Generation and detection of squeezed states via a synchronously pumped optical parametric oscillator

TL;DR

This work tackles stable generation and robust detection of pulsed squeezed states in a multimode, spectrally structured regime. It introduces a self-referenced local oscillator scheme where a counter-propagating beam used for cavity stabilization also serves as the LO for homodyne detection, paired with independent spectral shaping of pump and LO via spatial light modulators and dispersion management. The study reports up to -3.3 dB detected squeezing (−5.7 dB at the SPOPO output) with high homodyne visibility, in good agreement with a multimode singular-value-decomposition model that captures pump-power and LO-width dependencies across multiple supermodes. Overall, the work provides a robust, flexible platform for pulsed squeezed-light generation with potential for multimode quantum information applications and time/frequency multiplexed quantum networks.

Abstract

A synchronously pumped optical parametric oscillator (SPOPO) operating at 93 MHz is used to generate squeezed states at 1035 nm. The system features a counter-propagating beam at the same wavelength as the quantum state, which simultaneously actively stabilizes the cavity and, after transmission, acts as the local oscillator for homodyne detection. By deriving the local oscillator directly from the SPOPO cavity, the setup establishes an intrinsically excellent spatial mode overlap and high interference visibility, forming a distinctive self-referenced architecture. Two spatial light modulators enable precise spectral shaping of both the pump and the local oscillator in amplitude and phase, allowing investigation of the spectral properties of the generated states. The versatility of the setup further allows exploration of different SPOPO configurations, including regimes with varied finesse and escape efficiency. Representative measurements, including homodyne traces and squeezing levels as functions of pump power and local oscillator bandwidth, demonstrate the performance of the system. Theoretical simulations based on a multimode singular-value-decomposition model reproduce well the measured dependence of squeezing on pump power and LO bandwidth, confirming the accuracy of the description and the robustness of the setup. Measured squeezing levels up to -3.3 dB are achieved, corresponding to -5.7 dB at SPOPO output, evidencing the robustness and versatility of this platform for stable pulsed squeezed-light generation and advanced quantum optical applications.

Figures (4)

• Figure 1: Scheme of the experimental setup. Pump and LO are generated from a 93MHz single mode-locked laser, and independently spectrally shaped using two SLMs. The SPOPO is frequency-stabilized to the laser repetition rate, and both the quantum state and the LO originate from the same cavity mode. A balanced homodyne detection stage measures the frequency-resolved squeezing level. HWP half-waveplate, PBS polarizing beam splitter, LBO lithium-borate crystal, SLM$_\mathrm{i}$ spatial light modulators, PDH Pound-Drever-Hall electronics, M$_\mathrm{ic}$ and M$_\mathrm{oc}$ input and output couplers, M$_\mathrm{ch}$ chirped mirror, NLC nonlinear crystal, LO local oscillator, QS quantum state, PZT piezoelectric actuator, BS 50:50 beam splitter, FIL bandpass filter, AMP differential amplifier.
• Figure 2: a) Measured quadrature noise versus LO phase $\theta$, normalized to the shot-noise level (above), and corresponding squeezing level in dB (below). Here, $R_\mathrm{oc}=81%$, and intracavity dispersion is compensated with a chirped mirror of -900fs. b) Wigner function associated with the state outgoing the SPOPO as reconstructed from the experimental homodyne trace in the left panel of the figure and corresponding to a squeezed-thermal state with $72%$ purity and -5.7dB of squeezing (see the text for details).
• Figure 3: a) Squeezing and anti-squeezing levels as a function of pump power normalized to the SPOPO threshold. b) Squeezing and anti-squeezing levels as a function of LO spectral width. Lines correspond to theoretical predictions, while points are experimental values. Error bars ($\sim\pm0.2dB$) are within the marker size.
• Figure 4: Squeezing and anti-squeezing level contribution at the SPOPO output, $\sigma_\mathrm{sq,k}^2$, of the first $40$ supermodes $k$ (top), along with their projection, $\left|M_k\right|^2$, onto LOs with different spectral widths (bottom) of 1.0nm, 2.0nm, and 3.0nm FWHM. In this case, the fundamental supermode has a spectral width of 4.4nm-FWHM.