In the lab there is a laser that fires a pulse of light shorter than a billionth of a second. This pulse is no ordinary one: it is made of "squeezed vacuum," a quantum state in which the noise in one property of the light is suppressed while the noise in another is amplified. Such light carries up to ten trillion photons in a single mode, and its quantum nature shows up not in brightness but in a delicate superposition of possible fields. The matter the pulse hits must, within those instants, switch into a state that usually demands long and careful preparation.
For a long time it seemed that light alone was not enough: after the interaction, matter ends up in a classical mixture of states, as if the quantum patterns had been smeared out. The reason lies in light-matter entanglement: every possible outcome of the field leaves its own trace, and without extra information those traces add up to something average, stripped of quantum sharpness. But if, after the light has passed through, you perform a single measurement of its "quadrature" (that is, one component of the electric field), the picture changes. The measurement outcome acts as a filter that selects from the mixture only those states of matter consistent with the value you observed.
This selection is not arbitrary. For an ensemble of two-level systems (like atoms with two energy levels), it works as a Gaussian filter on the electric polarization: the state with zero polarization, known as the Dicke state, is selected most strongly. In it, all the particles collectively "look" in one direction, which yields a sensitivity to external influences that grows as the square of the number of particles. The quantum Fisher information, a measure of this sensitivity, reaches a value of N²/2+1, which indicates macroscopic entanglement.
Then the counter-rotating terms of the interaction come into play. They cause a stroboscopic transition: the Dicke state turns into a cat-like state, a superposition of two macroscopically distinct states, as in the famous Schrödinger's cat, except that instead of a cat being alive and dead, here there are two opposite directions of polarization. The preparation time is set by the inverse effective field strength, and under realistic parameters it drops into the attosecond range, that is, thousandths of a femtosecond.
Let us return to the laser in the lab. Its pulse is not just a bright flash but a tool that, in a single pass and a single measurement, carves the desired pattern out of quantum noise. That pattern can be used in sensors that detect changes in magnetic fields or gravity more precisely than the ordinary limit allows. And all of this happens faster than matter can lose its quantum coherence through interaction with its environment.
Bright squeezed vacuum can contain up to $10^{13}$ photons in a single mode, comparable to the number of stars in a small galaxy. Yet its quantum nature manifests in a superposition of coherent states, not merely in noise.
The idea of a macroscopic quantum superposition echoes Schrödinger's cat, and the creation of cat-like states of matter brings to mind stories about quantum computers, where such states are used for computation.