Variational simulation of Schwinger’s Hamiltonian with polarization qubits

Applied Physics Letters, Volume 118, Issue 14, April 2021. The numerical emulation of quantum physics and quantum chemistry often involves an intractable number of degrees of freedom and admits no known approximation in the general form. In practice, representing quantum-mechanical states using available numerical methods becomes exponentially more challenging with increasing system size. Recently, quantum algorithms implemented as variational models have been proposed to accelerate such simulations. Here, we study the effect of noise on the quantum phase transition in the Schwinger model within a variational framework. The experiments are built using a free space optical scheme to realize a pair of polarization qubits and enable any two-qubit state to be experimentally prepared up to machine tolerance. We specifically exploit the possibility to engineer noise and decoherence for polarization qubits to explore the limits of variational algorithms for noisy intermediate-scale quantum architectures in identifying and quantifying quantum phase transitions with noisy qubits. We find that despite the presence of noise, one can detect the phase transition of the Schwinger Hamiltonian even for a two-qubit system using variational quantum algorithms.