Demonstration of a Coherent Tunable Amplifier for All-Optical Ising Machines

Coherent Ising machines are a type of optical accelerators that can solve different optimization tasks by encoding the problem in the connection matrix of the network. So far, experimental realizations have been limited to time multiplexed solutions, in which one nonlinear node is present in a feedback loop. In Hewlett Packard Labs, we investigate the implementation of a spatially multiplexed solution, with an array of nominally identical nonlinear nodes. As this avoids the need for a long delayline, this makes the system more suitable for integration and hence mass production. In this paper, we demonstrate a phasesensitive amplifier, a critical component in integrated circuits for this type of all-optical computing, as it allows to overcome circuit losses in the passive connection matrix of the circuit and waveguide losses in the feedback loop. The amplifier is fabricated in amorphous silicon-on- insulator and relies on thermo-optic selfheating in a ring-loaded Mach-Zehnder interferometer, although the concept is transferable to other types of nonlinearities. The effective gain is tunable by controlling the power and phase of the bias input. While we propose this amplifier in the context of all-optical integrated coherent Ising machines, this amplifier can be used in other applications where coherent amplification is required without the need to add a gain material in the fabrication flow.