Room temperature conditional \(\pi\)-phase shifts mediated by simultaneously propagating single-photon level pulses

Abstract

Here we demonstrate the first room-temperature implementation of \(\pi\) phase shifts in single-photon-level probe light created by a simultaneously propagating few-photon triggering signal field. The photon-photon interaction is mediated by rubidium atoms in a double \(\Lambda\) atomic scheme. We use homodyne tomography and maximum likelihood estimation on the quadrature statistics of the input and phase-shifted photons to fully characterize their quantum states in the Fock state basis. For particular choices in control fields strengths and input phases, the input-output fidelity of the controlled \(\pi\) phase shift operation reaches \(\sim\)90\(\%\).