Quantum reservoir computing is an unconventional computing approach that exploits the quantumness of physical systems used as reservoirs to process information, combined with an easy training strategy. An overview is presented about a range of possibilities including quantum inputs, quantum physical substrates and quantum tasks. Recently, the framework of quantum reservoir computing has been proposed using Gaussian quantum states that can be realized e.g. in linear quantum optical systems. The universality and versatility of the system makes it particularly interesting for optical implementations. In particular, full potential of the proposed model can be reached even by encoding into quantum fluctuations, such as squeezed vacuum, instead of classical intense fields or thermal fluctuations. Some examples of the performance of this linear quantum reservoir in temporal tasks are reported.
Optical parametric oscillators emit light with non-classical correlations between opposite spatial modes (twin
beams). We consider these devices in presence of an intracavity photonic crystal, modeled by a spatial modulation
of the refractive index. The introduction of photonic crystals allows to control not only the macroscopic transverse
profile of the emitted light beam but also its quantum fluctuations. We employ the Q representation to study
pump and signal spatial correlations.
We analyze a rate equation model in the Langevin formulation for the two modes of the electric field and the
carrier density, modelling the spontaneous emission noise in a semiconductor ring laser biased in the bidirectional
regime. We analytically investigate the influence of complex backscattering coefficient when the two modes
are reinterpreted in terms of mode-intensity sum (I-Spectrum) and difference (D-spectrum). The D-spectrum
represents the energy exchange between the two counterpropagating modes and it is shaped by the noisy precursor
of a Hopf bifurcation influenced mainly by the conservative backscattering. The I-Spectrum reflects the energy
exchange between the total field and the medium and behaves similarly to the standard relative intensity noise
for single-mode semiconductor lasers. Good agreement between analytical approximation and numerical results
is found.
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