Near-Degenerate Quadrature-Squeezed Vacuum Generation on a Silicon-Nitride Chip

Squeezed states are a primary resource for continuous-variable (CV) quantum information processing. To implement CV protocols in a scalable and robust way, it is desirable to generate and manipulate squeezed states using an integrated photonics platform. In this Letter, we demonstrate the generation of quadrature-phase squeezed states in the radio-frequency carrier sideband using a small-footprint silicon-nitride microresonator with a dual-pumped four-wave-mixing process. We record a squeezed noise level of 1.34 dB ($\pm 0.16\mathrm{dB}$) below the photocurrent shot noise, which corresponds to 3.09 dB ($\pm 0.49\mathrm{dB}$) of quadrature squeezing on chip. We also show that it is critical to account for the nonlinear behavior of the pump fields to properly predict the squeezing that can be generated in this system. This technology represents a significant step toward creating and manipulating large-scale CV cluster states that can be used for quantum information applications, including universal quantum computing.

Figure 1

(a) The DPDC process used in ${\chi }^{(2)}$-based squeezing, and (b) the DPFWM process used in this experiment for on-silicon-chip squeezing. (c) Experimental schematic (not to scale). Two pump lasers at 1543 and 1559 nm are coupled onto the chip, which simultaneously generates a squeezed state and a LO. The two output fields are overlapped on a $50/50$ beam splitter with the arm length difference controlled by a piezo steering mirror. After the beam splitter, the pump fields are separated from the signal fields with two transmission gratings. The signal fields are then detected by a balanced photodetector, and the rf signal is recorded by a rf spectrum analyzer (RFSA). The full experiment schematic including pump preparation and stabilization is presented in the Supplemental Material [32], which includes Ref. [33].

Figure 2

Numerical simulation of equilibrium states in a dual-pumped ${\chi }^{(3)}$ cavity. For clarity, some equilibrium states are not shown. Stable states are plotted in solid lines and unstable states in dashed lines. Dark lines indicate the branch accessed in the experiment and light lines indicate the branches to avoid. The states corresponding to the final experimental condition are shown in circles. The simulation parameters are chosen as ${\delta }_r=2\pi \times 650\mathrm{MHz}$, $\gamma =1{\mathrm{W}}^{-1}{\mathrm{m}}^{-1}$, $L=230\mu \mathrm{m}$, $f_{\delta }=2\pi \times 500\mathrm{GHz}$, $P_b=48\mathrm{mW}$, $P_r=54\mathrm{mW}$, $\alpha =2\pi \times 150\mathrm{MHz}$, (a) $\theta =2\pi \times 150\mathrm{MHz}$, and (b) $\theta =2\pi \times 300\mathrm{MHz}$.

Figure 3

Output spectra from the oscillator (a) and the squeezer (b), respectively. The FSR of the devices is 4 nm (500 GHz).

Figure 4

(a) Piezoscan voltage and (b) detected rf noise power while the delay is scanned, (i) with the squeezed state, (ii) without the squeezed state, and (iii) without light. Shaded regions $A$ and $B$ correspond to the return part of the scan. After the third scan, we block the squeezed state to record an extra copy of the shot noise, which is shown in shaded region $C$.