Quantum Assisted Simulator

Quantum simulation offers a possibility to explore the exponentially large configuration space of quantum mechanical systems and thus help us study poorly understood topics such as high-temperature superconductivity and drug design. Here, we provide a novel hybrid quantum-classical algorithm for simulating the dynamics of quantum systems. Without loss of generality, the Hamiltonian is assumed to be a linear combination of unitaries and the Ansatz wavefunction is taken as a linear combination of quantum states. The quantum states are fixed, and the combination parameters are variationally adjusted. Unlike existing variational quantum simulation algorithms, our algorithm does not require any classical-quantum feedback loop and by construction bypasses the barren plateau problem. Moreover, our algorithm does not require any complicated measurements, such as the Hadamard test. The entire framework is compatible with existing experimental capabilities and thus can be implemented immediately. We also provide an extension of our algorithm to imaginary time evolution.