Yu Miyawaki

Quantum-enhanced stimulated Raman scattering (QE-SRS) is a promising technique for highly sensitive molecular vibrational imaging and spectroscopy surpassing the shot noise limit. However, the previous demonstrations of QE-SRS utilized rather weak optical power which hinders from competing with the sensitivity of state-of-the-art SRS microscopy and spectroscopy using relatively high-power optical pulses. Here, we demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, which works even when using high-power optical pulses. We used 4-ps pulses to generate pulsed squeezed vacuum at a wavelength of 844 nm with a squeezing level of −3.28 ± 0.12 dB generated from a periodically-poled stoichiometric LiTaO₃ waveguide. The squeezed vacuum was introduced to an SRS spectrometer employing a high-speed spectral scanner to acquire QE-SRS spectrum in the wavenumber range of 2000–2280 cm^-1 within 50 ms. Using SRS pump pulses with an average power of 11.3 mW, we successfully obtained QE-SRS spectrum whose SNR was better than classical SRS with balanced-detection by 2.27 dB.

Quantum-enhanced stimulated Raman scattering (QE-SRS) becomes a powerful method for high-sensitivity molecular vibrational spectroscopy and imaging by breaking the shot-noise-limited sensitivity of classical SRS. However, it’s still challenging to achieve quantum-enhancement with typical pulse power (<10-mW) for SRS. Although amplitude-squeezing technique was utilized in the previous QE-SRS, it limits the available optical power by its deamplification or optical loss at asymmetric beamsplitter. To circumvent this problem, we propose and demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, where squeezed vacuum (SQV) is combined with a strong local oscillator (LO) to enhance the sensitivity of SRS. We generated SQV at the central wavelength of 844-nm, using a periodically-poled stoichiometric LiTaO3 waveguide. Picosecond LO light is modulated by a spatial light modulator to achieve spatial matching with SQV , and resultant high squeezing level. At each photodiode, LO has ~10-mW optical power, and –3.1-dB squeezing level was observed. To obtain the QE-SRS spectrum of dimethyl sulfoxide-d6 (d-DMSO), we scanned the wavelength of Stokes laser by a high-speed spectral scanner composed of a grating and galvanometric scanner . The QE-SRS spectrum in the wavenumber range of 1950-2300-cm-1 was acquired with 2.1-dB signal-to-noise ratio (SNR) improvement compared with classical shot-noise-limited SRS under balanced-detection. The proposed QE-BD SRS paves the way for QE-SRS with high-power pulses, and shows a potential to exceed the SNR of state-of-the-art classical SRS system without quantum enhancement