EIT quantum memory in the presence of four-wave mixing

Nikolai Lauk, Christopher O’Brien, Michael Fleischhauer

We study the effects of four-wave mixing (4WM) in a quantum memory scheme based on electromagnetically induced transparency (EIT). We treat the problem of field propagation on the quantum mechanical level, which allows us to calculate the fidelity of propagation for a quantum light pulse such as a single photon. While 4WM can be beneficial for classical, all-optical information storage, the quantum noise associated with the signal amplification and idler generation is, in general, detrimental for a quantum memory. We identify a range of parameters where 4WM makes a single-photon quantum memory impossible.

G. Romanov, Irina V. Novikova

Halyne Silva Borges, Celso Villas-B oas

Qi Zhang, Guoxiang Huang

Quantum vacuum noise is very detrimental for photon propagation and optical quantum memory, which particularly occurs in optically thick atomic media working under the condition of electromagnetically induced transparency (EIT) accompanied by a process of four-wave mixing (FWM). The suppression of quantum vacuum noise becomes now an imperative task for advancing the research and application of quantum memory. Here we present a scheme to suppress quantum vacuum noise and hence to improve the fidelity of photon propagation, by assuming that signal and idler laser fields are initially prepared in a two-mode squeezed vacuum state. With such a scheme, we show that the normal gain mode inherent in the EIT-based FWM system, which contributes optical gain to both the signal and idler fields, can be largely inhibited, giving rise to a significant suppression of the quantum vacuum noise along with their amplification and hence a large enhancement of the fidelity for photon propagation. The results reported here may have promising applications for high-fidelity quantum information processing and transformation, especially for improving the quality of multimode optical quantum memory based on EIT techniques.

G. Romanov, Irina V. Novikova

Roberto Buffa, Stefano Cavalieri, Marco V. Tognetti

In their recent study [1] on a four-wave mixing (FWM) process, Deng et al. state that ‘‘many orders of magnitude enhancement in the amplitude of the generated wave’’ is obtained when electromagnetically induced transparency (EIT) is present. Here we will show that their claim is deeply misleading. In fact, according to the formulas reported in their paper, it is straightforward to show that the efficiency of the FWM generation process is actually reduced by the presence of EIT, and that the maximum FWM signal is obtained in the absence of EIT.