wikis, email lists, software repositories and other tools needed to support their projects. A few researchers joining to work together should be able with a few mouse clicks to create these and other necessary tools, and with a few clicks more to securely share their workspace with collaborators.

LIGO, together with Internet2, the iPlant Collaborative, and Project Bamboo and funded by the National Science Foundation, is building COmanage: a tool for collaborative organizations to manage effective and secure access to applications and services. Soon when LIGO and iPlant researchers collaborate with outside colleagues they will be able to create workspaces that include wikis, email lists, calendars, software repositories, and other applications. This means more freedom to share and collaborate without the intervention of system administrators. COmanage has been designed and is in development to support researchers from all disciplines and to foster more productive collaborations.

 

Interferometers as probes of Planckian quantum Ggometry

Technology type: Influences on other scientific disciplines

Originator: Craig J. Hogan
Contact: Sam Waldman sam.waldman(at)ligo.org
Institutions: University of Chicago and Fermilab
Supporting agency: U.S. Department of Energy
Technology source: Advanced LIGO

Theoretical studies of black hole thermodynamics have long suggested an underlying two-dimensional nature to our three-dimensional world. Recent studies in string theory suggest that gravity may exist in 11-space time dimensions, most of which exist at the Planck scale but some of which may be observable at macroscopic length scales. Moreover recent cosmological evidence points to a cosmological constant 120 orders of magnitude times smaller than predicted by a naive counting of the quantum mechanical vacuum. From this mix of puzzling observations, Hogan et al. (PRD 2008) conjectured that our three-dimensional world could be an emergent property of a Planck-scale space-time.

According to this conjecture, information about spatial coordinates is transmitted through time by a Planck wavelength carrier. The carrier encodes two coordinates of a test mass and as the test mass propagates through time and space, diffraction of the carrier's finite wavelength creates uncertainty in the coordinates. This effect is physically observable if two coordinates of the same test mass are measured at two different times by a photon. In the presence of the "holographic noise", the mass will appear to "jitter" in the plane of the two coordinates. The jitter is understood to appear as a random shift of the coordinates in the plane transverse to the carrier by a Planck length in a Planck time.

Gravitational wave interferometer technology could provide a means to confirm this conjecture. Considered in this framework, the beamsplitter of a Michelson interferometer is a test mass measured by photons at two times separated by the round trip time of the arms. The Michelson geometry measures the X and Y coordinates simultaneously. In the language of gravitational wave interferometry, the effective strain of holographic noise is given by the square root of the Planck time: h ≈ 10-22Hz-1/2

Since 2009, several members of the LIGO Laboratory have participated in the Fermilab Holometer, a purpose-built experiment designed to measure the holographic noise directly. [sw hqi]Based on an original design by LIGO's Prof. Rainer Weiss, the experiment correlates the output of two overlapping interferometers. If the holographic noise exists, the two interferometers' beamsplitters will appear to move in a correlated fashion as a result of the space-time uncertainty. Because the noise occurs at the Planck frequency, the noise signal will be broad band, extending up to the 4 MHz bandwidth of the interferometers. Crucially, the experiment includes a null configuration, in which the two interferometers remain co-located but are rotated 90 degrees with respect to each other, removing the holographic correlation.

The work of the gravitational wave community has enabled the Fermilab Holometer and its design is based directly on GEO 600 topology and the Caltech 40 m prototype interferometer construction. The Holometer project uses LIGO electronic circuit designs and has even borrowed initial LIGO optics including a pre-mode cleaner. In short, the Fermilab Holometer has substantially benefited from the direct contributions of gravitational wave community ideas, designs and experience.

References

Interferometers as Probes of Planckian Quantum Geometry FERMILAB-PUB-10-036-A-T
Indeterminacy of Holographic Quantum Geometry C.J. Hogan, Phys. Rev. D 78, 087501 (2008)