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Optical and Quantum Sensing and Precision Metrology II

Precise amplitude and phase matching by integrating spatial light modulation and digital holography for pulsed squeezing

Yuki Sano, Kenichi Oguchi, Zicong Xu, Yoshitaka Taguchi, Kazuhiro Katoh, Yasuyuki Ozeki

Published Mar 3, 2022• 2 citations

Abstract

Quantum-enhanced measurements using squeezed light provide sensitivities beyond the shot-noise limit and have attracted much attention in many fields such as spectroscopy, gravitational wave detection, and biological imaging. To achieve high squeezing levels, the precise matching of the amplitude and phase between squeezed vacuum and local oscillator (LO) is essential. Still, it is not a simple task, especially when optical pulses are used. For continuous-wave beams, a mode-cleaning cavity or a single-mode waveguide is commonly used to control the spatial profile of the two beams. However, the cavity must be large enough to couple the optical pulses, and the waveguide causes unwanted nonlinearities for intense optical pulses and has a limited variety of materials. This study implemented precise amplitude and phase matching by integrating phase-shift digital holography and spatial light modulator (SLM). Specifically, we used a periodically-poled stoichiometric lithium tantalate (PPSLT) waveguide for optical parametric amplification (OPA). We measured the spatially resolved phase difference and intensity profile of idler light and LO by analyzing their interference movie with phase- shifting digital holography. We performed spatial mode matching by controlling the amplitude and phase of LO using SLM. As a result, we achieved high visibility of 92.9% and observed a squeezing level of -3.61 dB.

DOI: 10.1117/12.2606157

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Quantum-enhanced measurements using squeezed light provide sensitivities beyond the shot-noise limit and have attracted much attention in many fields such as spectroscopy, gravitational wave detection, and biological imaging. To achieve high squeezing levels, the precise matching of the amplitude and phase between squeezed vacuum and local oscillator (LO) is essential. Still, it is not a simple task, especially when optical pulses are used. For continuous-wave beams, a mode-cleaning cavity or a single-mode waveguide is commonly used to control the spatial profile of the two beams. However, the cavity must be large enough to couple the optical pulses, and the waveguide causes unwanted nonlinearities for intense optical pulses and has a limited variety of materials. This study implemented precise amplitude and phase matching by integrating phase-shift digital holography and spatial light modulator (SLM). Specifically, we used a periodically-poled stoichiometric lithium tantalate (PPSLT) waveguide for optical parametric amplification (OPA). We measured the spatially resolved phase difference and intensity profile of idler light and LO by analyzing their interference movie with phase- shifting digital holography. We performed spatial mode matching by controlling the amplitude and phase of LO using SLM. As a result, we achieved high visibility of 92.9% and observed a squeezing level of -3.61 dB.

Pulsed squeezed light is a key resource for many quantum information processing tasks. Among the various methods for generating squeezed light, single-pass parametric amplification has various advantages such as broad bandwidth, compactness, and stability. The challenge in achieving high-level pulsed squeezing has been to improve detection efficiency. We used a spatial light modulator controlled by machine learning to improve the spatial mode matching between the local oscillator and squeezed light. We achieved pulsed squeezing of 5.88 dB generated by optical parametric amplification in a waveguide. To the best of our knowledge, this is the highest obtained pulsed squeezing, and we believe this achievement will lead to more compact and efficient quantum information technology.

We demonstrate the generation of

12.1 \pm 0.2

dB squeezed light from a periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). While single-pass OPAs offer squeezed light with THz-order bandwidths, loss from spatial mode mismatch between the squeezed light and the local oscillator (LO) previously capped the squeezing level at $\sim$10 dB [K. Hirota et al., Opt. Express 34, 7958 (2026)]. In this work, we minimize this loss by introducing a machine-learning-optimized spatial light modulator (SLM) in the path of the LO. Specifically, we employed a double-reflection configuration to increase the spatial degrees of freedom, and directly used the measured squeezing level as the optimization's objective function.

In high-precision optical measurements, squeezed vacuum states are a promising resource for reducing the shot noise. To utilize a squeezed vacuum, it is important to lock the phase of the local oscillator (LO) to the squeezed light. The coherent control sideband (CCSB) scheme has been established for the precise phase locking, while the previous CCSB scheme was designed for the squeezed vacuum generated with an optical parametric oscillator (OPO). Thus the previous CCSB scheme is not applicable to squeezing by a single-pass optical parametric amplifier (OPA), which is attractive for generating broadband squeezed vacuum states. In this study, we propose a variant of CCSB scheme, which is applicable to the squeezing by single-pass OPA. In this scheme, we inject pump light and frequency-shifted signal light into an OPA crystal in the same way as the previous CCSB scheme. The parametric process in the OPA crystal generates a squeezed vacuum, amplifies the signal light, generates an idler light, and causes the pump depletion reflecting the interference of the amplified signal light and the idler light. Through the lock-in detection of the pump depletion, we can phase-lock the injected signal light to the pump light. Then, after the heterodyne detection of the signal and the idler light, we get the error signal of LO and realize the precise phase locking of LO to the squeezed quadrature. We show the feasibility of the proposed scheme by deriving the signal-to-noise ratio (SNR) of the modulated pump signal. We experimentally demonstrate the proposed scheme on pulsed squeezing by a single-pass OPA.

Squeezed states of light enable enhanced measurement precision by reducing noise below the standard quantum limit. A key application of squeezed light is nonlinear microscopy, where state-of-the-art performance is limited by photodamage and quantum-limited noise. Such microscopes require bright, pulsed light for optimal operation, yet generating and detecting bright pulsed squeezing at high levels remains challenging. In this work, we present an efficient technique to generate high levels of bright picosecond pulsed squeezed light using a

χ^2

optical parametric amplification process in a waveguide. We measure

-3.2~\mathrm{dB}

of bright squeezing with optical power compatible with nonlinear microscopy, as well as

-3.6~\mathrm{dB}

of vacuum squeezing. Corrected for losses, these squeezing levels correspond to

-15.4^{+2.7}_{-8.7}~\mathrm{dB}

of squeezing generated in the waveguide. The measured level of bright amplitude pulsed squeezing is to our knowledge the highest reported to date, and will contribute to the broader adoption of quantum-enhanced nonlinear microscopy in biological studies.

The effect of mode matching on the preparation of the squeezed light at 1064 nm with periodically poled KTiOPO 4 (PPKTP) crystal is reported. We can derive the desired mode by improving the spatial mode matching into optical cavities, including the beam from the laser mode matched to the optical parametric amplifier cavity and mode cleaning cavity; also it is useful for the stability of the locking systems. By increasing the fringe visibility between the squeezed light and the local oscillator beam, a squeezing level of −8.75 dB or even higher could be observed, even with the same experimental parameters as described elsewhere.

Optical parametric amplification is expected as a key technique for high-speed optical fiber communication and optical quantum computing.We developed a high-gain and broadband optical parametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide, the single-mode core of which is directly bonded to a lithium tantalate substrate.The OPA is assembled as a pump-combiner-integrated module with four optical-fiber input/output ports.By improving the fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved over-30-dB phase sensitive amplification with a continuous-wave pump.By using the PPLN waveguide, we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz.This high-gain broadband OPA module will help build an advanced information society.