Have you ever tried to throw a die by each hand at once?

For usual dice, with numbers from 1 to 6, the probability to get the numbers equal will be 1/6. For more advanced dice, icosahedron-shaped, it will be 1/20.

A single photon is similar to a die before it is thrown: it is intrinsically probabilistic. Before detection, it has no definite position, or momentum, or polarization. And still, photon pairs can show perfect correlations, as if two dice showed the same numbers every time they were thrown together. This effect - entanglement - has important applications in metrology, imaging, and information transmission. BRISQ2 aims at making bright beams behave in the same way, with many interesting consequences.

BRISQ2 project

Although almost no one doubts nowadays the validity of quantum mechanics, a widely accepted viewpoint is that only microscopic objects behave in a truly quantum way. Counter-examples to this opinion are recent experiments with large atomic clouds, Bose condensates, superconducting circuits and some other material systems. Surprisingly, for the case of light there have been only few attempts to observe quantum effects on a macroscopic scale.

 

 

One of them addresses bright squeezed vacuum, a state that emerges at the output of an unseeded parametric amplifier. It is called vacuum because it has zero mean values of electric and magnetic fields or, in other words, it contains only fluctuations. However, these fluctuations amount to macroscopic numbers of photons and are therefore bright. More importantly, bright squeezed vacuum consists of light beams with perfect correlations in photon numbers (photon-number squeezing). For instance, it can involve vertically and horizontally polarized light beams with photon numbers hugely fluctuating but equal to each other up to a single photon.

BRISQ2 aims at investigating the properties of bright squeezed vacuum. In particular, under study is the amount of quantum features (nonclassicality, entanglement) contained in it, its mode structure and the perspectives of engineering this structure or filtering it. Moreover, the project implies application of bright squeezed vacuum to quantum information technologies such as quantum imaging, quantum metrology, and quantum key distribution.