Detection of 15 dB Squeezed States of Light and their Application for the Absolute Calibration of Photoelectric Quantum Efficiency

Squeezed states of light belong to the most prominent nonclassical resources. They have compelling applications in metrology, which has been demonstrated by their routine exploitation for improving the sensitivity of a gravitational-wave detector since 2010. Here, we report on the direct measurement of 15 dB squeezed vacuum states of light and their application to calibrate the quantum efficiency of photoelectric detection. The object of calibration is a customized InGaAs positive intrinsic negative (p-i-n) photodiode optimized for high external quantum efficiency. The calibration yields a value of 99.5% with a 0.5% (k = 2) uncertainty for a photon flux of the order 10^17/s at a wavelength of 1064 nm. The calibration neither requires any standard nor knowledge of the incident light power and thus represents a valuable application of squeezed states of light in quantum metrology.

💡 Research Summary

The authors report the direct observation of 15 dB of continuous‑wave squeezed vacuum at 1064 nm and demonstrate a novel, absolute calibration method for the external quantum efficiency (EQE) of a custom InGaAs p‑i‑n photodiode. Using a monolithic non‑planar Nd:YAG laser (2 W, 1064 nm) they generate 532 nm pump light (≈350 mW) which drives a doubly resonant optical parametric amplifier (OPA) based on a 9.3 mm periodically poled KTP crystal. The OPA cavity (≈40 mm length, finesse 54 at 1064 nm) is locked to resonance via Pound‑Drever‑Hall technique. With pump powers as low as 16 mW they achieve up to 15 dB of noise reduction, while the corresponding anti‑squeezing reaches about 30 dB.

Balanced homodyne detection with a 26.5 mW local oscillator (photon flux 1.4 × 10¹⁷ s⁻¹) provides a clearance of 28 dB between electronic dark noise and the vacuum noise reference. The measured squeezing spectra are fitted with a standard OPA model that includes total detection efficiency η_tot, pump‑to‑threshold ratio P/P_thr, side‑band frequency f, and an rms phase jitter θ_pn. The best‑fit parameters are η_tot = 0.975, θ_pn = 1.7 mrad, and an OPA escape efficiency η_esc = 99.05 %.

A detailed loss budget reveals an overall optical loss of 2.5 % (±0.1 %). Of this, 0.8 % stems from the homodyne fringe visibility (99.6 %), 0.2 % from lens transmission, and the remaining ≈1.5 % is attributed to the photodiode’s quantum efficiency. Consequently, the external EQE of the device under test is determined to be 99.5 % ± 0.5 % (k = 2). Importantly, this calibration requires no absolute power reference or calibrated detector; it relies solely on the measured squeezing/anti‑squeezing levels and the independently characterized optical losses.

The work has two major implications. First, achieving 10 dB of squeezing together with only 11 dB of anti‑squeezing demonstrates a low‑loss, low‑phase‑noise source suitable for injection into gravitational‑wave interferometers, where total optical loss must stay below 10 % to obtain a 10 dB quantum‑noise improvement. Second, the absolute EQE calibration method provides a trace‑free, high‑precision tool for characterizing photodetectors in quantum‑enhanced metrology, quantum communication, and precision spectroscopy. Further reductions in OPA escape loss and phase noise could push the calibration uncertainty into the 10⁻⁴ regime, making this technique competitive with traditional radiometric standards.

In summary, the paper establishes a record‑high 15 dB squeezed‑vacuum source, validates a loss‑budget model that isolates detector efficiency, and delivers a practical, standards‑free method for absolute photodiode calibration, thereby advancing both fundamental quantum optics and its applied metrological technologies.