Squeezed States of Light
Squeezed light is best understood by considering complex phasors for the representation of the state of light in one mode of the optical field. Classically, such a state can be represented by a certain phasor (or its end point in the complex plane). According to quantum optics, however, there is a quantum uncertainty, and any measurement of the complex amplitude of the light field can deliver different values within an uncertainty region. Moreover, there is an uncertainty relation for the quadrature components of the light field, saying that the product of the uncertainties in both components is at least some quantity times Planck's constant <$h$>.
Glauber's coherent states have circularly symmetric uncertainty regions, so that the uncertainty relation dictates some minimum noise amplitudes e.g. for the amplitude and phase. The area of that uncertainty region is independent of the average amplitude, i.e., it can not be reduced by attenuating the light. A further reduction in, e.g., amplitude noise is possible only by “squeezing” the uncertainty region, reducing its width in the amplitude direction while increasing it in the orthogonal direction, so that the phase uncertainty is increased. Such light is called amplitude-squeezed (see Figure 1, left). Conversely, phase-squeezed light (Figure 1, middle) has decreased phase fluctuations at the expense of increased amplitude fluctuations.
Of course, there are also squeezed states where the orientation of the uncertainty region is different from the cases shown, or where the shape of the uncertainty region is different from that of an ellipse. For example, there are photon number squeezed states with a reduced uncertainty of the photon number but possibly complete phase uncertainty. (An extreme case are Fock states, having a certain photon number.) In any case, some noise component is below the standard quantum limit.
There is also the so-called squeezed vacuum (Figure 1, right), where the center of the uncertainty region (corresponding to the average amplitude) is at the origin of the coordinate system, and the fluctuations are reduced in some direction. The mean photon number is larger than zero in this case; a squeezed vacuum is a “vacuum” only in the sense that the average amplitude (but not the average photon number) is zero. Squeezed light with a non-zero average amplitude is also called bright squeezed light.
Quantum noise also leads to fluctuations of the polarization, which are reduced in polarization-squeezed light.
Generation of Squeezed Light
Squeezed light is usually generated from light in a coherent state or vacuum state by using certain optical nonlinear interactions. For example, an optical parametric amplifier with a vacuum input can generate a squeezed vacuum with a reduction in the noise of one quadrature components by the order of 10 dB. A lower degree of squeezing in bright amplitude-squeezed light can under some circumstances be obtained with frequency doubling. The Kerr nonlinearity in optical fibers also allows the generation of amplitude-squeezed light. Semiconductor lasers can generate amplitude-squeezed light when operated with a carefully stabilized pump current. Squeezing can also arise from atom-light interactions.
Another possibility is to use optomechanical squeezing [20, 26]. Here, fluctuations of the radiation pressure, which are associated with the intensity noise, modulate the path length of light in an optical resonator and thus induce correlations between the amplitude and phase noise.
In principle, squeezed light can be used in a number of areas, as it allows for measurements with reduced quantum noise. An example is the ultraprecise measurement of lengths for the detection of gravitational waves with large-scale interferometers. In particular, the advanced LIGO Hanford setup has been equipped with that technology, which contributed a substantial enhancement of measurement sensitivity  before the first detection in 2015 succeeded .
So far, the use of squeezed light is not very widespread, basically because it is plagued with various difficulties. For example, any optical losses bring a squeezed state of light closer to a coherent state, i.e. tend to destroy the nonclassical properties. At least in fundamental quantum optics research, however, squeezed states of light play an important role.
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