As a crucial quantum light source, the squeezed state of light plays a pivotal role in advancing quantum information, quantum networks, quantum computing, and quantum-precision measurements. The emergence of optical frequency combs is of great significance in the field of precision measurements and offers a novel avenue for achieving high-precision and long-distance measurements. The optical frequency comb represents a new class of light sources characterized by periodic ultrashort pulses, resulting in a spectrum with equally spaced frequencies. Based on the special properties of the optical frequency comb, researchers applied it to the quantum field, obtaining the “quantum frequency comb,” which is a new quantum resource contributing to the development of the quantum field. The generation of a femtosecond squeezed state in an optical field is challenging because of the extended cavity and optical path of the resonator. Compared with continuous waves, femtosecond pulses are more sensitive to external noise, posing difficulties in achieving a stable squeezed state. This limitation hinders the practical application of femtosecond squeezed light in quantum information protocols. Thus, achieving a long-term stable femtosecond pulse-squeezed state of the light field is considered a fundamental requirement.

A titanium sapphire laser is used to generate pulsed light with a central wavelength of 850 nm, pulse width of 130 fs, and pulse repetition rate of 76 MHz. A femtosecond pulse-squeezed vacuum state of the light field is obtained without signal injection based on a synchronously pumped optical parametric oscillator below the threshold. Nevertheless, the primary hurdle is the effective suppression of noise through the implementation of phase-locking between the signal and local components. To address this, the relative phase between the pump and local field is locked to suppress the phase noise. A bismuth borate (BIBO) nonlinear crystal with a thickness of 2 mm is used to generate the second harmonic. The second harmonic light generated by the pump is interference-locked with the pump. The interference signal is segmented into two paths and subtracted using a half-wave plate and polarization beam splitter (PBS) to improve the signal-to-noise ratio. Finally, the error signal is directed toward the proportion integration differentiation (PID) controller, which drives the piezoelectric ceramic in the local field. This achieves effective phase synchronization between the local field and pump. This synchronization locks the experimental light path before the 50/50 beam splitter. By improving the relative phase stability of the beams, a long-term stable femtosecond pulse-squeezed vacuum state of light is achieved. The degree of phase drift of the squeezed light is measured and analyzed for both the locked and unlocked phase loops.

In the absence of locked phase loops, the measured squeezed vacuum noise fluctuates significantly owing to external factors. Long-term monitoring reveals a large left-right drift in the power spectrum line of the squeezed vacuum noise, with a phase jitter range of 0.507 rad (Fig. 2). After locking the relative phase between the local field and pump, the stability of the system improves. The stability of the noise power spectrum of the squeezed light is significantly enhanced compared with that of the unlocked situation (Fig. 6). Simultaneously, the left-right drift at the squeezed light in both cases is analyzed, and data points are extracted and plotted in a scatter plot. The standard deviation of the two sets of data reflects the degree of phase jitter. The analysis results show that the random phase jitter is suppressed by 68.05% during the experiment (Fig. 7). Finally, the noise signals in both cases are smoothed. Under long-term measurement, the long-term squeezing degree increases from 0.6 dB to 3.2 dB, effectively inhibiting random phase jitter and improving the quality and accuracy of squeeze test results (Fig. 8).

Based on a synchronously pumped optical parametric oscillator below the threshold, a femtosecond pulse-squeezed vacuum state of the light field is obtained without signal field injection. This study successfully achieves a long-term stable output of squeezed vacuum state of femtosecond pulse through a scheme of frequency multiplication of local light and interference locking of frequency multiplication light with the pump. The experimental results demonstrate the successful suppression of 68.05% of the stochastic phase jitter during the squeezing measurement process, leading to an increase in the squeezing degree from 0.6 dB to 3.2 dB. Thus, the stability of the system is significantly enhanced. This methodology addresses the challenges associated with femtosecond pulses, paving the way for advancements in quantum technology.