Protecting collective qubits from non-Markovian dephasing

Abstract

Collectively-encoded qubits, involving ensembles of atomic or solid-state emitters, present many practical advantages for quantum technologies. However, they suffer from uncontrolled inhomogeneous dephasing which couples them to a quasi-continuum of dark states. In most cases, this process cannot be encompassed in a standard master equation with time-independent coefficients, making its description either tedious or inaccurate. We show that it can be understood as a displacement in time-frequency phase space and accurately included in resource-efficient numerical simulations of the qubit's dynamics. This description unveils a regime where the qubit becomes protected from dephasing through a combination of strong driving and non-Markovianity. We experimentally investigate this regime using a Rydberg superatom and extend its coherent dynamics beyond the inhomogeneous-dephasing characteristic time by an order of magnitude.