Controlling irreversibility and directionality of light via atomic motion: optical transistor and quantum velocimeter

Abstract

The Doppler effect of moving atoms can create irreversibility of light. We show that the laser field in an electromagnetic induced transparency (EIT) scheme with atomic motion can control the directional propagation of two counter-propagating output probe fields in an atomic gas. Quantum coherence and the Doppler effect enable the system to function like an optical transistor with two outputs that can generate states analogous to the Bell basis. Interference of the two output fields from the gas provides useful features for determining the mean atomic velocity and can be used as a sensitive quantum velocimeter. Some subtle physics of EIT is also discussed. In particular, the sign of the dispersive phase in EIT is found to have a unique property, which helps to explain certain features in the interference.