Optimal detuning for writing warm-atomic-vapor quantum memory in the presence of collisional fluorescence and four-wave mixing

Abstract

Atomic vapor cells with buffer gas have a number of advantages when employed as quantum memory blocks based on the DLCZ (Duan-Lukin-Cirac-Zoller) protocol: operation slightly above room temperature, ease of handling, as well as commercial availability. Nevertheless, the signal-to-noise ratio in the current implementations is severely limited by the simultaneous presence of collisional fluorescence and the four-wave mixing noise. In our previous work, we have shown how to minimize the influence of the former on the writing process and provided an unambiguous demonstration of quantum memory lasting for 4 μs. An elegant approach to suppress the four-wave-mixing noise by pre-pumping to the state with the hyperfine sublevel with the maximum value was proposed by Walther et al., Int. J. Quantum Inform. 5, 51 (2007). Here we show that this approach is fundamentally limited by the cancellation of the Raman matrix elements involving the Fˊ= 1 and Fˊ= 2 levels, which occurs for all experimental conditions in the S → P transitions of all alkali atoms. A detuning that maximizes the signal-to-noise ratio is shown to exist for a given detector dark-count rate.