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- What do you see when you turn out the light ? - I can't tell you but I know it's mine...

The quantum vacuum is one of the most surprising features of quantum electrodynamics (QED). According to QED, empty space, also called "vacuum," is not a calm and quiet place at all. Rather, it is full of particles and anti-particles popping into existence and then vanishing in an instant. It also contains electromagnetic fields buzzing at every imaginable frequency, including optical frequencies. Paradoxically, these these so-called "vacuum fluctuations" can't be seen, neither by a camera nor by your eyes. How do we know they really exist ?

A clever way to study vacuum fluctuations is with interferences. We use balanced homodyne detection, in which the vacuum fluctuations meet a strong laser beam at a beamsplitter. If the vacuum fluctuations are in phase with the laser, they will give constructive interference, if they are out of phase, destructive. This makes the vacuum fluctuations visible to ordinary detection. What we observe is noise, because the quantum fluctuations are changing so rapidly. Here is a record of the quantum fluctuations from our lab in 2008.

(:quicktime Attach:VacLPWave2.mov AUTOPLAY=FALSE :)

Vacuum fluctuations can also be manipulated. Manipulating a quantum property of the electromagnetic field is what we refer to as "squeezing" of light. By passing ordinary vacuum fluctuations through a phase-sensitive amplifier, we can amplify certain phases of the vacuum noise, while de-amplifying other phases. This is done in an experiment like this one

Optical Parametric Oscillator

It allows us to reduce the strength of the vacuum fluctuations. You can hear the result here:

(:quicktime Attach:SqLPWave.mov AUTOPLAY=FALSE :)

Squeezing allows us to manipulate something very fundamental, the quantum vacuum. Does it have any applications ? Yes !

For one thing, many of our best instruments are based on interference. This includes gravitational wave detectors, atomic clocks, atomic magnetometers, and many others. Because they are based on interference, they are sensitive to vacuum fluctuations. And squeezing is a way to reduce this source of noise, and to make these instruments more sensitive.

In a completely different area, squeezing is a fantastically-efficient way to produce entanglement of particles. With great difficulty, it is possible to entangle a handful of ions, by interacting them one-by-one. In contrast, a typical squeezing experiment produces (at least) billions of entangled photons per second.