Demonstration of the spatial separation of the entangled quantum sidebands of an optical field

Quantum optics experiments on “bright” beams are based on the spectral analysis of field fluctuations and typically probe correlations between radio-frequency sideband modes. However, the extra degree of freedom represented by this dual-mode picture is generally ignored. We demonstrate the experimental operation of a device which can be used to separate the quantum sidebands of an optical field. We use this device to explicitly demonstrate the quantum entanglement between the sidebands of a squeezed beam.

Figure 1

A schematic diagram of the system along with spectra illustrating successful operation at the classical level. All the spectral measurements were made with a scanning confocal Fabry-Pérot cavity [21] with a free spectral range of 500 MHz and a linewidth of approximately 2 MHz. The truncated carrier is shown at zero frequency, the phase-modulation sidebands are at ±90.5 MHz and residual mode-mismatch peaks (with less than 1% power) are at ±250 MHz.

Figure 2

Homodyne measurements of the quantum noise limit (QNL) for the local oscillator power; the input to the UMZI $(“A_{\mathrm{in}}”)$, one output of the UMZI when locked to $\varphi =-\pi ∕2$ $(“A_{2,-\pi ∕2}”)$; and the predicted output of the UMZI based on the measured input amplitude and quadrature phase variances, as well as the measured homodyne detection fringe visibilities (0.89% and 0.92% for input and output, respectively [20]) and UMZI fringe visibility (“Theory”).

Figure 3

Characterization of the output beams of the UMZI when using squeezed light. The upper two graphs show the amplitude noise level of the individual beams (gray lines) compared to the respective quantum noise limit (black lines). The lower left graph shows the correlations in the amplitude quadrature (gray line) compared to the quantum noise limit (black line). The lower right graph shows the correlations in the phase quadrature (gray line) compared to the quantum noise limit (black line). All noise levels have been detected using a pair of spectrum analyzers (HP 8590) at a measurement frequency of 10.25 MHz, a resolution bandwidth of 300 kHz, and a video bandwidth of 30 Hz. All traces have been corrected for the electronic noise level, which was at about −77.9 dB m.