Cancellation of Internal Quantum Noise of an Amplifier by Quantum Correlation

Quantum noise is usually added in an amplification process through the internal degrees of the amplifier. Coupling the squeezed state to the internal degree can suppress the extra noise. Here, we demonstrate another method: when the internal degree of the amplifier is correlated with the input signal via quantum entanglement, quantum destructive interference between the input and the internal degree may result in noise reduction at the amplified output. We achieve a quantum noise reduction of 2.3 dB at the output and an improvement of $4.0\pm 0.2\mathrm{dB}$ in signal-to-noise ratio during the amplification process with a quantum noise gain of 4.5 dB.

Figure 1

Schematic diagram for an amplifier with its internal degrees in (a) vacuum; (b) squeezed states; (c) entangled states with the input. The dotted boxes indicate signal (arrow) with its noise (circles and ellipses).

Figure 2

Detailed experimental arrangement.

Figure 3

(a) Noise levels of the amplifier under various inputs: (i) shot noise level; (ii) amplified noise level with the input and idler in vacuum; (iii) input noise level for a thermal state; (iv) amplified noise level with the input in a thermal state and the idler in vacuum; (v) noise level of the amplifier with the input and the idler entangled; (vi) noise level of the amplifier with the phase between the two fields of the entangled source scanned. (b) Noise reduction of the amplifier with the entangled input versus the noise level of the entangled source for the noise gain of the amplifier at 4 dB (blue diamond), and 6 dB (red square), respectively.

Figure 4

Signal and noise levels of the input and output of an amplifier under different situations: (a) input signal at vacuum level and the internal degree of the amplifier is in vacuum; (b) input signal with extra noise and the amplifier is in vacuum; (c) input is the same as in (b) but the internal degree of the amplifier is entangled with the input.