We present a direct optical homodyne measurement for broadband squeezed light with ~80nm bandwidth, lifting the extreme bandwidth limitation of the standard homodyne measurement.   

Homodyne measurement is a corner stone of quantum optics and quantum information and is used in numerous applications, such as continues variable quantum computing and sub-shot noise measurement. Homodyne is the standard measurement of the two quadratures of light – the optical analog of quantum position and momentum. The standard homodyne technique uses square-law photo-detectors for mixing the optical signal to be measured and an external local-oscillator. As a result the homodyne bandwidth is extremely limited by the electrical bandwidth of the photo-detectors, typically MHz to GHz range. Wider bandwidth measurement requires two homodyne setups with phase and frequency locks between the local-oscillators, which are hard to prepare. Considering the growing interest in quantum optical squeezed states and the fact that two-mode squeezing with a separation of optical frequency can be generated just as easily as degenerate single-mode squeezing, this limitation is critical for exploration and utilization of quantum squeezing.

To overcome this inherent bandwidth limit, we replace the slow square-law photo-detectors in the homodyne process with a nonlinear optical medium, and use the same pump-laser, originally used for generating the squeezed state, as the homodyne local-oscillator. By this we achieve a direct optical homodyne measurement, where the homodyne process generates an optical output, offering effectively unlimited measurement bandwidth.

To demonstrate the optical homodyne measurement experimentally, we use parametric amplification with optical four-wave mixing in a photonic crystal fiber in a double pass configuration, pumped by a narrow band laser. In the first pass the parametric amplifier serves to generate broadband squeezed-light and in the second pass to perform the homodyne measurement of the incoming squeezing. The ability to use an identical mechanism for both the original generation and for the homodyne measurement is a great advantage and is applicable in many experiments. In our case, the original four-wave mixing signal-idler spectrum has a ~50nm bandwidth each with ~140THz mean frequency separation, many orders of magnitude over the standard homodyne bandwidth limit. We observe a reduction of ~1.5dB under the vacuum level, over the entire spectrum with our method. This is the first time such a large squeezing bandwidth has been measured simultaneously.

References:

1. Y. Shaked, R. Pomerantz, R. Z. Vered and A. Pe'er, "Observing the nonclassical nature of ultra broadband bi-photons at ultra-fast speed", New J. Phys. 16, 053012 (2014).
2. R. Z. Vered, Y. Shaked, Y. Ben-Or, M. Rosenbluh, and A. Pe’er, "Classical-to-quantum transition with broadband four-wave mixing", Phys. Rev. Lett. 114, 063902 (2015).