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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Ultra-fast Continuous-wave Homodyne Measurement of Schrödlnger Cat States with 40-ps Wavepackets

Katsuki Nakashima, Tatsuki Sonoyama, Akito Kawasaki, Takumi Suzuki, Takahiro Kashiwazaki, Taichi Yamashima, Asuka Inoue, Takeshi Umeki, Masahiro Yabuno, Shigehito Miki, Hirotaka Terai, Kan Takase, Warit Asavanant, Mamoru Endo, Akira Furusawa

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Light has a high carrier frequency of hundreds of THz and has the potential to realize ultrafast quantum information processing. Time-domain multiplexed continuous-variable optical quantum computation has recently attracted much attention [1]. In this method, it is important to encode optical quantum states in wavepackets with short temporal widths because the width of the quantum state determines the clock frequency of the computation. So far, in the continuous-wave (CW) light field, this wavepacket width has been limited by the bandwidth of homodyne measurements, which read out the quadratures of the quantum state. Recently, a technique using phase-sensitive amplifiers (PSAs) and broadband homodyne detectors with CW light has been developed [2]. Using that, the wavepacket width of quantum entangled states was successfully shortened to the picosecond scale [3]. However, the wavepacket width of the Schrödinger cat state, which is a phase-sensitive non-Gaussian state, was limited to the sub-nanosecond scale due to the timing jitter of the photon detector [4].

DOI
10.1109/cleo/europe-eqec65582.2025.11109412

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Light has a high carrier frequency of hundreds of THz and has the potential to realize ultrafast quantum information processing. Time-domain multiplexed continuous-variable optical quantum computation has recently attracted much attention [1]. In this method, it is important to encode optical quantum states in wavepackets with short temporal widths because the width of the quantum state determines the clock frequency of the computation. So far, in the continuous-wave (CW) light field, this wavepacket width has been limited by the bandwidth of homodyne measurements, which read out the quadratures of the quantum state. Recently, a technique using phase-sensitive amplifiers (PSAs) and broadband homodyne detectors with CW light has been developed [2]. Using that, the wavepacket width of quantum entangled states was successfully shortened to the picosecond scale [3]. However, the wavepacket width of the Schrödinger cat state, which is a phase-sensitive non-Gaussian state, was limited to the sub-nanosecond scale due to the timing jitter of the photon detector [4].
Entanglement is a fundamental resource for various optical quantum information processing (QIP) applications. To achieve high-speed QIP systems, entanglement should be encoded in short wavepackets. Here we report the real-time observation of ultrafast optical Einstein–Podolsky–Rosen correlation at a picosecond timescale in a continuous-wave system. Optical phase-sensitive amplification using a 6-THz-bandwidth waveguide-based optical parametric amplifier enhances the effective efficiency of 70-GHz-bandwidth homodyne detectors, mainly used in 5G telecommunication, enabling its use in real-time quantum state measurement. Although power measurement using frequency scanning, such as an optical spectrum analyser, is not performed in real time, our observation is demonstrated through the real-time amplitude measurement and can be directly used in QIP applications. The observed Einstein–Podolsky–Rosen states show quantum correlation of 4.5 dB below the shot-noise level encoded in wavepackets with 40 ps period, equivalent to 25 GHz repetition—103 times faster than previous entanglement observation in continuous-wave systems. The quantum correlation of 4.5 dB is already sufficient for several QIP applications, and our system can be readily extended to large-scale entanglement. Moreover, our scheme has high compatibility with optical communication technology such as wavelength-division multiplexing, and femtosecond-timescale observation is also feasible. Our demonstration is a paradigm shift in accelerating accessible quantum correlation—the foundational resource of all quantum applications—from the nanosecond to picosecond timescales, enabling ultrafast optical QIP. Optical Einstein–Podolsky–Rosen correlation is observed on a picosecond timescale in a continuous-wave system. By introducing waveguide optical parametric amplifiers and balanced detectors, the quantum correlation 4.5 dB below the shot-noise level is observed.
We propose a pulse and continuous wave (CW) hybrid architecture of continuous-variable measurement-based optical quantum computation utilizing the strengths of both pulsed and CW light. In this architecture, input and ancillary non-Gaussian quantum states necessary for fault-tolerance and universality are generated with pulsed light, whereas quantum processors including continuous-variable cluster states and homodyne measurement systems are operated with CW light. This architecture is expected to enable both generation of quantum states with shorter optical wavepackets for ultrafast computation and low-loss manipulation and measurement of these states. In this study, as a proof-of-principle, an ultrafast homodyne measurement using a CW local oscillator was performed on single-photon states generated with pulsed light. The measured single-photon state's temporal width was around 70 ps and the value of the Wigner function at the origin was W(0,0)=−0.153±0.003W(0,0) = -0.153\pm0.003W(0,0)=−0.153±0.003, which is highly non-classical. This will be a core technology for high-speed optical quantum information processing.
Quantum information processors greatly benefit from high clock frequency to fully harnessing the quantum advantages before they get washed out by the decoherence. In this pursuit, all-optical systems offer unique advantages due to their inherent 100 THz carrier frequency, permitting one to develop THz clock frequency processors. In practice, the bandwidth of the quantum light sources and the measurement devices has been limited to the MHz range and the generation rate of nonclassical states to kHz order -- a tiny fraction of what can be achieved. In this work, we go beyond this limitation by utilizing optical parametric amplifier (OPA) as a squeezed-light source and optical phase-sensitive amplifiers (PSA) to realize high-rate generation of broadband non-Gaussian states and their quantum tomography. Our state generation and measurement system consists of a 6-THz squeezed-light source, a 6-THz PSA, and a 66-GHz homodyne detector. With this system, we have successfully demonstrated non-Gaussian state generation at a 0.9 MHz rate -- almost three orders of magnitude higher than the current state-of-the-art experiments -- with a sub-nanosecond wave packet using continuous-wave laser. The performance is constrained only by the superconducting detector's jitter which currently limits the usable bandwidth of the squeezed light to 1 GHz, rather than the optical and electronic systems. Therefore, if we can overcome the limitation of the timing jitter of superconducting detector, non-Gaussian state generation and detection at GHz rate, or even THz rate, for optical quantum processors might be possible with OPAs.
Quantum information processors benefit from high clock frequencies to fully harness quantum advantages before they are lost to decoherence. All-optical systems offer unique benefits due to their inherent 100-THz carrier frequency, enabling the development of THz-clock frequency processors. However, the bandwidth of quantum light sources and measurement devices has been limited to the MHz range, with nonclassical state generation rates in the kHz range. In this study, we demonstrated broadband generation and quantum tomography of non-Gaussian states using an optical parametric amplifier (OPA) as a squeezed light source and an optical phase-sensitive amplifier (PSA). Our system includes a 6-THz squeezed-light source, a 6-THz PSA, and a 66-GHz homodyne detector. We successfully generated non-Gaussian states at a 0.9 MHz rate with sub-nanosecond wave packets using a continuous-wave laser. The performance is currently limited by the jitter of superconducting detectors, restricting the usable bandwidth to 1 GHz. Our technique extends the bandwidth to GHz, potentially increasing non-Gaussian state generation rates for practical optical quantum processors using OPAs. The use of optical phase sensitive amplifiers (PSA) has been shown to allow fast quantum tomography of Gaussian states, but non-Gaussian states are a key component for quantum computation using optical continuous-variable systems. Here, the authors extend PSA-enhanced high-bandwidth homodyne detection to non-Gaussian states.
Phase-sensitive detection is the essential projective measurement for measurement-based continuous-variable quantum information processing. The bandwidth of conventional electrical phase-sensitive detectors is up to several gigahertz, which would limit the speed of quantum computation. It is theoretically proposed to realize terahertz-order detection bandwidth by using all-optical phase-sensitive detection with an optical parametric amplifier (OPA). However, there have been experimental obstacles to achieve large parametric gain for continuous waves, which is required for use in quantum computation. Here, we adopt a fiber-coupled χ (2) OPA made of a periodically poled LiNbO 3 waveguide with high durability for intense continuous-wave pump light. Thanks to that, we manage to detect quadrature amplitudes of broadband continuous-wave squeezed light. 3 dB of squeezing is measured up to 3 THz of sideband frequency with an optical spectrum analyzer. Furthermore, we demonstrate the phase-locking and dispersion compensation of the broadband continuous-wave squeezed light, so that the phase of the squeezed light is maintained over 1 THz. The ultra-broadband continuous-wave detection method and dispersion compensation would help to realize all-optical quantum computation with over-THz clock frequency.
A balanced homodyne detection scheme with nanosecond time resolution and sub-shot-noise sensitivity has been developed and successfully tested yielding an efficient detection scheme for high-speed quantum-optical measurements and communication protocols, for example, quantum cryptography. The parameters of the detector and its precise balancing allow complete characterization of quantum states created by femtosecond light pulses that include the measurement of photon number, optical phase, and statistical properties with a high signal-to-noise ratio for the whole bandwidth from DC to several tens of megahertz. The electronic part of the detector is based on a commercially available amplifier that provides ease of construction and use while yielding good performance.