Enhanced intensity-difference squeezing via energy-level modulations in hot atomic media

Narrow-band intensity-difference squeezing (IDS) beams have important applications in quantum metrology and gravitational wave detection. The best way to generate narrow-band IDS is to employ a parametrically amplified (PA) four-wave mixing (FWM) process in high-gain atomic media. Such IDS can be further enhanced by cascading multiple PA FWM processes in separate atomic media. The complicated experimental setup, added losses, and mechanical stability can limit the wide use of such a scheme in practical applications. Here we show that by modulating the internal energy level(s) with an additional laser (or lasers), the degree of original IDS can be substantially increased. With an initial IDS of (-3.6 +/- 0.4) dB using a PA nondegenerate FWM process in a three-level A-type configuration, the degree of IDS can be enhanced to (-7.0 +/- 0.4) dB or (-9.0 +/- 0.4) dB when we use one (two) laser beam (beams) to modulate the involved ground (excited) state (states). Our results show a low-loss, robust, and efficient way to produce a high degree of IDS and facilitate its potential applications.