Geometric phases are well known in classical electromagnetism and quantummechanics since the early works of Pantcharatnam and Berry. Their origin relieson the geometric nature of state spaces and has been studied in many differentsystems such as spins, polarized light and atomic physics. Recent works haveexplored their application in interferometry and quantum computation. Earlierworks suggest how to observe these phases in single quantum systemsadiabatically driven by external classical devices or sources, where, byclassical, we mean any system whose state does not change considerably duringthe interaction time: an intense magnetic field interacting with a spin 1/2, ora birefringent medium interacting with polarized light. Here we propose afeasible experiment to investigate quantum effects in these phases, arisingwhen this classical source drives not a single quantum system, but twointeracting ones. In particular, we show how to observe a signature of fieldquantization through a vacuum effect in Berry's phase. To do so, we describethe interaction of an atom and a quantized cavity mode altogether driven by anexternal quasi-classical field.
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