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Journal of the Optical Society of America B

Quantum-enhanced balanced detection for ultrasensitive transmission measurement

Yasuyuki Ozeki, Yu Miyawaki, Yoshitaka Taguchi

Published Sep 14, 2020• 32 citations
Publisher

Abstract

Balanced detection is a popular method to cancel out the effect of laser intensity noise in optical measurements and spectroscopy. However, the signal-to-noise ratio (SNR) that can be achieved with balanced detection is constrained by the standard quantum limit (SQL). Here, we propose quantum-enhanced balanced detection (QBD), which allows us to improve the SNR beyond the SQL to realize ultrasensitive transmission measurement. In QBD, squeezed vacuum is injected to one of the input ports of a beamsplitter (BS) used in balanced detection to produce a pair of light waves whose shot noises are entangled with each other. Compared with previous quantum-enhanced measurement methods, QBD is advantageous because it can handle a higher optical power without sacrificing the degree of sensitivity enhancement. We present the theory of QBD and discuss the effects of the splitting ratio of the BS and the optical loss caused by the sample under test. We also describe the application of QBD to the sensitivity enhancement of molecular vibrational imaging based on stimulated Raman scattering microscopy.

DOI
10.1364/josab.403222

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Balanced detection is a popular method to cancel out the effect of laser intensity noise in optical measurements and spectroscopy. However, the signal-to-noise ratio (SNR) that can be achieved with balanced detection is constrained by the standard quantum limit (SQL). Here, we propose quantum-enhanced balanced detection (QBD), which allows us to improve the SNR beyond the SQL to realize ultrasensitive transmission measurement. In QBD, squeezed vacuum is injected to one of the input ports of a beamsplitter (BS) used in balanced detection to produce a pair of light waves whose shot noises are entangled with each other. Compared with previous quantum-enhanced measurement methods, QBD is advantageous because it can handle a higher optical power without sacrificing the degree of sensitivity enhancement. We present the theory of QBD and discuss the effects of the splitting ratio of the BS and the optical loss caused by the sample under test. We also describe the application of QBD to the sensitivity enhancement of molecular vibrational imaging based on stimulated Raman scattering microscopy.
Stimulated Raman scattering (SRS) microscopy is a powerful technique that allows for the visualization of molecular vibrational images for label-free imaging, metabolic imaging, and supermultiplex imaging. However, its sensitivity is mainly limited by the shot noise of laser pulses, and long acquisition times are sometimes required to detect weak SRS signals hidden by the shot noise. To overcome this limitation, quantum-enhanced (QE-) SRS microscopy has been demonstrated, while the low optical power of squeezed pulses limits the sensitivity. In this work, we present QE-SRS microscopy using quantum-enhanced balanced detection (QE-BD) scheme, where the sensitivity of balanced detection SRS microscope is enhanced by injecting squeezed vacuum, allowing for QE-SRS imaging with high power SRS pump pulses (typically several tens of milliwatt), while balanced detection causes 3 dB drawback in the signal-to-noise ratio. We experimentally demonstrate QE-SRS imaging with 2.6 dB noise reduction compared with shot-noise-limited balanced detection SRS. We also demonstrate hyperspectral QE-SRS imaging by fast wavelength tuning of Stokes pulses. These results show the potential feasibility of high-power QE-SRS whose sensitivity is beyond that of classical shot-noise-limited SRS microscopes.
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
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.
Benefitting from the sub-Poissonian statistics, squeezed light has been applied to push the shot-noise-limited sensitivity of stimulated Raman scattering spectroscopy and microscopy to a sub-shot-noise level [5]. However, the balanced detection regime requires a high squeezing level to counteract the 3-dB signal-to-noise ratio (SNR) drawback. In this presentation, we introduce dual-polarization quantum-enhanced stimulated Raman scattering (QESRS), which is also suitable for high-power operation [6]. This approach utilizes a symmetric beamsplitter but detects squeezed light on two independent photodetectors, thus avoiding the 3-dB decrease in SNR.
Quantum-enhanced stimulated Raman scattering (QESRS) microscopy is expected to realize molecular vibrational imaging with sub-shot-noise sensitivity, so that weak signals buried in the laser shot noise can be uncovered. Nevertheless, the sensitivity of previous QESRS did not exceed that of state-of-the-art stimulated Raman scattering (SOA-SRS) microscopes mainly because of the low optical power (3 mW) of amplitude squeezed light [Nature594, 201 (2021)10.1038/s41586-021-03528-w]. Here, we present QESRS based on quantum-enhanced balanced detection (QE-BD). This method allows us to operate QESRS in a high-power regime (>30 mW) that is comparable to SOA-SRS microscopes, at the expense of 3 dB sensitivity drawback due to balanced detection. We demonstrate QESRS imaging with 2.89 dB noise reduction compared with classical balanced detection scheme. The present demonstration confirms that QESRS with QE-BD can work in the high-power regime, and paves the way for breaking the sensitivity of SOA-SRS microscopes.
Squeezed light has been applied in stimulated Raman scattering (SRS) spectroscopy [1] and microscopy [2] to break the shot-noise-limited sensitivity. The sub-shot-noise sensitivity achieved in quantum-enhanced SRS (QE-SRS) systems is advantageous for uncovering weak signals and achieving faster imaging speed, which attract the attention of biomedical scientists. To break the sensitivity of state-of-the-art SRS microscopes, the applied squeezed light power in QE-SRS systems should be high enough (>10mW). Recently, high-power picosecond-pulsed QE-SRS spectroscopy [3] based on balanced detection scheme [4] was reported, and a sensitivity that is comparable to state-of-the-art SRS microscopes was firstly demonstrated in QE-SRS microscopy based on the same scheme [5]. An important issue of QE-SRS based on balanced detection is the 3-dB additional noise, so >3-dB squeezing level is required to demonstrate an absolute sensitivity improvement compared with state-of-the-art SRS microscopes in single-photodiode detection scheme. However, it's hard to generate highly squeezed pulsed light because of the difficulty in temporal and spatial mode matching between squeezed vacuum (SQV) and local oscillator (LO). Here, we introduce QE-SRS in dual-polarization scheme, which can be implemented in high-power regime but is free from the 3-dB signal-to-noise ratio drawback.