  Real-time quantum simulation of quantum field theory in (3+1)D requires large
quantum computing resources. With a few-qubit quantum computer, we develop a
novel algorithm and experimentally study the Schwinger effect, the
electron-positron pair production in a strong electric field, in (3+1)D. The
resource reduction is achieved by treating the electric field as a background
field, working in Fourier space transverse to the electric field direction, and
considering parity symmetry, such that we successfully map the three spatial
dimension problems into one spatial dimension problems. We observe that the
rate of pair production of electrons and positrons is consistent with the
theoretical predication of the Schwinger effect. Our work paves the way towards
exploring quantum simulation of quantum field theory beyond one spatial
dimension.
