The Impact of Quantum Noise in Neuromorphic Systems

The quantum dynamics of driven-dissipative systems with a stable classical fixed-point are well understood, however dynamics far from equilibrium and in regimes without a fixed-point are less studied. Considerable theoretical challenges arise because such situations often involve strong transient excitation while the role of quantum fluctuations remains significant. This scenario is realized in neuromorphic optical systems which respond to a weak input above threshold with a robust large amplitude pulse. The quantum dynamics of neuromorphic systems can thus not be described by the standard small fluctuation expansion around a classical steady-state, and a full quantum modeling is out of question due to large transients. We present a general theoretical approach based on quantum stochastic differential equations which captures quantum noise about classical trajectories and apply it to two physical realizations of neuromorphic dynamics: an excitable laser and a superconducting circuit. Contrary to previous findings, fundamental quantum noise drives large fluctuations in pulse response times, while the amplitude response remains robust. In addition, quantum noise softens the bifurcation to a self-sustained pulsation regime by exciting the system in the absence of an input.