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Applied Physics Letters

A highly efficient photodetector for squeezed light measurement in the gigahertz range

Dennis Wilken, J. Junker, M. Heurs

Published Jan 1, 2025• 0 citations
Publisher

Abstract

Squeezed light plays a crucial role in state-of-the-art quantum metrology and quantum information experiments. There is significant interest in utilizing squeezed states at high MHz and GHz frequencies. However, past efforts to build suitable photodetectors at these frequencies have yet to yield the required high quantum efficiency. Here, we present the development of a high-frequency balanced photodetector with near-unity quantum efficiency, realized with off-the-shelf components. The detector operates in balanced mode up to approximately 500 MHz, above which the differential frequency response limits its performance. To obtain high sensitivity above 500 MHz, the detector can be efficiently used in an unbalanced homodyne detection scheme. We employ our detector in this unbalanced mode to measure a squeezing comb up to 6.4 GHz, achieving a squeezing level of up to 10.7 dB. By sharing our experience, specifically in identifying the unequal frequency response as a limiting factor, we aim to enable and advance further developments in the field.

DOI
10.3204/pubdb-2026-00285

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Squeezed light plays a crucial role in state-of-the-art quantum metrology and quantum information experiments. There is significant interest in utilizing squeezed states at high MHz and GHz frequencies. However, past efforts to build suitable photodetectors at these frequencies have yet to yield the required high quantum efficiency. Here, we present the development of a high-frequency balanced photodetector with near-unity quantum efficiency, realized with off-the-shelf components. The detector operates in balanced mode up to approximately 500 MHz, above which the differential frequency response limits its performance. To obtain high sensitivity above 500 MHz, the detector can be efficiently used in an unbalanced homodyne detection scheme. We employ our detector in this unbalanced mode to measure a squeezing comb up to 6.4 GHz, achieving a squeezing level of up to 10.7 dB. By sharing our experience, specifically in identifying the unequal frequency response as a limiting factor, we aim to enable and advance further developments in the field.
Squeezed light plays a crucial role in state-of-the-art quantum metrology and quantum information experiments. There is significant interest in utilizing squeezed states at high MHz and GHz frequencies. However, past efforts to build suitable photodetectors at these frequencies have yet to yield the required high quantum efficiency. Here, we present the development of a high-frequency balanced photodetector with near-unity quantum efficiency, realized with off-the-shelf components. The detector operates in balanced mode up to approximately 500 MHz, above which the differential frequency response limits its performance. To obtain high sensitivity above 500 MHz, the detector can be efficiently used in an unbalanced homodyne detection scheme. We employ our detector in this unbalanced mode to measure a squeezing comb up to 6.4 GHz, achieving a squeezing level of up to 10.7 dB. By sharing our experience, specifically in identifying the unequal frequency response as a limiting factor, we aim to enable and advance further developments in the field.
Squeezing-based quantum metrology has been extensively applied in the field of precision measurement. Performance characteristics of squeezed light are obtained by a balanced homodyne detector. Here, we report a large dynamic range balanced homodyne detector with low-noise and high-signal-to-noise-ratio (SNR) for broadband squeezed light measurement. We perform a comprehensive theoretical analysis of the SNR in a wide bandwidth and experimentally analyze the important printed circuit board (PCB) layout and electronic components. Consequently, in the 100 MHz frequency band, the SNR and common mode rejection ratio (CMRR) rise over 12.6 dB and 55 dB, respectively, and the electronic noise is suppressed below -90 dB. The directly observed squeezing level at 1550 nm with the specified detector is 11.8 dB at 2 MHz and 3.1 dB at 100 MHz.
A high-performance resonant balanced homodyne detector is a key element for the measurement of picosecond pulsed squeezed light, which is an important quantum resource in quantum-enhanced microscopic imaging. In this paper, we design and demonstrate a resonant photodetector for homodyne measurement with a maximum signal-to-noise ratio of 22.42 dB at resonant frequency of 20 MHz. Using this detector to pulsed regime, a maximum signal-to-noise ratio at resonant frequency is 10.02 dB, where a 40 dB subtraction capability at 80 MHz repetition rate of pulsed laser is obtained. With this design, the quantum noise of picosecond pulsed squeezed light is measured and the best squeezing level −1.7 dB below the shot noise level is clearly observed at 20 MHz.
As an essential quantum resource, the intensity-difference squeezed state based on four-wave mixing (FWM) in atomic vapor is widely applied in quantum information processing. In particular, a high intensity-difference squeezing bandwidth is vital for the realization of high-speed information processing. However, limited by the bandwidth of photodetectors, broadband intensity-difference squeezed state based on this system has not yet been reported. Here, we developed a transimpedance broadband balanced homodyne detector at 895 nm, achieving a bandwidth greater than 100 MHz and a maximum signal-to-noise ratio of 15 dB with 4 mW optical power. Utilizing this detector in a nondegenerate FWM process based on cesium vapor, we experimentally achieved broadband intensity-difference squeezing with a bandwidth of 100 MHz, which yielded a maximum squeezing of −7.17 ± 0.8 dB between 20 and 40 MHz. Meanwhile, using this detector, we experimentally investigated the cavity-enhanced FWM process, achieving a squeezing level of −6.07 ± 0.5 dB within a 4 MHz frequency range, which is limited by the cavity bandwidth. This work provides a reliable detection tool and experimental foundation for the research and application of broadband squeezed light sources based on FWM.
Balanced Homodyne Detection (BHD), introduced in the 1970s, enables highly sensitive detection of phase and amplitude variations in weak light. Initially developed as an advanced optical detection technique, it rapidly became essential in quantum optics for accurately quantifying quadrature components and detecting squeezed states at the shot-noise limit. Today, BHD plays a critical role in diverse fields such as high-precision quantum metrology, gigabit speed optical communications, gravitational wave detection, and large-scale quantum information systems. In this study, a BHD simulation was implemented with a 90 phase difference between the local oscillator and the signal, thus enabling the extraction of the phase quadratureP^component of the optical field.
High-frequency squeezed states are important for quantum metrology and information processing, but quadrature measurements at gigahertz frequencies remain challenging. Balanced homodyne detection (BHD), the standard approach, requires closely matched complex transfer functions in both photodetection channels to suppress local-oscillator (LO) noise and maintain a well-defined readout quadrature. This matching becomes increasingly difficult at high bandwidths. Strongly asymmetric single-photodiode homodyne detection avoids this requirement, but achieving high signal efficiency and shot-noise clearance typically requires hundreds of milliwatts of LO power and does not suppress technical LO sidebands. I propose cavity-assisted homodyne detection, in which the quantum field and LO enter separate ports of an impedance-matched traveling-wave cavity. The resonant LO carrier is transmitted to the photodetector, where it beats with off-resonant signal sidebands reflected from the cavity. At the same time, technical LO sidebands outside the cavity linewidth are suppressed. I derive the quantum input-output relations and linearized photocurrent including intracavity loss, and show that near-unity signal-transfer efficiency can be achieved at moderate incident LO power. The cavity linewidth and free spectral range determine the usable detection band.