Filtering multiphoton emission from state-of-the-art cavity quantum electrodynamics

Engineering multiphoton states is an outstanding challenge with applications in multiple fields such as quantum metrology, quantum lithography, or even biological sensing. State-of-the-art methods to obtain them rely on post-selection, multi-level systems, or Rydberg atomic ensembles. Recently, it was shown that a strongly driven two-level system interacting with a detuned cavity mode can be engineered to continuously emit n-photon states. In the present work, we show that spectral filtering of its emission relaxes considerably the requirements on the system parameters even to the more accessible bad-cavity situation, opening up the possibility of implementing this protocol in a much wider landscape of different platforms. This improvement is based on a key observation: in the imperfect case where only a certain fraction of emission is composed of n-photon states, these have a well-defined energy separated from the rest of the signal, which allows one to reveal and purify multiphoton emission just by frequency filtering. We demonstrate these results by obtaining analytical expressions for the relevant figures of merit of multiphoton emission, such as the n-photon coupling rate between cavity and emitter, the fraction of light emitted as n-photon states, and n-photon emission rates. This allows us to make a systematic study of such figures of merit as a function of the system parameters and demonstrate the viability of the protocol in several relevant types of cavity quantum electrodynamics setups, where we take into account the impact of their respective experimental limitations. (c) 2018 Optical Society of America