Ultra Compact low cost two mode squeezed light source

Quantum-correlated states of light, such as squeezed states, constitute a fundamental resource for quantum technologies, enabling enhanced performance in quantum metrology, quantum information processing, and quantum communications. The practical deployment of such technologies requires squeezed-light sources that are compact, efficient, low-cost, and robust. Here we report a compact narrowband source of two-mode squeezed light at 795 nm based on four-wave mixing in hot 85Rb atomic vapor. The source is implemented in a small, modular architecture featuring a single fiber-coupled input, an electro-optic phase modulator combined with a single Fabry-Perot etalon for probe generation, and two free-space output modes corresponding to the signal and conjugate fields. Optimized for low pump power, the system achieves up to -8 dB of intensity-difference squeezing at an analysis frequency of 0.8 MHz with a pump power of only 300 mW. The intrinsic narrowband character of the generated quantum states makes this source particularly well suited for atomic-based quantum sensing and quantum networking, including interfaces with atomic quantum memories. Our results establish a versatile and portable platform for low-SWaP squeezed-light generation, paving the way toward deployable quantum-enhanced technologies.

💡 Research Summary

The authors present a compact, low‑cost, and low‑SWaP source of narrowband two‑mode squeezed light at 795 nm, generated via four‑wave mixing (FWM) in hot ^85Rb vapor using a double‑Λ configuration. The central innovation lies in simplifying the probe‑beam generation stage: a single electro‑optic phase modulator (EOPM) driven at 3.04 GHz creates the required frequency‑shifted sideband, and a single Fabry‑Perot etalon (FSR ≈ 15 GHz, finesse > 30) filters out all unwanted sidebands, delivering a clean probe beam with minimal added intensity noise. This architecture replaces more complex schemes such as double‑pass acousto‑optic modulators (AOMs) or Mach‑Zehnder electro‑optic modulators (MZ‑EOMs) that typically require multiple crystals, interferometric bias control, and several etalons, all of which increase loss, technical noise, and system footprint.

In the experimental setup, a 795 nm external‑cavity diode laser is amplified to 300 mW by a tapered amplifier and serves as the pump. A small fraction of the pump is frequency‑down‑shifted by the EOPM to generate the probe, which is then injected together with the pump into a 12.5 mm isotopically pure ^85Rb vapor cell heated to ≈110 °C. The pump and probe are orthogonally polarized and intersect at a small angle (~0.5°) inside the cell, satisfying the two‑photon Raman resonance (Δ₂ ≈ –8 MHz) while the one‑photon detuning is set to 0.9 GHz. The FWM process amplifies the probe (gain ≈ 15) and creates a conjugate beam at the complementary frequency. After the cell, a polarizing beam splitter removes residual pump light, and the probe and conjugate are directed to a balanced photodetector. The intensity‑difference noise is measured with a spectrum analyzer (RBW = 30 kHz, VBW = 300 Hz). A delay line in the conjugate arm compensates for slow‑light effects.

The key performance metrics are: (i) up to –8 dB of intensity‑difference squeezing relative to the shot‑noise limit (SNL) at an analysis frequency of 0.8 MHz; (ii) a squeezing bandwidth extending to ≈3 MHz; (iii) total detected optical power of ≈270 µW (probe ≈ 150 µW, conjugate ≈ 120 µW) after amplification; and (iv) system noise contributing roughly 1 dB of degradation, primarily from residual pump scattering and electronic detector noise. The authors model internal atomic absorption as fictitious beam splitters with transmissivity η, showing that the achievable squeezing is fundamentally limited by these intrinsic losses rather than by the probe‑generation stage.

Comparative measurements of probe‑beam noise demonstrate that the EOPM + single‑etalon configuration yields the lowest initial intensity noise among three tested methods (double‑pass AOM, MZ‑EOM with three etalons, and the proposed EOPM scheme). The reduction in optical components directly translates into lower insertion loss, fewer sources of excess phase and amplitude noise, and eliminates the need for interferometric stabilization.

From a practical perspective, the entire optical train—including fiber‑coupled input, EOPM, etalon, vapor cell, and free‑space output—fits within a footprint of less than 10 cm and consumes under 1 W of RF power in addition to the 300 mW optical pump. This makes the source amenable to field deployment, integration with portable atomic clocks, quantum memories, and quantum‑enhanced sensors that require narrowband light matched to atomic transitions.

In conclusion, the work establishes that high‑quality squeezed light does not necessitate elaborate, power‑hungry modulation schemes. By adopting a minimalistic yet carefully engineered architecture, the authors achieve near‑optimal squeezing limited only by the intrinsic properties of the hot‑Rb medium. This paradigm offers a clear pathway toward truly deployable quantum‑enhanced technologies, bridging the gap between laboratory‑scale demonstrations and real‑world applications in navigation, timing, precision spectroscopy, and secure communications.