Chargeless, massless, yet undeniably there, light is one of the most fascinating and mysterious substances in the cosmos. Without it we could see nothing, but what do we see in it?  Philosophers and scientists have debated light’s nature for centuries. With the advent in the past century of quantum theory, we now know enough to start exploiting it. In ever more spheres – from communications to computing and cryptography – we are turning light’s astonishing and unusual properties to our advantage.
Squeezing light - the very idea seems nonsensical. How do you grab something as intangible as light and throttle it down to something smaller?
Understanding how this is possible goes to the heart of what we now know light to be. By the early 20th century, light had been successfully described as an oscillating wave, one that carries the electromagnetic force. That conclusion seems obvious when you see, for example, that light bends and spreads out, or diffracts, on meeting an obstacle, just as a water wave does.
  In 1921, however, Albert Einstein won a Nobel prize for explaining the emission of electrons from certain metals when they were illuminated - the photoelectric effect - by assuming light was made of particles. These tiny, massless packets of energy were later called photons. 
So which is it: wave or particle? The answer, courtesy of one of the weirdest of all quantum weirdnesses, wave-particle duality, seems to be that it is both. Light sometimes behaves as one, sometimes the other depending on the situation. 
Quantum theory also dictates that there are fundamental limits to what we can know about the world, an idea embodied by Heisenberg’s uncertainty principle. One consequence is that a light wave is not described by a smoothly oscillating sine curve. The photon side of the dual description brings a smudging both to the wave’s amplitude (the size of its oscillations) and its phase (their timing). 
Starting in the 1960s, physicist Roy Glauber of Harvard University whipped up these elements into a Nobel-prizewinning quantum theory of light. He derived the existence of an optimal “coherent” state of light with the least quantum smudging, and showed how you could squeeze down the uncertainty in either amplitude or phase by allowing that of the other to increase. 
Why do we care? Because signals made from less unruly, squeezed-down photons would be like radio signals with less static, making optical communication networks less error-prone. They would also be a boon to metrology, since the best way to measure distances with high precision is to compare them with the wavelength of a well-defined wave of light. 
The current record for photon squeezing is held by Roman Schnabel and his colleagues at the Max Planck Institute for Gravitational Physics in Hannover, Germany, with greater than 10-fold increases in the signal-to-noise ratio of their laser beams. That advance is now being used to upgrade the sensitivity of huge instruments called interferometers. These are looking for gravitational waves - ripples in space-time predicted by Einstein’s theory of general relativity. 
In facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is split between two sites in Louisiana and Washington state, and GEO600 near Hannover, laser beams ping back and forth over kilometres to detect the tiny displacements, of around 10-18 metres, that gravitational waves should cause in any matter they encounter - as yet without success. Squeezed light could make the decisive difference.

Chargeless, massless, yet undeniably there, light is one of the most fascinating and mysterious substances in the cosmos. Without it we could see nothing, but what do we see in it?

Philosophers and scientists have debated light’s nature for centuries. With the advent in the past century of quantum theory, we now know enough to start exploiting it. In ever more spheres – from communications to computing and cryptography – we are turning light’s astonishing and unusual properties to our advantage.

Squeezing light - the very idea seems nonsensical. How do you grab something as intangible as light and throttle it down to something smaller?

Understanding how this is possible goes to the heart of what we now know light to be. By the early 20th century, light had been successfully described as an oscillating wave, one that carries the electromagnetic force. That conclusion seems obvious when you see, for example, that light bends and spreads out, or diffracts, on meeting an obstacle, just as a water wave does.

  In 1921, however, Albert Einstein won a Nobel prize for explaining the emission of electrons from certain metals when they were illuminated - the photoelectric effect - by assuming light was made of particles. These tiny, massless packets of energy were later called photons. 

So which is it: wave or particle? The answer, courtesy of one of the weirdest of all quantum weirdnesses, wave-particle duality, seems to be that it is both. Light sometimes behaves as one, sometimes the other depending on the situation. 

Quantum theory also dictates that there are fundamental limits to what we can know about the world, an idea embodied by Heisenberg’s uncertainty principle. One consequence is that a light wave is not described by a smoothly oscillating sine curve. The photon side of the dual description brings a smudging both to the wave’s amplitude (the size of its oscillations) and its phase (their timing). 

Starting in the 1960s, physicist Roy Glauber of Harvard University whipped up these elements into a Nobel-prizewinning quantum theory of light. He derived the existence of an optimal “coherent” state of light with the least quantum smudging, and showed how you could squeeze down the uncertainty in either amplitude or phase by allowing that of the other to increase. 

Why do we care? Because signals made from less unruly, squeezed-down photons would be like radio signals with less static, making optical communication networks less error-prone. They would also be a boon to metrology, since the best way to measure distances with high precision is to compare them with the wavelength of a well-defined wave of light. 

The current record for photon squeezing is held by Roman Schnabel and his colleagues at the Max Planck Institute for Gravitational Physics in Hannover, Germany, with greater than 10-fold increases in the signal-to-noise ratio of their laser beams. That advance is now being used to upgrade the sensitivity of huge instruments called interferometers. These are looking for gravitational waves - ripples in space-time predicted by Einstein’s theory of general relativity. 

In facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), which is split between two sites in Louisiana and Washington state, and GEO600 near Hannover, laser beams ping back and forth over kilometres to detect the tiny displacements, of around 10-18 metres, that gravitational waves should cause in any matter they encounter - as yet without success. Squeezed light could make the decisive difference.


Posted 2 years ago with 222 notes
Tagged:lightphotonsphysicsquantum mechanics

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