Universal Optical Amplification without Nonlinearity

We propose and experimentally realize a new scheme for universal phase-insensitive optical amplification. The presented scheme relies only on linear optics and homodyne detection, thus circumventing the need for nonlinear interaction between a pump field and the signal field. The amplifier demonstrates near optimal quantum noise limited performance for a wide range of amplification factors.

Figure 1

Conceptual diagram of the amplifier setup. The gain is determined by the transmission of the equivalent beam splitter (BS) [realized here by a combination of a half wave plate and a polarizing beam splitter (PBS)]. AM and PM are amplitude and phase modulators, respectively. Also shown are the phase space diagrams of the input coherent state and the amplified output state.

Figure 2

Power spectra showing the operation of the amplifier for conjugate quadratures. The beam splitter was set to $1:2$ enabling an optical gain of $G=1.5(1.8\mathrm{dB})$. The mean value of the field is amplified by 1.8 dB and the noise is consequently increased by 3.2 dB, which is very close to the ideal noise level of 3 dB above the shot noise. The noise figure is $\mathrm{NF}=0.7$. The resolution bandwidth is 10 kHz and the video bandwidth is 30 Hz.

Figure 3

The noise figure, NF, as a function of the gain, $G$. The black ( gray) dots represent the experimental data for the amplitude (phase) quadrature. The solid line represents the quantum noise limit [given by Eq. (2)], whereas the predicted noise figure for our device with imperfect detectors is shown by the dashed line. For comparison, the dotted line corresponds to an amplifier with two vacuum units of extra technical noise. Errors mainly stem from the inaccuracy in determining the quantum efficiency of the photodiodes.

Figure 4

Proposed scheme for a phase conjugating amplifier with the nonlinearity put off-line. The displacements, indicated by $D$, can be performed as shown in Fig. 1.