Table of contents:
- Helpful Webpages
- Link to inclusion in public webpage:
- Speaker notes:
- Introduction slide / General speaker notes:
- Slide 1: Gravitational waves and LIGO
- Slide 2: LIGO and "squeezed light" (I)
- Slide 3: LIGO and "squeezed light" (II)
Helpful Webpages
Link to inclusion in public webpage:
Speaker notes:
Introduction slide / General speaker notes:
Synopsis of work:
Mavalvala is a key member is the Laser Interferometric Gravitational Wave Observatory (LIGO) collaboration. She began by developing a tabletop prototype of LIGO as a proof-of-concept, and contributed to both the design and creation of the detector. Mavalvala is an expert in precision measurement, required to measure the incredibly small perturbations caused in the interferometer by gravitational waves, and mainly works in the field of quantum optics. Laser-cooling techniques are used to optically cool and trap the massive interferometer pendulum mirrors, and has enabled observations of quantum phenomena in macroscopic objects. Mavalvala also works on the development of squeezed coherent states of light, which are injected into the LIGO detector to reduce quantum noise. Not only does Mavalvala's work allow for progress at LIGO, but is also pushing the bounds of experimental physics in many diverse applications.
Researcher's background:
Mavalvala was born and raised in Pakistan, and moved to the United States in 1986 to enroll at Wellesley College, where she obtained her bachelors degree in physics and astronomy. She then joined the group of Dr. Rainer Weiss at MIT to pursue a PhD, which she defended in 1997, and then moved to Caltech to work on the Laser Interferometric Gravitational Wave Observatory for her postdoc. Eventually she joined the Physics faculty at MIT in 2002, and was appointed Associate Department Head of Physics in 2015. In 2020, Mavalvala was named Dean of MIT's School of Science.
Mavalva describes herself as an "out, queer person of colour", and has two children with her wife in Cambridge, Massachusetts.
Societal relevance:
The detection of gravitational waves at LIGO confirmed a major prediction of Einstein's theory of general relativity, and was a historic milestone in both experimental and theoretical physics. Gravitational wave detections at LIGO are an entirely new way to observe the universe, and have provided insights into the darkest and most extreme objects in space – black holes.
Mavalvala's success in bringing massive objects into their ground states, and observing their quantum phenomena, has not only been pivotal for LIGO, but has laid the foundations for observing quantum behaviour in human-scale objects.
After the addition of the squeezed states of light in the LIGO detector, the frequency of discoveries increased from around one event a month to one a week, also enabling events at wider ranges of frequencies as well. As LIGO becomes more sensitive, events at higher redshifts will be able to be detected as well, allowing the detector to peer further and further back in time. Squeezed states of light can also be used in radiometry to calibrate the quantum efficiency of photo-electric photo detectors, as well as to produce Einstein-Podolsky-Rosen-entangled light (a resource for a high quality level of quantum key distribution, a secure communication method used to encrypt and decrypt messages).
General citations and resources:
https://en.wikipedia.org/wiki/Nergis_Mavalvala
https://www.sciencemag.org/careers/2012/06/gravitational-wave-researcher-succeeds-being-herself
https://en.wikipedia.org/wiki/Squeezed_states_of_light
https://en.wikipedia.org/wiki/Quantum_key_distribution
Slide 1: Gravitational waves and LIGO
Science details:
In 1916 Albert Einstein predicted the existence of gravitational waves in his general theory of relativity. Massive accelerating objects would disrupt space-time in such a way that 'waves' of undulating space-time would propagate in all directions away from the source. The ripples would theoretically travel at the speed of light, and carry information about both their origin and their travelling medium (space-time itself). There exist objects so massive that should they collide they would disrupt spacetime so violently that the ripples could be detected across the universe. These objects are neutron stars and black holes, the densest objects known. When these objects collide they first spiral inwards, producing characteristic gravitational waves prior to their final collision.
The Laser Interferometric Gravitational Wave Observatory (LIGO) is able to detect these collisions by detecting the oscillations of spacetime passing through the detector, causing the detector to also oscillate. The detector is an extremely large scale interferometer, sensitive to perturbations smaller than the width of an atom. The perpendicular 'L'-shaped 4km arms of the detector contain powerful lasers which are reflected back by perfect mirrors. The passing of gravitational waves causes one arm to differ in length compared to the other, causing interference patterns where the lasers meet which reveal minute details of the objects or phenomena causing the waves.
Citations and resources:
https://www.ligo.caltech.edu/page/ligos-ifo
https://www.ligo.caltech.edu/page/what-are-gw
Figures:
Top: An artist's impression of gravitational waves generated by binary neutron stars. https://www.ligo.caltech.edu/video/gravitational-waves
Bottom: Simplified diagram of how LIGO works, with a demonstration of interference caused by gravitational waves. https://spaceplace.nasa.gov/gravitational-waves/en/
Slide 2: LIGO and "squeezed light" (I)
Science details:
- sensitivity required for ligo
At its most sensitive, LIGO is able to detect a change in distance between its mirrors 1/10,000th the width of a proton. Measurements at this level of precision are possible thanks to the use of "squeezed light", which allows for more accurate measurements of the arrival time of incident photons on the mirrors.
The Heisenberg uncertainty principle restricts an observer from knowing the exact position and momentum of a particle simultaneously, and therefore there are unavoidable uncertainties in both the amplitude and phase of a photon. Interestingly, a state without the presence of any light, a perfect 'vacuum state', can be described by an electro-magnetic field which obeys the Heisenberg uncertainty principle in the same way. The uncertainty of the amplitude and phase of these 'zero point fluctuations' are distributed equally.
Through the use of a special crystal with particular non-linear optical properties, one can prepare a state of light where most of the uncertainty if concentrated only in one of the two variables (phase or amplitude) creating a "squeezed vacuum" with phase fluctuations smaller than a normal vacuum.
By reducing the quantum phase fluctuations of the background vacuum noise, fainter and more distant gravitational waves were uncovered that would otherwise be buried beneath this noise.
Citations and resources:
https://www.ligo.caltech.edu/news/ligo20191211b?highlight=squeezed light
https://www.ligo.org/science/Publication-SqueezedVacuum/index.php
https://news.mit.edu/2019/ligo-reach-quantum-noise-wave-1205
Figures:
Right: Video representing the size of the fluctuations detected at LIGO. This video zooms in on the proton of a hydrogen atom. https://www.ligo.caltech.edu/video/ligo20160211v8
Left: Quantum vacuum noise can be represented by sphere of uncertainty with two main axes. Squeezing this sphere like a stress-ball means constricting the sphere along one axis, while allowing it to expand along the other axis. When researchers direct a beam of light through a squeezing device, interactions between the laser and the quantum vacuum are facilitated in a way that rearranges the properties of phase versus amplitude, creating a squeezed vacuum.
Slide 3: LIGO and "squeezed light" (II)
Science details:
Shot noise, also known and Poisson or Photon noise, is a basic form of uncertainty associated with the measurement of light due to its quantized nature and the independence of photon detections. In order to make measurements below this inherent shot-noise, squeezed light must be used.
The squeezed light states are manipulated to have higher uncertainty in momentum (number) of photons in order to facilitate a lower uncertainty in position (timing) of the photons. This is done through the use of optomechanical systems made up of stationary and movable mirrors that allow scientists to establish a correlation between the two quantum properties.
At room temperature, the shot noise is overwhelmed by jitters caused by the thermal energy of the system, and so most squeezing has needed to be realized at extremely cold temperature (~10 Kelvin). Mavalva and her team have successfully created optomechanical squeezers that are able to operate at room temperature, making the system much more compact and portable.
The mirrors in this system were fabricated from crystals with very ordered atomic structures, meaning electrons in the material have less places to dissipate energy, unlike disordered materials where electrons often collide and generate thermal motion. These materials absorb and emit very little thermal energy, allowing for the system to operate at room temperature without thermal fluctuations overwhelming the measurements.
Citations and resources:
https://news.mit.edu/2020/quantum-noise-laser-precision-wave-detection-0707
https://arxiv.org/pdf/1812.09942.pdf
https://people.csail.mit.edu/hasinoff/pubs/hasinoff-photon-2012-preprint.pdf
Figures:
Left: Quantum vacuum noise can be represented by sphere of uncertainty with two main axes. Squeezing this sphere like a stress-ball means constricting the sphere along one axis, while allowing it to expand along the other axis. When researchers direct a beam of light through a squeezing device, interactions between the laser and the quantum vacuum are facilitated in a way that rearranges the properties of phase versus amplitude, creating a squeezed vacuum.
Right: A measured spectrum relative to measured shot noise of the squeezed light created by the system discussed, Aggarwal et al. The measured noise clearly dips below shot noise level between 30 and 60 kHz, with maximum squeezing of 0.7 dB at 45 kHz (a 15% reduction). The total budgeted noise refers to the quadrature sum of quantum noise, thermal noise, classical laser noise, cavity-feedback noise and differential phase noise. https://arxiv.org/pdf/1812.09942.pdf