The scientific background is the following:
It was the year 1996 when Davis and coworkers  set the cornerstone of Femtosecond Laser Micromachining (FLM), demonstrating that the optical properties of a small volume inside a glass substrate could be modified in a permanent way by tightly focused ultrashort laser pulses. In particular, for low pulse energies, a smooth refractive index change may be produced inside the material, allowing one to write optical waveguides and more complex photonic devices by translating the sample under the focused laser beam along the desired path.
It is worth noting that FLM is not limited to micromachining of glass substrates. In fact, relying on the nonlinear nature of the process, FLM stands out for its unique flexibility in processing any kind of transparent material (glass, polymers, crystals) in a three-dimensional geometry [2,3].
 K. M. Davis et al. “Writing waveguides in glass with a femtosecond laser”, Optics letters, 21(21), 1729-1731, (1996).
 R. R. Gattass et al. ”Femtosecond laser micromachining in transparent materials”, Nature photonics, 2(4), 219-225, (2008).
 R. Osellame et al. “Femtosecond laser micromachining, photonic and microfluidic devices in transparent material”, Springer (2012).
Let us take a black-box device with several inputs, where classical massive particles (say, billiard balls) can enter, bounce around and then exit from a set of outputs with given probabilities. By measuring the output distributions of single balls inserted in one input at a time, the behaviour of the system when multiple balls are injected simultaneously in different inputs is easily predicted.
This wouldn’t actually be the case, if an analogous experiment were performed with quantum particles such as photons. In fact, let us now consider an optical multi-mode interferometer, which is exactly a device with several inputs and outputs where photons can bounce and exchange paths. The task of calculating, or even approximating, the collective output distribution when several identical photons are injected in the device is named Boson Sampling. Accomplishing such task with a classical computer takes a computational time that increases exponentially with the number of photons involved and rapidly becomes intractable.
A Boson Sampling experiment [1, 2, 3, 4], where a large enough number of photons is injected in an interferometer with a large enough number of ports, may produce experimental results that are impossible to simulate classically, and may represent the first viable demonstration of the superior computational power of quantum hardware.
 M. Broome et al. “Photonic boson sampling in a tunable circuit”, Science 339, 794-798 (2013).
 J. Spring et al. “Boson sampling on a photonic chip”, Science 339, 798-801 (2013).
 M. Tillmann et al. “Experimental boson sampling”, Nature Photonics 7, 540 (2013).
 A. Crespi et al. “Integrated multimode interferometers with arbitrary designs for photonic boson sampling”, Nature Photonics 7, 545 (2013).
Efficient and long-lived quantum memories are crucial devices for the development of quantum technologies, as they allow synchronizing quantum mechanical events, e.g. single photon emission or logic gate operations, which typically occur in a probabilistic fashion .
Among the various ways devised for realising optical quantum memories, the mapping of single photons in the collective excitation of rare-earth-doped crystalline matrices is particularly promising, and several milestone achievements have been demonstrated in bulk systems of this kind . In addition, rare-earth-doped crystals are naturally suitable for the integration of quantum memories in waveguide-based integrated optical circuits . Therefore, they are the ideal platform for including quantum memory operations in integrated quantum photonics architectures.
 N. Sangouard et al. “Quantum repeaters based on atomic ensembles and linear optics”, Reviews of Modern Physics, 83(1), 33 (2011).
 M. P. Hedges et al. “Efficient quantum memory for light”, Nature 465(7301), 1052 (2010).
 A. Seri et al. “Laser-written integrated platform for quantum storage of heralded single photons”, Optica, 5(8), 934-941 (2018).