Fig. 1. (a) Schematic of transmitting the quantum coherence of a Gaussian state in a thermal noise channel. (b) Experimental setup. The 1080 nm and the 540 nm laser outputs from the Nd:YAP/LBO laser pass through two mode cleaners and are injected into the NOPA as signal light and pump light, respectively. The output modes of the PBS behind the NOPA are an amplitude squeezed state (transmitted mode) and a phase squeezed state (reflected mode) or EPR entanglement state, when the HWP behind the NOPA is set to 22.5° or 0°, respectively. We use homodyne detectors to measure the output modes and a digital storage oscilloscope to record the experimental data. The interference efficiencies of homodyne detectors are 99%, and the quantum efficiencies of photodiodes (LASER COMPONENTS, InGaAs-PD-500um) are 99.6%. AM, amplitude modulator; PM, phase modulator.

Fig. 2. Experimental results in a lossy channel. (a) Dependence of squeezing (red solid line) and antisqueezing (blue dotted line) of the squeezed state on the loss. The dashed line is the shot noise limit (SNL). (b) Dependence of PPT value of the EPR entangled state on the loss. The dashed line is the boundary of the entangled and separable states. (c) and (d) Dependence of the quantum coherence of the squeezed state and the EPR entangled state on the loss, respectively. The initial squeezed and antisqueezed noise levels are −2.95 dB and 4.15 dB, respectively. The error bars represent one standard deviation and are obtained based on the statistics of the data.

Fig. 3. Experimental results in a noisy channel. (a) Dependence of the squeezing (red solid line) and antisqueezing (blue dotted line) of the squeezed state on the excess noise. The dashed line is SNL. (b) Dependence of the PPT value of the EPR entangled state on the excess noise. The dashed line is the boundary of the entangled and separable states. (c) and (d) Dependence of the quantum coherence of the squeezed state and the EPR entangled state on the excess noise, respectively. (e) and (f) Quantum coherence of the squeezed state and the EPR entangled state parameterized by loss and excess noise, respectively. The red dots represent the experimental results in (c) and (d).

Fig. 4. Schematic of transmitting an EPR entangled state in two Gaussian thermal noise channels.

Fig. 5. (a) and (b) Dependence of PPT values of the EPR entangled state on the loss and the excess noise, respectively. The dashed line is the boundary of the entangled and separable states. (c) and (d) Dependence of quantum coherence of the EPR entangled state on the loss and the excess noise, respectively. (e) Quantum coherence of the EPR entangled state parameterized by loss and excess noise.