Gravitational Quantum Sensors


LIGO and Virgo, the joint gravitational wave consortia and related detector complexes, based, respectively, in the USA (Washington State and Louisiana) and in Europe (Italy), can now measure more subtle gravitational episodes thanks to a quantum optics improvement in their devices.

A gravitational wave detector is a huge interferometer (stretching along a few kilometers), not much dissimilar to the table-top models used in an undergraduate physics laboratory class. An interferometer is a device that measures interference fringes. It is easy to picture interference in matter waves even in nature, such as looking at sea waves or the ripples of a stone thrown in a pond. For light, which is an electromagnetic wave, this is less intuitive, and it is easier to witness interference with the aid of modern devices, such as lasers. In Virgo, a laser beam is split into two, and the two resulting laser rays take different paths, along the two “arms” of the interferometer. They bump onto two mirrors, and on the way back they interfere with each other. Like giant lightsabers, although the beams seamlessly cross each other, apparently with no change, but actually interfering in phase due to the different paths each beam has taken.

In gravitational interferometers, the change in length of one of the arms is due to a modification of spacetime induced by a gravitational wave. Distant events such as the collapse of two neutron stars, or black holes, reverberate in space with ripples in spacetime. This change in length is very tiny, and for this reason a large interferometer is needed. In the future, even larger interferometers will be sent to space. Several gravitational wave detectors allow to pinpoint the target “event” in space, with triangulation. The blips in LIGO have already been used to check whether these events are accompanied with detectable light emission, opening the era of “multi-messenger astronomy”.

Quantum technology enters the picture twice through quantum optics. Classical optics studies the properties of light, and its characteristics when many photons are present. Quantum optics studies the properties of light at the single (or few) particle level. This description of light-matter interaction led Einstein to predict the mechanisms that is behind the laser itself, including the lasers used in LIGO and Virgo. Moreover, now another quantum mechanical effect, called “light squeezing”, will be used to enhance the sensitivities of these gravitational detectors.

Quantum squeezing is a macroscopic effect that allows to measure with more precision than otherwise possible a given quantity, such as the position of an object. Squeezing is derived from Heisenberg’s uncertainty principle, which states that the uncertainty in the measurement of two joint observables cannot be reduced to less than a given constant, set forth by the fabric of quantum mechanics. The trick used in quantum squeezing devices comes from the observation that while Heisenberg’s limit certifies the limit for the joint measurement sensitivities (say, position and momentum), it does not provide a constraint on either one of them can become more sensitive, at the expenses of the other one, for which the uncertainty grows proportionally.

Quantum squeezing has been mastered in table-top experiments over several decades now in research laboratories and generally relies on some non-linear optical process to occur, such as by shining laser light through a special type of crystal, whose anisotropic structure can affect the properties of light. The and it is now being injected in the laser beams of the LIGO device to reduce the uncertainty in the displacement of the mirrors produced by gravitational waves. Link

Both LIGO and Virgo reported improving their sensitivity by exploiting squeezed light produced by an optical parametric oscillator. Entangled photons were generated by this device to go beyond the so-called “standard quantum limit”. The idea is that, by injecting correlated noise in the interferometer, one can reduce the otherwise uncorrelated noise of the apparatus, given both by shot noise, arising from quantum fluctuations of the electromagnetic field (usually uncorrelated), and the noise given by the radiation pressure of light. Link

This enhancement in the gravitational setup is an application of quantum technology to sensing and metrology (which provides more precise standards for measurements). According to the EU Quantum Flagship, the open consortium coordinating the European research efforts in the forthcoming years, sensing is one of the pillars of application of quantum technology together with quantum communication, quantum computing, and the quantum simulation of physical systems. While quantum-technology applications are underway, we can already witness the deployment of quantum squeezing to the benefit of scientific discovery: expect quantum-resolution limited detection of astrophysical events, such as black hole dynamics, neutron star mergers and more, in the upcoming months and years, thanks to quantum-technology-powered gravitational-wave detectors. Link


This is a collection of my articles on quantum technology, part of my Quantum Tech Newsletter. You can read the original posts also on Medium:

  1. Gravitational Quantum Sensors
  2. Quantum Advantage
  3. Analog Computing
  4. Quantum Internet
  5. Quantum Games
  6. Open-Source Quantum Tech
  7. Quantum Machine Learning
  8. Space Quantum Communication

© Nathan Shammah — 2017 and beyond.

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