  The vacuum of the lattice Schwinger model is prepared on up to 100 qubits of
IBM's Eagle-processor quantum computers. A new algorithm to prepare the ground
state of a gapped translationally-invariant system on a quantum computer is
presented, which we call Scalable Circuits ADAPT-VQE (SC-ADAPT-VQE). This
algorithm uses the exponential decay of correlations between distant regions of
the ground state, together with ADAPT-VQE, to construct quantum circuits for
state preparation that can be scaled to arbitrarily large systems. SC-ADAPT-VQE
is applied to the Schwinger model, and shown to be systematically improvable,
with an accuracy that converges exponentially with circuit depth. Both the
structure of the circuits and the deviations of prepared wavefunctions are
found to become independent of the number of spatial sites, $L$. This allows
for a controlled extrapolation of the circuits, determined using small or
modest-sized systems, to arbitrarily large $L$. The circuits for the Schwinger
model are determined on lattices up to $L=14$ (28 qubits) with the qiskit
classical simulator, and subsequently scaled up to prepare the $L=50$ (100
qubits) vacuum on IBM's 127 superconducting-qubit quantum computers
ibm_brisbane and ibm_cusco. After applying an improved error-mitigation
technique, which we call Operator Decoherence Renormalization, the chiral
condensate and charge-charge correlators obtained from the quantum computers
are found to be in good agreement with classical Matrix Product State
simulations.

Quantum Physics;High Energy Physics - Lattice;High Energy Physics - Phenomenology;Nuclear Theory