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Concept
Squeezed coherent state
Summary
In physics, a squeezed coherent state is a quantum state that is usually described by two non-commuting observables having continuous spectra of eigenvalues. Examples are position and momentum of a particle, and the (dimension-less) electric field in the amplitude (phase 0) and in the mode (phase 90°) of a light wave (the wave's quadratures). The product of the standard deviations of two such operators obeys the uncertainty principle:
and , respectively.
Trivial examples, which are in fact not squeezed, are the ground state of the quantum harmonic oscillator and the family of coherent states . These states saturate the uncertainty above and have a symmetric distribution of the operator uncertainties with in "natural oscillator units" and . (In literature different normalizations for the quadrature amplitudes are used. Here we use the normalization for which the sum of the ground state variances of the quadrature amplitudes directly provide the zero point quantum number ).
The term squeezed state is actually used for states with a standard deviation below that of the ground state for one of the operators or for a linear combination of the two. The idea behind this is that the circle denoting the uncertainty of a coherent state in the quadrature phase space (see right) has been "squeezed" to an ellipse of the same area. Note that a squeezed state does not need to saturate the uncertainty principle.
Squeezed states of light were first produced in the mid 1980s. At that time, quantum noise squeezing by up to a factor of about 2 (3 dB) in variance was achieved, i.e. . As of 2017, squeeze factors larger than 10 (10 dB) have been directly observed.
The most general wave function that satisfies the identity above is the squeezed coherent state (we work in units with )
where are constants (a normalization constant, the center of the wavepacket, its width, and the expectation value of its momentum). The new feature relative to a coherent state is the free value of the width , which is the reason why the state is called "squeezed".
Official source
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