Observation of quantum noise reduction in a Raman amplifier via quantum correlation between atom and light

Abstract

Any amplifier requires coupling to its internal degrees of freedom for energy gain. This coupling introduces extra quantum noise to the output. On the other hand, if the internal degree of the amplifier can be accessed and manipulated, we can manage and even reduce the quantum noise of the amplifier's output. In this paper, we present an experiment to reduce the quantum noise of a Raman amplifier by preparing the atomic medium in a correlated state with the Stokes light field. We report an observation of quantum noise reduction of more than 3.5 dB in the atomic Raman amplification process. From another perspective, the Raman amplifier at high gain in turn serves as a measurement tool for the quantum correlation between the atom and light. Furthermore, such a scheme, when viewed as a whole, also forms a quantum-entangled atom-light hybrid interferometer that can lead to quantum-enhanced sensors.

Figures (5)

• Figure 1: Conceptual schemes for (a) quantum noise reduction of an amplifier by correlating internal degree with the input field; (b) measurement of quantum correlation/entanglement between atomic spin wave and Stokes light field by a Raman amplifier; (c) formation of a hybrid atom-light SU(1,1) interferometer using Raman amplifiers (RA) as beam splitters for the input Stokes field.
• Figure 2: Experimental schematic for a Raman amplifier (RA2) with the atoms and light field prepared in a quantum correlated state by another Raman process (RA1). M: mirror; PBS: polarization beam splitter; PZT: piezo-electric transducer. HWP: half wave plate. LO: local oscillator. S: Stokes field. PD: photo diode. OP: Optical pumping.
• Figure 3: Quantum noise reduction of a Raman amplifier. (a) Noise levels for uncorrelated input (blue), correlated input with a phase scan (red) and the vacuum or shot noise level is -73.6 dB; (b) Noise reduction as a function of pump power of RA1 for various gains of RA2.
• Figure 4: Noise level reduction as a function of the gains of RA2 for different degrees of correlation between atom and light. (a1-a4) are fitted via Eq.(\ref{['R3']}) to get the quantum correlation coefficients at different powers of W1. (b) Solid red points are the quantum correlation $\Delta^2 X_+$ in log-scale with 0 dB corresponding to vacuum, evaluated using best fit parameters obtained in (a). (c) The fitting values of $\mu$, the Stokes field loss $L_1$, and the atomic spin wave loss $L_2$ obtained from Eq.(\ref{['R3']}) at different powers of W1.
• Figure 5: Interference fringes (red dots with blue trace fit to sine-function) as a function of phase scan (gray), measured at the output of RA2. Red trace: injected Stokes level at input of RA1; Black trace: background level.